الرئيسية The Third Chimpanzee for Young People: On the Evolution and Future of the Human Animal
The Third Chimpanzee for Young People: On the Evolution and Future of the Human AnimalJared Diamond, Rebecca Stefoff
At some point during the last 100,000 years, humans began exhibiting traits and behavior that distinguished us from other animals, eventually creating language, art, religion, bicycles, spacecraft, and nuclear weapons—all within a heartbeat of evolutionary time. Now, faced with the threat of nuclear weapons and the effects of climate change, it seems our innate tendencies for violence and invention have led us to a crucial fork in our road. Where did these traits come from? Are they part of our species immutable destiny? Or is there hope for our species’ future if we change?
With fascinating facts and his unparalleled readability, Diamond intended his book to improve the world that today’s young people will inherit. Triangle Square’s The Third Chimpanzee for Young People is a book for future generation and the future they’ll help build.
With fascinating facts and his unparalleled readability, Diamond intended his book to improve the world that today’s young people will inherit. Triangle Square’s The Third Chimpanzee for Young People is a book for future generation and the future they’ll help build.
You may be interested in
Most frequently terms
A SEVEN STORIES PRESS FIRST EDITION Copyright © 2014 by Jared Diamond All rights reserved. No part of this book may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, including mechanical, electric, photocopying, recording, or otherwise, without the prior written permission of the publisher. SEVEN STORIES PRESS 140 Watts Street New York, NY 10013 www.sevenstories.com College professors may order examination copies of Seven Stories Press titles for free. To order, visit www.sevenstories.com/ textbook or send a fax on school letterhead to (212) 226-1411. A teaching guide and lesson plan for The Third Chimpanzee for Young People, as well as other select titles within the Young People’s series from Triangle Square / Seven Stories Press, is available for free download at www.sevenstories.com. Book design by Pollen/Stewart Cauley, New York Library of Congress Cataloging-in-Publication Data Stefoff, Rebecca, 1951- Third chimpanzee for young people / by Jared Diamond ; adapted by Rebecca Stefoff. -- Seven Stories Press first edition. pages cm Adapted from Jared M. Diamond’s The third chimpanzee. Includes index. ISBN 978-1-60980-522-7 1. Human evolution--Juvenile literature. 2. Social evolution- juvenile literature. 3. Nature--Effect of human beings on-Juvenile literature. I. Diamond, Jared M. II. Diamond, Jared M. Third chimpanzee. III. Title. GN281.S785 2014 573.2--dc23 2013040205 Printed in the United States 9 8 7 6 5 4 3 2 1 Contents Introduction WHAT MAKES US HUMAN? PART ONE JUST ANOTHER BIG MAMMAL CHAPTER 1: A TALE OF THREE CHIMPS CHAPTER 2: THE GREAT LEAP FORWARD PART TWO A STRANGE LIFE CYCLE CHAPTER 3: HUMAN SEXUALITY CHAPTER 4: THE ORIGIN OF HUMAN RACES CHAPTER 5: WHY DO WE GROW OLD AND DIE? PART THREE UNIQUELY HUMAN CHAPTER 6: THE MYSTERY OF LANGUAGE CHAPTER 7: ANIMAL ORIGINS OF ART CHAPTER 8: AGRICULTURE, FOR BETTER AND WORSE CHAPTER 9: WHY DO WE SMOKE, DRINK, AND USE DANGEROUS DRUGS? CHAPTER 1O: ALONE IN A CROWDED UNIVERSE PART FOUR WORLD CONQUERORS CHAPTER 11: THE LAST FIRST CONTACTS CHAPTER 12: ACCIDENTAL CONQUERORS CHAPTER 13: IN BLACK AND WHITE PART FIVE REVERSING OUR PROGRESS OVERNIGHT CHAPTER 14: THE GOLDEN AGE THAT NEVER WAS CHAPTER 15: BLITZKRIEG AND THANKSGIVING IN THE NEW WORLD CHAPTER 16: THE SECOND CLOUD Afterword NOTHING LEARNED, EVERYTHING FORGOTTEN? Glossary Photograph and Illustration Credits About the Authors Introduction WHAT MAKES US HUMAN? HUMANS ARE DIFFERENT FROM ALL ANIMALS. At the same time, humans are animals—a species of big mammal. This contradiction is our most fascinating feature. We still have a hard time understanding what it means and how it came to be. On one hand, between us and all other species lies a gulf that leads us to call them “animals” and to see them as separate from us. We think that centipedes, chimpanzees, and clams share some animal features that we don’t have, or that we have human features that they don’t share. Those human features include communicating through language, enjoying art, making complex tools, wearing clothes, and darker traits such as killing mass numbers of our own and other species. On the other hand, we have the same body parts, molecules, and genes as other animals. It’s even clear what type of animal we are. As long ago as the eighteenth century, scientists who studied anatomy (the structure of the body) saw that humans are very similar to chimpanzees, animals that live in Africa. We recognize two species of chimpanzees: the common chimp and the bonobo, sometimes called the pygmy chimp. A scientist from outer space would immediately classify humans as a third species of chimpanzee. Scientists right here on Earth know that we share more than 98 percent of our genetic makeup with the other two chimps. The difference between our genes and chimps’ genes is small. Yet that small difference must have been responsible for the things that make humans unique. And all those changes happened fairly recently in our genetic history. Somehow, within a few tens of thousands of years, we started to show the features that make humans unique and fragile. This book takes a close look at how and why we developed those features, both good and bad—from language, art, and our life cycle to our ability to destroy our own and other species. How This Book Came to Be My own interests and background shaped this book. As a child, I wanted to be a doctor. By my last year in college, that goal had gently changed, and I wanted to become a medical researcher. I trained in physiology, which is the study of how living systems function, from cells to animals. Afterward I went on to teach and do research at the University of California Medical School in Los Angeles. But I had other interests as well. Birdwatching had attracted me since the age of seven, and I had also been lucky to attend a school that let me plunge into languages and history. I did not like the idea of spending the rest of my life on physiology alone. Then I had the chance to spend a summer in the highlands of New Guinea, a large tropical island north of Australia. The purpose of the trip was to measure how successfully birds were nesting. That project collapsed when I was unable to locate even a single bird’s nest in the jungle, but the trip fed my thirst for adventure and birdwatching in one of the wildest remaining parts of the world. After that first trip to New Guinea, I developed a second career, focused on birds, evolution, and biogeography. I’ve returned to New Guinea and the neighboring Pacific islands many times to pursue my bird research. As I saw human activity destroying the forests and birds I loved, I became involved in conservation, helping governments design national parks to protect ecosystems and plant and animal species. Finally, it was hard to study the evolution and extinction of birds without wanting to understand the evolution and possible extinction of the most interesting species of all, the species that includes you, me, and everyone on Earth—Homo sapiens, the modern human. This book was the result. It begins with a look at our origins several million years ago. It ends with some thoughts about our future, and about ways we can learn from our past. Building a Big Picture The story of how we became human spans millions of years, and it pulls together information and ideas from many branches of science. In writing this book, I drew on my own experiences and the sciences I have studied, and also on the work of many scientists in other fields, from archaeology to zoology. Pieces of the story come from fields as different as paleopathology, the study of ancient diseases, and paleobotany, the science of fossil plants. As you’ve seen, my background started with anatomy and physiology, then moved on to the study of birds, especially their ecology—that is, the ways bird interact with other species around them and with their environment. As a biogeographer, I’m interested in the relationships between geography and living things. Biogeographers ask questions such as: Why are some species spread out across almost the entire world, while others live only in a single tree? As you’ll see in this book, biogeography has played a big role in the history of our species. I am also an evolutionary biologist. This means that I look at animals and plants in terms of evolution, the process of change in life on Earth over time, as new species develop and old ones become extinct. (In chapter 4, you’ll read about how this happens.) In this book, I use the framework of evolutionary biology to examine human features and behavior. Seeing Ourselves in a New Way From a scientist’s point of view, things often look different from the way they look in everyday life. Take the question of how people are attracted to each other. What do you find attractive in another person? There are as many answers to that question as there are individuals in the world. But to an evolutionary biologist, the question takes on another dimension. Because we see the human species as part of the natural world, we assume that people are shaped by the same forces that shape other species. By looking for patterns in the way birds and mice and apes choose their mates, as I do in chapter 3, we expect to learn something about our own behavior. In evolutionary terms, successful features and behaviors let parents produce the greatest number of children, who will eventually produce children of their own, passing the parents’ genes on to new generations. This doesn’t mean that evolutionary biology is the complete explanation, or the only explanation, for everything people do. It does mean that seeing ourselves as part of the evolutionary history of life enlarges our knowledge. Looking at our own species in the same way we look at others can bring new understanding of human behavior that may seem confusing or mysterious, or make us uncomfortable. It is a way of knowing ourselves better—and the quest for selfknowledge is a very human characteristic. PART ONE JUST ANOTHER BIG MAMMAL (Left to right: Gibbon, Human, Chimpanzee, Gorilla, Orangutan.) Five members of the primate family: Homo sapiens and four kinds of apes. The similar anatomy of human and ape skeletons had been recognized for centuries, but DNA studies confirmed that chimpanzees are our closest relatives, and we are theirs. WHEN, WHY, AND HOW DID WE STOP BEING JUST another species of big mammal? Clues come from three types of evidence, all explored in the next two chapters. Fossil bones and preserved tools are traditional evidence from archaeology, the study of the past through physical remains. A newer kind of evidence comes from the science of molecular biology, which examines our genetic heritage and traces our descent from an apelike ancestor. One basic question concerns the differences between us and chimpanzees. Just looking at humans and chimps and counting visible differences doesn’t help, because many genetic changes have effects that can’t be seen, while other changes have very obvious effects. A Great Dane and a Chihuahua look much more different from each other than a chimp and a human being do. Yet all dogs belong to the same species, but chimps and humans are different species. So how can we tell our genetic distance from chimps? The problem has been solved by molecular biologists. They have discovered that the gene difference between us and chimps is greater than the difference between any two living human populations or any two breeds of dogs. But the gene difference between us and chimps is small compared with differences between many other pairs of related species. This means that only a small change in the chimpanzee genes led to enormous changes in humans’ behavior. Next we’ll consider what we can learn from the bones and tools left by creatures along the way between our apelike ancestor and modern humans. Fossil bones show the switch from our walking on all fours to walking upright, and our increase in brain size. Our large brain was surely necessary for the development of human language and inventiveness. In fact, we might expect the fossil record to show our tools getting better as our brains got bigger. But the greatest surprise and puzzle of human evolution is that stone tools remained very crude for hundreds of thousands of years after our brains had expanded almost to their present size. Sixty thousand years ago, Neanderthals had brains even larger than those of modern humans, yet their tools show no signs of inventiveness or art. Neanderthals were still just another species of big mammal. Even for tens of thousands of years after some other human populations had evolved skeletons like those of modern people, their tools remained as boring as Neanderthals’ tools. Within the small percentage of difference between our genes and chimpanzee genes, there must have been an even smaller percentage that was not involved in the shapes of our bones but that gave us the human traits of inventiveness, artistic creativity, and the use of complex tools. In Europe, at least, those traits appeared suddenly at the time Neanderthals were replaced by the early modern humans known as Cro-Magnons. That’s when we finally stopped being just another species of big mammal. At the end of part 1, I’ll talk about what triggered our steep rise to human status. CHAPTER I A TALE OF THREE CHIMPS THE NEXT TIME YOU VISIT A ZOO, WALK PAST THE ape cages. Imagine that the apes had lost most of their hair and that next to them was a cage holding some unfortunate people who had lost their clothes and couldn’t speak but were normal in every other way. Now try guessing how different the apes’ genes are from the humans’ genes. Would you guess that a chimpanzee shares 10 percent, 50 percent, or 99 percent of its genetic makeup with humans? In recent decades, science has answered that question. Even though many other questions remain unanswered, we now know more about our origins than ever before. Every human society has felt a deep need to make sense of its origins, and has met that need with its own story of creation. The creation story of our time is the tale of three chimps. Three Questions For centuries it’s been clear roughly where we fit into the animal kingdom. We are mammals, part of the group of animals that have hair and nurse their young. Among mammals, we are primates, the group of mammals that includes monkeys and apes. We share primate features that most non-primates do not have, including flat fingernails and toenails (rather than claws), hands for gripping, and thumbs that can move in the opposite direction from our fingers. Within the primates, we are more similar to apes (gorillas, chimpanzees, orangutans, and gibbons) than to monkeys. For one thing, monkeys have tails, but apes and humans do not. Gibbons stand out from the other apes because they are small and have very long arms. Gorillas, chimpanzees, orangutans, and humans are more closely related to one another than any of them is to gibbons. Going further into our primate relationships proved difficult for scientists. It led to an intense debate centered on three questions: * What is the detailed family tree of relationships among humans, the living ape species, and the extinct ape species that were our ancestors? If we knew the answer to this question, we would know which living ape is our closest relative. * When did we and our closest living relative last share the same ancestor? This would tell us how long ago the human line branched off the family tree. * How much of our genetic makeup do we share with our closest living relative? This would tell us what percentage of our genes is uniquely human. Fossil evidence might answer the first two questions, except for one unfortunate fact. Almost no ape fossils have been found for the crucial period between five and fourteen million years ago in Africa. Instead, the answers to the questions came from an unexpected source: a project to sort out relationships among bird species. A Clue from the Bird World In the 1960s, molecular biologists began to realize that the chemicals that make up plants and animals might provide “clocks” to measure the genetic distances between species, and to tell how long ago those species separated from each other on the evolutionary tree. Take lions and tigers, for example. Suppose we knew from fossils that lions and tigers separated five million years ago. Suppose that a certain molecule in lions was 1 percent different from the same molecule in tigers. That would mean that 1 percent of genetic difference equaled five million years of separate evolution. Then, if scientists wanted to compare two living species but had no fossils to show those species’ evolutionary history, they could look at that same molecule in both species. If the difference between the two molecules was 3 percent, they would know that the species separated from their shared ancestor about fifteen million years ago—that is, three times five million. In the 1970s, two scientists named Charles Sibley and Jon Ahlquist used the idea of a molecular clock based on changes in DNA to study the evolutionary relationships of about 1,700 bird species, nearly a fifth of all living birds. A decade later, they used the same techniques to study primate evolution. For this project they examined the DNA of humans and all our closest relatives: common chimpanzees, bonobos (or pygmy chimpanzees), gorillas, orangutans, two species of gibbons, and seven species of monkeys. Their results gave us a new understanding of the primate family tree. A CLOCK MADE OF DNA THE MOLECULAR CLOCK WORKS THIS WAY: Suppose some type of molecule existed in all species, but had a unique structure in each species. Suppose that structure changed slowly over millions of years because of genetic mutations, and suppose the rate of change was the same for all species. Two species that descended from the same ancestor would start off with identical forms of the molecule, inherited from their ancestor. Over time, though, mutations would occur independently in each line of descent. These mutations would change the structure of that molecule in each of the two species. We could measure the present difference in the molecule’s structure between the two. Then, if we knew how many structural changes occurred, on average, every million years, the present difference between the two species would serve as a “clock,” telling us how much time had passed since the two species shared a common ancestor. By around 1970, molecular biologists had found that the best “clock” molecule is deoxyribonucleic acid, or DNA. It occurs in all living things but is unique to each species. DNA is made up of two long chains of molecules. Each chain is made up of four types of small molecules. The sequence, or order, of those small molecules carries all the genetic information that is passed from parents to offspring. To measure changes in DNA structure, scientists use a method called DNA hybridization. They mix the DNA from two species, then measure the melting point of this mixed, or hybrid, DNA. The next step is to compare the melting point of the hybrid DNA with the melting point of pure DNA from a single species. A difference of about one degree centigrade means that the two species differ by about 1 percent of their DNA. The final stage is to calibrate, or set, the DNA clock. This means linking DNA change to the passage of time. We might know that two species’ DNA differs by 1 percent, but until we know how DNA changes over time, we can’t know how long the two species have been evolving separately. To calibrate the DNA clock, scientists use species whose evolutionary history is known from fossils that can be accurately dated. In the case of birds, studies of both fossils and DNA from living bird species reveal that one gene in DNA (the gene called cytochrome b) appears to change by 1 percent every one million years. Using this information, scientists can measure the differences in cytochrome b in any two living bird species and tell how long ago those species separated from the ancestor they shared. A key to understanding connections among living things lies in DNA, the genetic material within our cells. It is made up of two long strands of molecules linked by shorter pairs of molecules, like a ladder with many rungs that has been twisted into a spiral—a shape known as a double helix. FIGURE I A family tree of evolutionary relationships among primates, including humans. Black dots represent the last time two groups shared the same ancestor. The scale on the right measures time and the scale on the left measures differences in the DNA of living species. Start with the dot on the bottom right—that’s the point, more than 30 million years ago, when apes separated from the monkeys of Eurasia and Africa. The monkeys kept evolving right up to the present. As the apes evolved, gibbons split off into their own line about 20 million years ago. The black dot marking that split is at 5 percent on the DNA scale, because gibbons differ in 5 percent of their DNA from other apes and humans. The second black dot from the left shows humans and chimpanzees splitting about 7 million years ago, with a difference of less than 2 percent in the DNA of humans and chimpanzees today. The Primate Family Tree When scientists studied the molecular clock in primate DNA, they found that the biggest genetic difference is between monkeys on one hand and apes and humans on the other. This came as no surprise. Ever since apes became known to science, everybody has agreed that humans and apes are more closely related to each other than either of them is to monkeys. The molecular clock showed that monkeys differ from humans and apes in 7 percent of their DNA structure. The clock also confirmed that gibbons are the most distinct apes. They differ from the other apes and humans in 5 percent of their DNA structure. Orangutans differ in 3.6 percent from gorillas, chimpanzees, and humans. These findings show that gibbons and orangutans separated from the rest of the ape family long ago. Today, gibbons and orangutans are found only in Southeast Asia. In contrast, gorillas and chimpanzees are found only in Africa, which was also the home of the earliest humans. Among apes, the most closely related living species are the two types of chimpanzees, common chimps and bonobos. Their DNA is 99.3 percent identical. What about humans? We differ from gorillas in about 2.3 percent of our DNA, and from chimps of both species by about 1.6 percent. This means that we share 98.4 percent of our DNA with chimpanzees, our closest living relatives. Put another way, the chimpanzee’s closest relative is not the gorilla—it is the human. Calibrated for primate species, the DNA clock shows that gorillas separated from the line leading to chimps and humans about ten million years ago. Ancestral humans separated from chimps about seven million years ago. in other words, humans have been evolving on their own for something like seven million years. The genetic distance separating us from chimps is less than the distance between two species of gibbons (2.2 percent). in an example from the bird world, the red-eyed vireo and white-eyed vireo are species of songbirds. Both belong to the same genus, or cluster of closely related species. But they differ in 2.9 percent of their DNA—much more than the difference between us and chimps. in terms of genetic distance, humans, common chimps, and bonobos should be grouped in the same genus. Looked at this way, humans are a third species of chimpanzee. HOW SHOULD WE TREAT APES? NOW THAT WE KNOW HOW SMALL A GENETIC distance separates us from chimps, our ideas about the places of humans and apes may change over time. One area that might change is the way we treat apes. Ethical issues—that is, questions of what is right and wrong—are involved. It’s considered acceptable to exhibit caged apes in zoos, but it’s not acceptable to do the same thing with humans. Yet if it were not for the interest in apes that many people gain by visiting zoos, the public might contribute much less money to protect apes in the wild. How will we balance our desire to collect chimpanzees and other apes in zoos with our knowledge that we are so closely related to chimps? Medical experimentation on chimpanzees is a controversial subject. It is unethical, or wrong, to perform experiments on humans without their knowledge and consent. Why is it okay to perform such experiments on chimps? If we say it is because chimps are animals, then we are saying they are no different from insects and bacteria in terms of what we can do with them, because insects and bacteria are also animals. But if we consider intelligence, social organization, and the ability to feel pain, it becomes hard to draw an all-or-nothing line between all humans and all animals. Instead, different ethical rules should apply to research on different species. If there is any animal species now used in medical research for which we could argue that all experimentation should stop, that species is surely the chimpanzee. Making matters worse, chimps used for research are often caged under cruel conditions. The first research chimp that I saw had been injected with a slow-acting deadly virus. It was being kept alone for several years, in a small indoor cage, with nothing to play with, until it died. And the capture of wild chimps for research usually means that several wild chimps are killed to capture one, which is often a youngster being carried by its mother. Yet the very reason medical researchers use chimps is that they are so genetically similar to us. Experiments on apes are a far better way to improve medical treatments than experiments on any other animals. Researchers are now studying certain diseases in captive chimps. How would we explain to parents whose children are at risk of dying from those diseases that their children are less important than chimps? Ultimately, we the public, not just scientists, will have to make these terrible choices. Our view of man and apes will determine our decision. The Differences between Chimps and Humans How could just 1.6 percent of genetic difference change chimpanzees into humans? Exactly which genes changed? To answer those questions we need to understand what DNA, our genetic material, does. Much of our DNA has no known function. Of the DNA that does have known functions, the main functions have to do with proteins, which are long chains of amino acids. Parts of our functional DNA govern the creation of proteins. It works this way: The sequences of small molecules in our DNA specify, or direct, the order of amino acids in our proteins. Certain proteins make up our hair and tissue, while other proteins are enzymes that make and break down the other molecules in our bodies. The genetic features that are easiest to understand arise from single proteins and single genes, or chunks of DNA. For example, our blood’s oxygen-carrying protein, hemoglobin, is made of two amino acid chains, each specified by a single gene. But other genes influence more than one trait. For example, the fatal genetic disease Tay-Sachs causes many visible traits: drooling, abnormal skull growth, yellowish skin, and more. We know that all these effects come from changes in a single enzyme specified by the Tay-Sachs gene, but we don’t know how. Scientists understand the functions of many individual genes that specify individual proteins, but we know much less about how genes shape complex traits, such as behavior. Human hallmarks—that is, characteristics that distinguish us as human—such as art, language, or aggression, are not likely to depend on a single gene. in addition, human behavior is influenced by family, culture, nutrition, and other aspects of each person’s environment. it’s very controversial what role genes play in the individual differences among humans. But for differences in behavior between all chimpanzees and all humans, genetic differences are likely to play a role. The ability of humans but not chimps to speak, for example, must be related to differences in genes that specify the structure of the voice box (larynx) and the wiring of the brain. A young chimpanzee that was raised in a psychologist’s home along with the psychologist’s daughter of the same age didn’t learn to speak or walk upright, although the girl did. Humans grow up to speak, no doubt because of our genetic program. But whether an individual human grows up to speak english or Korean has nothing to do with genes. it depends on what languages the growing child hears spoken. We don’t yet know which chunks of our DNA are responsible for the significant differences between humans and chimps that are covered in the next four chapters. All we can say for sure is that those differences must come from some part of 1.6 percent of our genes. We do know that just one or a few genes can have big impacts. The many big, visible differences between Tay- Sachs patients and those without the disease come from one change in one enzyme. Cichlid fish, popular for aquariums, also show the impact of small genetic changes. Africa’s Lake victoria has about two hundred species of cichlids. All of them evolved from a single ancestor over a period of about two hundred thousand years. These species differ in their food habits as much as tigers and cows do. Some cichlids graze on algae, some catch insects, some nibble the scales off other fish, some crush snails, and some snatch fish embryos from mother fish. Yet all those species differ from one another by less than half of 1 percent of their DNA. it took fewer genetic mutations to turn a snail crusher into a baby snatcher than it took to produce us from an ape. Horses, bulls, deer, and other animals leap and run on the walls of a set of caves in Lascaux, in southern France. Created by people of the Late ice Age, around seventeen thousand years ago, the prehistoric artworks of Lascaux became known to the modern world in 1940, when four teenage boys discovered and explored the caves. CHAPTER 2 THE GREAT LEAP FORWARD FOR MOST OF THE MILLIONS OF YEARS SINCE the human line separated from the apes, we remained little more than glorified chimpanzees. As recently as sixty thousand years ago, western europe was still occupied by Neanderthals, a human species that scarcely knew of art or progress. Then came an abrupt change. Anatomically modern humans—that is, people who looked like us—appeared in europe, bringing with them art, musical instruments, trade, and progress. Soon the Neanderthals were gone. If there is any one time when it could be said that we became human, it was at the time of that Great Leap Forward, sixty thousand years ago. That leap was probably the result of another leap that took place in Africa and the Middle east. That earlier leap spanned a few tens of millennia. (One millennium is a thousand years; ten millennia is ten thousand years.) even several dozen millennia, though, is a tiny fraction, less than 1 percent, of our long history apart from ape history. After the Great Leap Forward, we were only a few dozen millennia from domesticating animals, developing agriculture and metalworking, and inventing writing. From there it was a short step to the monuments of civilization, such as the Mona Lisa, Beethoven’s symphonies, the eiffel Tower, the international Space Station, and weapons of mass destruction. What made our sudden, steep rise to humanity possible? What held back the Neanderthals, and what was their fate? Did two species of humans ever meet, and how did they behave toward each other? in short, what made us human, and why did our particular branch on the family tree, Homo sapiens, become the last humans standing? Becoming Human Life on earth originated several billion years ago. Dinosaurs became extinct about sixty-five million years ago. it was only between ten and six million years ago that our ancestors became distinct from the ancestors of chimpanzees. Human history is just a tiny percentage of the history of life. Science fiction films that show cavemen running from dinosaurs are just that: science fiction. The shared ancestor of gorillas, chimps, and humans lived in Africa. Gorillas and chimps are still found only in Africa, and humans remained confined there for millions of years. At first our own ancestors would have been classified as just another species of ape, but a series of three changes launched us in the direction of modern humans. The first change occurred about four million years ago. Fossils from that time show that our ancestors were regularly walking upright on two legs, unlike gorillas and chimpanzees, which usually walk on four legs and only occasionally on two. When our ancestors began walking upright, their front limbs were freed to do other things—most important, to make tools. The second change occurred around three million years ago. All modern humans belong to the same species, Homo sapiens, but on perhaps several occasions in the past, our lineage—that is, the line of descent leading from our ancestors to us—split into at least two species that lived at the same time. Around three million years ago, our lineage divided into two species. One was a man-ape with a heavy skull and big side teeth. it probably ate coarse plant food. We call that species Australopithecus robustus, “the robust southern ape.” The other was a man-ape with a thinner, lighter skull and smaller teeth. it probably ate a wide variety of foods. it is called Australopithecus africanus, “the southern ape of Africa.” Australopithecus africanus evolved into a larger- brained form called Homo habilis, “man the handyman.” But Homo habilis was not the only branch of our family tree living in Africa several million years ago. We now have fossil evidence that several different species of protohumans, or early forms of humans, existed at that time and place. The third big change that made our ancestors more human and less apelike was the regular use of stone tools. This is a human trait with clear origins in the animal world. Woodpecker finches, egyptian vultures, and sea otters are among the other animals that evolved to use tools such as stones or twigs to capture or process food. None of them, though, depend on tools as much as we do. Common chimpanzees also occasionally use tools, including stone tools, but not so many as to litter the landscape. But by around two and a half million years ago, very crude stone tools appear in large numbers in parts of east Africa where protohumans lived. Since there were multiple protohuman species, who made the tools? Most likely the earliest tools were made by the one protohuman species that survived and kept evolving. Shakedown in Africa With two or three protohuman species living in Africa at the same time, but only one human species surviving today, it’s clear that some species must have become extinct. Which species survived to become our ancestor? The winner was the light-skulled Homo habilis, who went on to increase in brain size and body size. By 1.7 million years ago enough differences had appeared that scientists give our lineage a new name: Homo erectus, “upright-walking man.” (Homo erectus fossils were found before all the older fossils i’ve mentioned, so the scientists who named this species didn’t yet know that earlier protohumans also walked upright.) Meanwhile, the lineage ofAustralopithecus robustus, the “robust man-ape,” disappeared sometime after 1.2 million years ago. Other protohuman species, if any, must have died out around the same time. Why did the robust man-ape and other protohumans go extinct? Perhaps they could no longer compete with Homo erectus, who ate both meat and plants and had tools and a larger brain. it’s also possible that Homo erectus gave his relatives a push toward extinction by killing them for meat. The shakedown in Africa left Homo erectus as the only protohuman on the stage. By that time Homo erectus had already expanded his horizons, starting around two million years ago. His stone tools and bones show that he reached the Middle east, then east Asia. He continued to evolve in our direction, developing a larger brain and a rounder skull. By about half a million years ago, some of our ancestors looked enough like us, and enough different from Homo erectus, that they are classified as our own species, Homo sapiens, even though they had thicker skulls and heavier bony eyebrow ridges than ours. Wasn’t the appearance of Homo sapiens half a million years ago a Great Leap Forward? Not at all. it was a nonevent. Cave paintings, houses, and bows and arrows still lay hundreds ofthousands ofyears in the future. Stone tools continued to be the same crude ones that Homo erectus had been making for nearly a million years. The extra brain size of early Homo sapiens had no dramatic effect on our way oflife. Our rise to humanity was not in direct relation to the changes in our genes. Some vital ingredient still had to be added before the third chimpanzee could come up with the idea of painting the Mona Lisa. FIGURE 2 The many-branched human family tree reminds us that for long stretches of prehistory we shared the world with other human species.Our hominid ancestors began evolving separately from apes around 7 million years ago. By 4 million years ago some hominids walked on two legs. Around 3 million years ago, the human line split into two branches, Australopithecus africanus and A. robustus. The A. robustus line died out. The A. africanus line split into branches of its own. The Homo habilis branch led to Homo erectus and later Homo sapiens. (Fossils suggest that another branch of A. africanus, sometimes called the “Third Man,” lived in Africa before dying out.) The H. sapien line eventually split into three branches. One branch led to anatomically modern humans in Africa and on to today’s global human population. One branch led to the Neanderthals, who became extinct. Scientists are 29 just starting to piece together the history of the third, Asian branch. MAN, THE NOT-SO-GREAT HUNTER? HOW DID OUR ANCESTORS MAKE THEIR LIVING during the million and a half years between the appearance of Homo erectus and the appearance of Homo sapiens? The only tools from this period are crude stone artifacts. They vary in size and shape. Archaeologists have used these differences to give the tools names such as hand axe and chopper. In reality, though, these may be just guesses. Wear marks show that all sizes and shapes of tools were used to cut meat, bone, hides, wood, and plants. The tools were not finely made or specialized for particular uses, as later stone tools were. What food did our ancestors get with these crude tools, and how did they get it? A common image in books about our origins is Man the Hunter. Some anthropologists, the scientists who study human societies, have suggested that big-game hunting was what led protohumans to cooperate with one another, develop language and big brains, join into bands, and share food. Western writers and scientists are not the only people with an exaggerated view of hunting. In New Guinea, I lived with real hunters, men who had just recently come out of the Stone Age. At campfires, they talked for hours about hunting. You would think they ate fresh kangaroo meat every day and did little except hunt. But when pressed for details, most admitted that they had bagged only a few kangaroos in their whole lives. One morning I set out with a group of a dozen men armed with bows and arrows. As we passed a fallen tree, the men began shouting. Convinced that an enraged boar or kangaroo was about to come out fighting, I looked for a tree to climb. Then I heard triumphant shrieks, and out of the brush came two mighty hunters holding their prey: a pair of baby wrens, barely able to fly. The rest of the day’s catch consisted of a few frogs and many mushrooms. There’s no doubt that our early ancestors ate some meat. Their tools left marks on animal bones, and cutting meat from those bones left marks on their tools. But how much big-game hunting did they do, and how much of their meat did they get by scavenging the carcasses of animals that were already dead? The oldest really good evidence of human hunting comes from about one hundred thousand years ago—and it’s clear that humans then were not yet very effective as hunters. Hundreds of thousands of years earlier, they must have been even worse at it. Studies of modern hunter-gatherer people— who have far better weapons than early Homo sapiens had—show that most of a family’s calories come from plant foods gathered by women. Occasionally the men bag a large animal, which contributes significantly to protein intake, but only in the Arctic is big-game hunting the main food source. I question the usual view that hunting was the driving force behind our uniquely human brain and societies. My guess is that big-game hunting made only small contributions to our diet until after we had evolved fully modern anatomy and behavior. For most of our history we were not mighty hunters but skilled chimps, using stone tools to acquire and prepare plants and small animals, bagging big animals only occasionally. Ice Age Neanderthals In the period just before the Great Leap Forward, at least three distinct populations of humans lived in different parts of the world. These were the last truly primitive humans, replaced by modern humans at the time of the Great Leap. The best-known of these populations lived in an area that stretched from western Europe through southern Russia and the Middle East to Central Asia. They were the Neanderthals. The earliest fully Neanderthal skulls and bones date to around 130,000 years ago, although some older bones have features pointing toward the Neanderthals. Most Neanderthal remains come from later than 74,000 years ago. The last Neanderthals died sometime after 60,000 years ago. This means that the Neanderthals flourished when Europe and Asia were in the grip of the last Ice Age. They must have been adapted to life in the cold, but only within limits. They got no farther north than southern Britain, northern Germany, and the Caspian Sea (in Central Asia). The first people to enter Siberia and the Arctic were later, fully modern humans. Neanderthals’ heads were shaped so differently from ours that even in a business suit or designer dress, a Neanderthal would draw stares on the streets of New York or London. Their eyebrows rested on bulging bony ridges, and their noses, jaws, and teeth stuck out farther than the upper part of their faces. Their foreheads were low and slanted, and their lower jaws sloped back without chins. Still, their brains were nearly 10 percent larger than ours! They were heavily muscled in their shoulders and necks, and the bones of their arms and legs were thicker than ours. Even their hands were much more powerful than ours. A Neanderthal’s handshake would have been literally bone-crushing. Besides their bones, our main source of information about Neanderthals is their stone tools. Like earlier human tools, Neanderthal tools may have been simple hand-held stones, not mounted on separate parts such as handles. The tools don’t fall into distinct types with specialized functions. Some stone blades were undoubtedly used to make wooden tools, but few wooden objects have survived. One exception is an eight-foot-long thrusting spear of wood, found in Germany in the ribs of a long-extinct species of elephant. In spite of that success, though, Neanderthals were probably not very good at big-game hunting. Their numbers were few, and even anatomically modern people living in Africa at the same time as the Neanderthals were not great hunters. Neanderthals must have built shelters against the cold climate in which they lived, but those shelters must have been crude. All that remains is a few piles of stones and some holes that once held wooden posts, compared with the elaborate remains of houses built later by the Cro-Magnons. Neanderthals must also have worn skins or furs, but they had no needles for sewing fitted garments. They also lacked boats, long-distance trade, and probably art. Finally, the Neanderthals seem to have lacked that most important human quality, innovation, or the ability to invent new things. We know this because their tools did not vary across time or place. Tools that Neanderthals used in Europe sixty thousand years ago look just like the ones they used a hundred thousand years ago in the Middle East. Despite the Neanderthals’ big brains, something was missing. Yet there are ways in which we can relate to the Neanderthals as human. For one thing, they are the first people to leave clear evidence that they regularly used fire. Well-preserved Neanderthal caves have small areas of ash and charcoal, signs of simple fireplaces. For another, they may also have been the first people who regularly buried their dead, although the evidence on that point is not yet completely clear. An early scientific drawing of ancient stone tools found in southeast England. For a million years or more, such tools were the cutting edge of human technology. Although some have been given labels such as “hand axe,” scientists do not know how their makers used them. Sharp- edged blades such as these could have cut through roots and branches, or skins and animal flesh. We do know, though, that Neanderthals took care of the sick and aged people among them. Most skeletons of older individuals show signs of serious physical weaknesses, such as withered arms, missing teeth, and poorly healed broken bones. Only care by young Neanderthals could have kept older ones alive under such conditions. This lets us feel a spark of kinship with these strange creatures of the last Ice Age—nearly human in form and yet not really human in spirit. Did Neanderthals belong to the same species as we do? That depends on whether a modern human could and would have mated and reared a child with a Neanderthal man or woman. As we will see, there was a chance for that to happen about sixty thousand years ago, at the time of the Great Leap Forward. The Other Human Populations The Neanderthals were just one of at least three humans species occupying different parts of the Old World—the continents of Africa and Eurasia—between one hundred thousand and fifty thousand years ago. Another population lived in eastern Asia. A few fossils from that region show that people there differed from both Neanderthals and modern humans, but we have not yet found enough bones to describe these people in detail. We know much more about the humans who lived in Africa one hundred thousand years ago. Some of them had skulls very similar in structure to our own. Yet the stone tools of these modern-looking Africans were just like those of the unmodern-looking Neanderthals. These Middle Stone Age Africans, as they are called, still lacked bows and arrows, nets, fishhooks, art, and innovation. Bones that were mostly modern, and also genes that we assume were also mostly modern, were not enough to produce modern human behavior. Caves occupied by Middle Stone Age Africans give us our first reliable information about what these people ate. Remains found in coastal caves show that they ate seals, penguins, and shellfish. Hunters also captured middle-size animals, especially eland, a kind of antelope. Because eland of all ages were killed, it’s possible that hunters managed at times to drive a whole herd of them over a cliff. Bones of more dangerous prey, such as elephants and rhinos, are not found among the food remains. There are buffalo bones, but only from very young and very old animals. Middle Stone Age Africans were big-game hunters, but only barely. I suspect that plants and small game made up most of their diet. By fifty thousand years ago, then, the world had three known human populations: Neanderthals in Europe and western Asia, increasingly modern-looking people in Africa, and a third variety in eastern Asia. The stage was set for the Great Leap Forward. Which population would take the leap? The Rise of Modern Humans The evidence for a sudden rise is clearest in France and Spain. There, around sixty thousand years ago, modern humans appeared where before there had been Neanderthals. These people are often called Cro-Magnons, from the name of the French site where their bones were first identified. Their anatomy was like our own. If a Cro-Magnon dressed in modern clothes strolled down a Paris street, he or she would not stand out from the rest of the crowd in any way. Archaeologists are as interested in the Cro- Magnons’ tools as in their skeletons. The tools suggest that, in Cro-Magnons, modern anatomy was finally joined by modern innovation. These humans of the Late Ice Age made tools in a wide variety of forms, with obvious uses, including needles, fishhooks, mortars and pestles, barbed harpoons, and bows and arrows. Bone and antler tools appeared for the first time. So did compound tools—that is, several pieces tied or glued together, such as axe heads fitted onto wooden handles. By making their cutting tools out of thin blades of stone struck off larger stones, Cro-Magnon toolmakers got ten times more cutting edge than Neanderthals got from the same amount of raw stone. Late Ice Age sites are much more numerous than those of Neanderthals or Middle Stone Age Africans. The Late Ice Age population must have been significantly larger than the other two groups, which suggests that the people of the Late Ice Age had more success in obtaining food. In fact, they were good big-game hunters. Many species of big animals that had survived many previous ice ages became extinct at the end of the last Ice Age, possibly exterminated by human hunters’ new skills. (We will explore this possibility in more detail in chapters 14 and 15.) Improved technology also let people occupy new environments, such as Australia, northern Russia, and Siberia. In Europe, people practiced long-distance trade. Modern archaeologists have found tools of high-quality stone, such as obsidian and flint, hundreds of miles from where those stones were quarried. Amber, a form of hardened tree resin used as a gem, reached southern Europe from the shores of the Baltic Sea, in northern Europe. Shells from the Mediterranean reached inland parts of France, Spain, and Ukraine. The trade in ornamental materials shows that Late Ice Age people had a sense of art and beauty. This relates to the Cro-Magnon achievement we most admire: their art. In addition to covering cave walls with stunning multicolored paintings of now-extinct animals, they carved figurines, jewelry, and musical instruments such as flutes and rattles. Advances in tools and art did not all appear at the same time. Different innovations appeared in different times and places. Beads and pendants, for example, appeared before cave paintings. Only people in France painted woolly rhinos on cave walls, and only those in Ukraine built houses out of mammoth bones. These variations in culture in time and space are totally unlike the unchanging, universal Neanderthal culture. The variations represent the most important new element in our rise to humanity—the capacity for innovation. What Happened to the Neanderthals? Did Neanderthals evolve into Cro-Magnons in Europe? Not likely. The last Neanderthal skeletons, from sometime after sixty thousand years ago, were still fully Neanderthal, while the first Cro-Magnons appearing in Europe at that time were fully modern. Anatomically modern people had been living in Africa and the Middle East tens of thousands of years earlier. It is likely that modern people invaded Europe from that direction rather than evolving in Europe. What happened when the invading Cro- Magnons met the resident Neanderthals? We can be certain only of the end result. Within a short time, no more Neanderthals. It seems clear to me that the Cro-Magnon arrival somehow caused the Neanderthal extinction. My guess is that what happened in Europe at the time of the Great Leap Forward was similar to what has happened many times in the modern world whenever a numerous people with advanced technology invades or colonizes the lands of a less-numerous people with less- advanced technology. When European colonists invaded North America, for instance, most native North Americans died of diseases carried by the invaders. Most of the rest were killed or driven off their land. Some survivors adopted European technology (horses and guns) and resisted for a while. Others were pushed onto land the Europeans didn’t want, or else intermarried with them. Like the Great Plains Indians, who used horses and guns to fight the Europeans, some Neanderthals may have learned Cro-Magnon ways and resisted for a while. What about mating and interbreeding? No convincing skeletal remains that could be considered Neanderthal- Cro-Magnon hybrids are known. If Neanderthal behavior was as primitive as I suspect, and Neanderthals looked as different from Cro-Magnons as their skeletons suggest, both groups might have had little interest in interbreeding. I think it happened rarely. We know that it did happen in one part of the world, for a short time. Scientists have recently been able to reconstruct Neanderthal DNA and compare it with human DNA. The results show that when modern humans first starting living in the Middle East, some of them interbred with Neanderthals there. As a result, all people alive today appear to have inherited a small amount of their genetic program from the Neanderthals— about i percent of our total DNA. After that first small period of interbreeding, there is no evidence that modern humans continued to interbreed with Neanderthals once they moved beyond the Middle East and settled in Europe. Much of the clearest evidence of the Great Leap Forward comes from western Europe, but modern humans replaced Neanderthals a bit earlier in eastern Europe. In the Middle East, possession of the same area shifted back and forth between Neanderthals and modern humans for about thirty thousand years, between ninety thousand and sixty thousand years ago. The First Great Leap The Great Leap Forward really began in Africa. It appears that anatomically modern people arose there more than one hundred thousand years ago. At first, they had the same tools as Neanderthals and had no advantage over them. But by around sixty thousand years ago, some magic twist of behavior had been added to the modern anatomy. That twist produced innovative, fully modern people who spread into the Middle East, Europe, and Asia, replacing the earlier people in those regions. Two million years ago, several protohuman species lived side by side in Africa until a shakedown left only one. It appears that a similar shakedown among human populations took place within the past sixty thousand years. All of us alive in the world today are descended from the winners of that shakedown. What was the last missing ingredient, the magic twist, that allowed our ancestor to win? THE LITTLE PEOPLE OF FLORES ISLAND AN ASTONISHING DISCOVERY ABOUT HUMAN origins was made in 2004. On the island of Flores, in the Southeast Asian nation of Indonesia, researchers unearthed fossil bones of tiny primitive humans. Flores is famous among biologists. It is the home of the world’s largest living lizard, the Komodo dragon. It was once the home of a now-extinct species of dwarf elephant. It turns out that, until recently, Flores was also home to humans barely three feet tall, with brains only one-quarter the size of modern human brains—about the size of a chimpanzee’s. Scientists are still debating the meaning of these fossil finds. Some believe that the little people of Flores Island were related to Homo erectus, the long-extinct forerunner of humans, and that these relatives of Homo erectus lived on for tens of thousands of years after modern humans arrived in Indonesia. Other scientists think that the fossils might be of modern humans dwarfed by a disease or genetic abnormality, and not of a primitive separate species. My own guess is that the fossils really do represent primitive humans who, like elephants, evolved into a small size after reaching Flores Island, then were quickly exterminated when modern humans arrived. We shall have to see what further discoveries tell us—but the Flores fossils show why it is exciting to live in a time of rapid scientific advances. A map of the Indonesian Archipelago—can you find the island of Flores? Tiny Change, Big Leap The ingredient that produced the Great Leap Forward is an archaeological puzzle without an accepted answer. The missing ingredient doesn’t show up in fossil skeletons. It may have been a change in only one-tenth of i percent of our DNA. What tiny change in our genes could have had such enormous results? Like some other scientists who have pondered this question, I can think of only one good answer: language. Anatomical or physical changes made complex spoken language possible. To understand how such a change could trigger a rapid burst of human innovation, take a look at how apes use language. Chimpanzees, gorillas, and even monkeys are capable of symbolic communication, in which a symbol or a sound represents something else—such as a picture of an apple representing a piece of fruit, or a particular cry meaning “snake!” As we will see in chapter 6, apes have learned to use sign language, plastic symbols, and computers to communicate. Some have mastered “vocabularies” of hundreds of symbols. And wild vervets (or green monkeys) have a natural form of symbolic communication based on grunts, with slightly different grunts to mean “leopard,” “eagle,” and “snake.” If these primates are capable of symbolic communication, why have they not gone on to develop much more complex natural languages of their own? The answer seems to involve the structure of the larynx (or voice box), the tongue, and all the muscles that give us fine control over spoken sounds. Our ability to speak depends on the perfect functioning of many structures and muscles. If, like apes, we could utter only a few consonants and vowels, our vocabularies would be greatly reduced. The missing ingredient that finally made us fully human could well have been some changes to the protohuman vocal tract—changes that gave us finer control and let us make a much greater variety of sounds. Such small changes to muscle and soft tissue need not show up in fossil skulls. It’s easy to see how a tiny change in anatomy, creating the capacity for speech, would produce a huge change in behavior. With language, it takes only a few seconds to communicate the message, “Turn sharp right at the fourth tree and drive the male antelope toward the red boulder, where I’ll spear it.” Without language, two protohumans could not brainstorm together about how to organize a hunt or invent a better tool. Without language, even one protohuman would have had difficulty thinking about inventing a better tool. I don’t suggest that the Great Leap Forward began as soon as the mutations for changes in the larynx and tongue appeared. Even with the right anatomy, it must have taken humans thousands of years to develop the structure of language as we know it. But if the missing ingredient did consist of changes that permitted fine control of sounds, then the capacity for innovation would have followed eventually. The spoken word made us free. Until the Great Leap Forward, human culture had developed at a snail’s pace for millions of years. That slow pace was dictated by the pace of genetic change. Our culture and behavior changed only when a mutation occurred to give rise to the change. After the Great Leap, cultural development no longer depended on genetic change. People could think, innovate, and communicate in a new way. They could pass ideas and knowledge on to other groups and to the next generation. Even though our anatomy has barely changed over the past sixty thousand years, human culture has evolved far more since the Great Leap than in the millions of years before. PART TWO A STRANGE LIFE CYCLE Many human cultures have been polygamous. This photograph, taken around 1900, shows Joseph Smith, founder of the Church of Jesus Christ of Latter-Day Saints, or Mormon religion, with his large family, including his children’s spouses. Smith had multiple wives, a practice eventually banned by the Mormon Church. OUR EVOLUTION OF LARGE BRAINS AND UPRIGHT posture was needed before we could develop language and art, but it wasn’t enough by itself. Human bones don’t guarantee humanity. Our rise to humanity also required drastic changes in our life cycle. Every species has what biologists call a life cycle. It is made up of traits such as the number of offspring produced in each litter or birth, the amount of care that the mother or father gives to the offspring, the way adults form social relationships, how males and females select mates, and how long individuals typically live. We take the human forms of these traits for granted, as if they were normal. But our life cycle is bizarre by animal standards. To mention just a few examples, most animals produce litters much larger than one baby at a time, most animal fathers provide no parental care to their offspring, and few other animals live even a fraction of seventy years, which is not an unusual life span for a human. Apes share some of these unusual features. Unlike cats, dogs, songbirds, and goldfish, apes usually have one baby at a time, and they live for several decades. In other ways, however, we’re greatly different even from apes. Young chimpanzees are cared for by their mothers, but among humans, most fathers as well as mothers are closely involved in caring for their young. Our infants require a long period of being fed, trained, and protected—a much greater investment of time and energy than ape mothers face. Human fathers who want their children to live and grow up have generally helped their mates raise them. Our life cycle differs from that ofwild apes in other ways. Human females often live for many years after menopause, the point in their lives beyond which they can no longer have children. This is almost unheard of among other mammals. Humans are unusual in their sexual activities, too. Apes engage in sex publicly, in front of other members oftheir group, and only when the female is ready to bear young. Among humans, sexual activity is usually private, and childbearing is not the only reason for it. Human society and child-rearing rest on both the skeletal changes mentioned in part 1 and also these remarkable new features of our life cycle. Unlike our skeletal changes, however, our life cycle changes left no fossils. We do know that our life cycle traits have some genetic basis. Among those 1.6 percent of our genes that are different from chimpanzees’ genes and that have any function, a significant part is likely to be involved in shaping our life cycle. Three aspects of our distinctly human life cycle are explored in the next three chapters. The first is human social organization and sexuality. Next is racial variation, the visible differences among humans native to different parts of the globe. I’ll argue that these differences arose as a result ofthe way we humans choose our mates. Finally, I’ll ask why we grow old and die. Aging is a part of our life cycle that we take for granted: of course everyone grows old and eventually dies. But why do we have to age, when our bodies are able to repair themselves to a great extent? Here, more than anywhere else in this book, it is important to think in terms of “trade-offs.” In the animal world there’s nothing that’s free or purely good. Everything involves not just benefits but also costs, by using space, time, or energy that could have been devoted to something else. In the framework of evolutionary biology, success is measured in terms of leaving more offspring. As you’ll see in chapter 5, this view of success helps explain why it just wouldn’t pay for us to make the increased investment in self-repair that we would need to live longer lives. The idea of the trade-off also explains the puzzle of menopause: a shutdown of childbearing so that women can leave more surviving children. CHAPTER 3 HUMAN SEXUALITY THE HUMAN LIFE CYCLE INCLUDES SEXUALITY and family life. It is not always easy, though, to investigate these subjects. One problem is that, where people’s sexual behavior is concerned, there are limits to the scientific approach. We can’t do controlled experiments the way we can with diet or teeth-brushing habits. Another is that the subject can be a sensitive one. Scientists did not begin to study human sexuality seriously until recently, and it still can be difficult to view the subject with a scientific attitude. Most people regard their relationships with loved ones—family relationships such as parent-child ties, and also romantic and sexual relationships—as deeply meaningful, personal, and private. It can seem cold-blooded, even harsh, to put those relationships under a microscope, so to speak, and look at them with a scientist’s eye. Some people might even be offended to see their interactions with the people in their lives compared with the childrearing behavior of apes or the courtship habits of birds. As you read this chapter and those that follow, keep two important points in mind. First, we are looking at the human life cycle through the particular framework of evolutionary biology. It does not necessarily explain everything about why people do the things they do—it is simply one of many tools that can help us understand ourselves. Second, our focus is on the entire human species, not on specific examples. There are always many exceptions to every rule, and many people behave very differently than science might predict. Our concern is with general trends, not with people as individuals. In spite of the challenges of studying human sexuality, we are beginning to understand how it is intertwined with other human characteristics, such as tool use, large brains, and child-rearing practices. Our shift from being just another species of big mammal to being uniquely human involved changes not just in our bones and skulls but also in our family lives and sex lives. Food and Family Life To understand how human sexuality got to be the way it is, we have to understand the evolution of our diet and our society. From the vegetarian diet of our ape ancestors, we separated within the last several million years to eat meat as well as plant foods, yet our teeth and claws remained those of apes, not of tigers. Instead of on teeth and claws, our success in hunting depended on large brains. By using tools and hunting in organized groups, our ancestors could hunt, and they regularly shared food with one another. We started using tools to gather roots and berries as well, so even the vegetarian part of our diets required large brains. Human children took years to acquire the information and experience they needed to be efficient hunter-gatherers, just as today they take years to learn how to be farmers or computer programmers. For many years after they are weaned—after they stop nursing on milk from the mother’s body and start to eat food—they are too ignorant and helpless to take care of themselves. Human children depend on parents to bring food to them. This seems natural to us, but it is exceptional in the primate world. Baby apes gather food for themselves as soon as they are weaned. Human infants are terrible food gatherers for two reasons. One reason is mechanical. Making and using the tools needed to find food requires fine finger coordination that children take years to develop. Just as my sons couldn’t tie their shoelaces when they were four years old, four- year-old hunter-gatherer children can’t sharpen a stone axe or build a dugout fishing canoe. The second reason is mental. More than other animals, we depend on brainpower when we look for food, because we have a much more varied diet and more complicated food-gathering techniques. New Guineans whom I work with typically have separate names for about a thousand different plant and animal species in their vicinity. For each species, they know something about where to find it, whether it is edible or useful, and how to capture or harvest it. All this information takes years to acquire. Weaned human infants not only need adults to feed them; they need adults to teach them for a decade or two. As with so many human hallmarks, these needs occur in other species. Young lions, for example, must be taught how to hunt by their parents. Chimpanzees, like humans, have a varied diet and use a number of techniques to obtain food. Chimpanzee parents do help their young find food, and common chimps also make some use of tools, although bonobos do not. But for humans, the necessary survival skills, and the burden on the parents, are much greater than for lions or chimps. Family life in the species Homo sapiens is organized around a simple fact: human children cannot fend for themselves at birth or for a long time afterward. They need parental care, not just for food, shelter, and protection, but to learn the skills they will need to survive in society. That parental burden means that care by the father as well as the mother is important if the child is to survive. Orangutan fathers provide their offspring with nothing. Gorilla, chimpanzee, and gibbon fathers do more, providing some protection for their young. Human hunter-gatherer fathers do even more, providing some food and much teaching. Our complex food-gathering habits require a social system in which a male has a longterm relationship with a female, so that he can help rear their child. Otherwise the child will be less likely to survive, and the father will be less likely to pass on his genes. A Social System That Meets Our Needs The orangutan system, in which the father simply departs as soon as he and his female partner have mated, wouldn’t work for us. The chimpanzee system also wouldn’t work for us. Among chimpanzees, a female who is ready to be fertilized and become pregnant is likely to mate with several adult males within a short time. As a result, a chimpanzee male has no idea which infants in the troop he has fathered. This is no big loss to a chimp father, because males don’t do a great deal for the troop infants. A human father, though, spends a lot of time and energy on the care of his child. From the point of view of evolution, a human male had better have some confidence that the child is his, or his child care contributions may help pass on some other man’s genes. Confidence about fatherhood would be no problem if humans, like gibbons, were scattered across the landscape in isolated couples, so that a female almost never saw a male other than her mate. Almost all human populations, however, have consisted of groups of adults. Hunting and gathering often involve cooperative group efforts among men, women, or both. Groups also offer protection against predators and enemies, especially other humans. To meet our need for both confidence in fatherhood and group living, humans evolved a social system that seems normal to us, although it is strange by ape standards. Adult orangutans are solitary. Adult gibbons live as solitary male-female pairs. Gorillas live in harems consisting of several adult females and usually one dominant adult male. Common chimpanzees live in communities of scattered females plus a group of males, in which individuals mate with more than one partner. Pygmy chimps, or bonobos, form colonies of both sexes that are even more promiscuous, meaning that individuals have multiple sex partners. Human societies resemble none of these primate societies. Like our food habits, our social system resembles more that of lions or wolves. We live in bands containing many adult males and females. Among lions, however, any male can and does mate with any female, meaning that the fatherhood of lion cubs is unknowable. Among humans, males and females are paired off with each other. The closest thing to our social system in the animal world is large colonies of seabirds, such as gulls and penguins, which also organize into male-female pairs. Officially, at least, in most modern political states human pairing is more or less monogamous, meaning that each individual has a single partner. Among hunter-gatherer bands, which are better models for how humans lived over the last million years, most men can support only a single family, but a few powerful men have several wives. The huge harems some human rulers have maintained weren’t possible until the rise of agriculture and centralized government let a few princes tax everyone else to feed the royal harem’s babies. Why Men Are Bigger Than Women Adult men are, on average, slightly bigger than women of the same age. Although there are many individual exceptions, across a whole population men weigh about 20 percent more than women and are about 8 percent taller. Why is this the case? The answer lies in our social and sexual organization. The typical hunter-gatherer social system, in which most men have one partner but a few men have several wives, can be called “mildly polygynous.” (Polygynous means “with multiple wives.”) Because humans were hunter- gatherers for many millennia before the rise of agriculture, that particular social organization explains why men are bigger than women. Among polygynous mammals, the average difference in size between males and females is related to the number of females that mate with a single male, and only with that male. The more females in a male’s harem, the greater the size difference between the sexes. The biggest harems are seen in species with males much larger than females. Three examples from the animal world show how this works. Surrounded by his harem of smaller females, a large male elephant seal basks on the not-so-balmy beach of King George Island, Antarctica. Gibbons are monogamous. Each individual has only one partner. The male gibbon has no harem, and there is no average size difference between the sexes. Males and females are the same size. Male gorillas, on the other hand, typically have harems of three to six wives. This is reflected in a size difference: a male weighs about twice as much as a female. The average harem of a southern elephant seal is forty-eight females. With such a big harem size, you would expect a big difference in size between the sexes, and you would be right. A three-ton (six-thousand-pound) male seal dwarfs his seven-hundred-pound wives. The explanation is that in a monogamous species, every male has the opportunity to win a female. In a very polygynous species, such as the elephant seal, many males go without mates, because a few dominant males have succeeded in rounding up all the females into their harems. The bigger the harem, the fiercer the competition among males—and the more important it is for a male to be big, because the bigger male generally wins a fight. We humans, with our slightly bigger males and slight polygyny, fit this pattern. At some point in human evolution, though, male intelligence and personality came to count for more than size. Big men don’t tend to have more wives than smaller ones. Our Unusual Sex Lives Human sexual activity is freakish by the standards of other mammals. For one thing, most mammals are sexually inactive most of the time. They copulate, or engage in sex, only when the female is estrous. That means that she has entered estrus, the part of her biological cycle when she is ovulating—her ovaries are preparing to release an egg. During this time she can be fertilized and become pregnant. Depending upon the species, females enter estrus at various time intervals, from every few weeks to a few times a year. Females of some species enter estrus just once a year. Then and only then are they willing to copulate with males, and they advertise this fact to males through behavior and, sometimes, changes in their appearance. Human sexual cycles are quite different. Instead of being limited to a short estrus phase, a sexually mature woman, like a man, may choose to engage in sexual activity whenever she decides to do so. Women ovulate once a month, but unlike other primate females, they do not advertise this fertile phase to males with changes in their appearance or behavior. In fact, human ovulation is so well concealed, from women as well as men, that doctors only began to understand its timing in the 1930s. Concealed ovulation, together with the fact that women have the ability to be sexually active when they want and not just during the time of the month when they are fertile, means that most sexual encounters by humans are at the wrong time for conceiving a child. Whatever the main biological function of human sex might be, it isn’t to produce children. In no species besides humans has sex become so unconnected to conception. For animals, copulation is a dangerous luxury. Animals in the act of mating burn energy and neglect opportunities to obtain food. They are also vulnerable to predators who want to eat them or rivals who want to take over their territory. Copulation is something to be accomplished in the minimum time needed for fertilization. If we regarded human sex as a means of achieving fertilization, it would be a colossal failure in evolutionary terms. It gets the job done, but it consumes a great deal of time and energy, because humans engage in much sexual activity at times when fertilization is impossible or unlikely. If we had kept an estrus cycle like other mammals, including our close primate relatives, our hunter-gatherer ancestors could have spent that wasted time butchering more mastodons and gathering more berries. The most hotly debated question in the evolution of human sexuality is how we wound up with concealed ovulation, and what good all those mistimed copulations do us. Sex is pleasurable, but evolution made it that way. If our species weren’t getting some evolutionary benefit from our sexual activities, the world would have been taken over by mutant humans who evolved not to enjoy sex. Closely related to the question of concealed ovulation is that of concealed copulation. All other animals that live in groups—whether individuals are monogamous or have multiple partners— copulate in full view of the other animals in their group. Why are humans unique in our strong preference for keeping our sexual activity private? Biologists are currently arguing over theories to explain the origin of concealed ovulation and copulation in humans. From an evolutionary point of view, some factor or combination of factors caused us to evolve these traits in the distant past. The traits would not still be present today if some factors weren’t keeping them alive. The factors responsible for concealed ovulation and copulation today don’t necessarily have to be the same factors that caused those traits to appear in the first place. Three of the explanations biologists have suggested for the origin of our unusual sex lives, however, seem to me to be still in operation today. Those explanations are: * Concealed ovulation and copulation evolved to reduce aggression and increase cooperation among males; * Concealed ovulation and copulation strengthen the bonds between particular couples, laying the foundations of the human family; and * Women evolved concealed ovulation to encourage men to bond permanently with their partners, which in turn makes men more confident that they are the fathers of the children their partners bear. All these explanations reflect a key feature of human social organization. A man and woman who want their child (and their genes) to survive must cooperate with each other for a long time to rear that child. At the same time, they must cooperate economically with other couples living close by. Regular sexual relations between the man and woman create a bond that is closer than the couple’s ties to their friends and neighbors. These close bonds between couples are a kind of social cement, not just a mechanism of fertilization. Our sex lives take place in private to emphasize the difference between sexual and nonsexual partners within the same close group. The Science of Adultery The human mating system is based on male and female pairs that rear children together and form a lasting bond. Another primate species also forms lasting pairs: the little apes called gibbons. Their mating system is different from that of humans, however. Gibbon couples live alone, apart from other gibbons, not in groups or communities. Gibbons who have formed a pair bond do not have sex outside the bond. Humans live in social groups, not solitary pairs. They also, at times, engage in sexual activity with people other than their partners. Sexual activity by married people outside their marriage, called adultery or extramarital sex, is an exception to our “normal” pattern of married sex. Adultery can be a heartbreaking, life- wrecking matter. Why, then, do humans do it? Like other behaviors, adultery can be examined from the point of view of evolution. Remember that when we use the framework of evolutionary biology, we look at patterns across whole species, and evolution is only one of the forces that drive human behavior. If life is viewed as an evolutionary contest, the winners are those who leave the greatest number of offspring. Different species have different strategies for winning the contest. Some species are purely monogamous, others are highly promiscuous (meaning that individuals may mate with many partners), and some follow a mixed strategy: monogamous, with exceptions. Within any species, the best strategy for males may not be the same as the best strategy for females. That is the case with humans. For men, the minimum effort needed to produce an offspring is copulation, which requires only a little time and energy. For women, the minimum effort is copulation plus nine months of pregnancy—and, throughout most of human history, several years of nursing as well. That’s a huge investment of time and energy. As a result, a man can potentially produce far more offspring in his lifetime than a woman can. The record lifetime number of offspring for a man is 888, sired by Emperor Ismail the Bloodthirsty of Morocco. The record for a woman is 69 children, born to a nineteenth-century Russian woman who had multiple sets of triplets. Few women have topped twenty children, while some men in polygynous societies easily do so. This biological difference means that a man can potentially gain much more from extramarital sex than a woman can—if the only measure of success is the number of offspring. That could be one reason a man might seek sex outside marriage. Why would a woman? Research in many parts of the world suggests that women’s motives for sex outside marriage often include dissatisfaction with their marriages and a desire to find a new lasting relationship. Does all this mean that extramarital sex is “only natural” and should be accepted? Not at all. Understanding and explaining a behavior is not the same thing as defending or accepting it. The goal of all human activity can’t be reduced to an evolutionary drive. We humans are able to choose other goals. Many people are simply not interested in relationships with people other than their partners. For others, goals such as honoring a promise to be faithful to one’s partner, staying true to religious and moral beliefs, or protecting the family are stronger than the urge to engage in extramarital sex. Among our species, success and happiness are not measured only by the number of offspring we leave. For evidence, consider the fact that many successful and happy women and men choose to limit their family size, or not to have children. Consider also that many humans form same-sex bonds, or have gender identities beyond traditional male and female roles. MONOGAMOUS BIRDS—OR ARE THEY? IN THE ANIMAL WORLD, THE CLOSEST MATCH to the human mating system is found in the nesting colonies of certain birds. Herons and gulls, for example, breed and rear their young in dense colonies of male-female pairs that appear monogamous. Successful chick rearing calls for two parents. One bird cannot raise a chick alone, because an unguarded nest will probably be destroyed while the parent is off gathering food, and a male cannot feed and guard two nests at the same time. In a study of great blue herons and great egrets in Texas, observers watched males who were left guarding their nests while their mates went offto find food. For the first day or two after pairing with their mates, the males often courted other passing females, although they did not copulate with them. The courting seemed to be a kind of “insurance,” in which a male tried to line up a backup mate in case his female deserted him (which happens in 20 percent of cases). The passing “backup” female is a single bird seeking a mate. She does not know the male already has a mate until his spouse returns to chase her away. Eventually the male gains complete confidence that his spouse will not desert him, and he stops courting the passing females. Ardea herodias or great blue herons, and their nests. Herring gulls follow a different strategy. A study of these birds in Lake Michigan found that 35 percent of the mated males engaged in extramarital sex. All mated female gulls, however, rejected the advances of males other than their mates, and they never flirted with neighboring males when their mates were away. All the male gulls who committed “adultery” did so with single females. At the same time, these males were “good providers”—they brought their mates plenty of food. Studies such as these have shown that so-called “monogamous” birds are not always monogamous. In some species, adulterous males try to have it both ways: keeping their spouses faithful while siring and their nests. offspring with other females. A Choice among Goals We are not mere slaves to the traits we have evolved, not even to those encoded in our genes. Modern civilization is fairly successful at ending ancient behaviors such as stealing brides from rival tribes or murdering children. While the evolutionary viewpoint is valuable for understanding how human social and sexual practices originated, it is not the only way to understand the way we behave today. Once human culture was firmly in place, it acquired new goals. Questions of sexual faithfulness or promiscuity are not decided simply by our evolutionary heritage. They are also ethical questions, involving our ideas and beliefs about right and wrong behavior. Like other animals, we evolved to win at the contest of leaving as many descendants as possible, but we have also chosen to pursue ethical goals, which can direct our behavior in different ways. Having that choice among goals is one of the biggest differences between us and other animals. How We Pick Our Mates One final piece of the puzzle that is human sexuality concerns the mystery of attraction. What draws us to one possible partner instead of another? How do we choose our mates? Psychologists have tackled this question by examining many married couples, measuring everything conceivable about them and then trying to make sense out of who married whom. Not surprisingly, most husbands and wives turn out to share the same ethnic background (although racially and ethnically mixed marriages are on the rise), religious beliefs, and political views. Spouses also tend to match each other reasonably well in intelligence and in personality qualities such as neatness. What about physical appearance? It turns out that if you measure enough couples, you make an unexpected discovery. There are many exceptions, but on average, spouses resemble each other slightly—but enough to be statistically significant—in almost every physical feature. This is true for the obvious physical features we think of first when describing people: height, weight, and color of eyes, hair, and skin. But it is also true for dozens of less obvious traits, such as breadth of nose, length of earlobe or middle finger, distance between eyes or around the wrist, and lung volume! Experimenters have made this finding for people as far apart as Poles in Poland, Americans in Michigan, and Africans in Chad. In each case, spouses were not identical, but they were more alike than they would have been if they had been randomly paired. In spite of the old saying “opposites attract,” people on average tend to marry people who are more like them than they are different. One reason for this is that we tend to spend a lot of time around people who are similar to us. Many people live in neighborhoods defined by ethnic background, or religion, or social and economic status. We meet people of the same religion in church. Family friends often share our own family’s interests, political views, and social and economic status. Those contacts give us many opportunities to meet and fall in love with someone similar to ourselves. But we don’t live in neighborhoods grouped by length of earlobe, so there must be some other reason spouses tend to be matched that way, too. The answer lies in physical attraction based on appearance. All kinds of traits—the obvious ones such as height and hair color, and the less obvious ones such as earlobe length and eye spacing—come together to form a search image, a mental picture of our ideal mate. We may not be consciously aware that we have such an image, but it is what makes us feel that “He’s my type” or “She’s not my type” when we meet someone new. We are attracted to people who look somewhat like us because our search images are based on people who share half our genes: our parents and siblings. We begin to develop our search image of a future partner when we are very young, between birth and age six. The image is heavily influenced by the people of the opposite sex whom we see most often. For most of us, that’s our mother or father, sister or brother, and close childhood friends. We may have evolved to form an early search image of the ideal mate, but researchers have found time and time again that factors such as personality, intelligence, and religion have more influence than physical appearance on our choice of spouses. Like other aspects of our social and sexual lives, our feeling of attraction to possible romantic partners, and our eventual choice of partners, whether of the opposite sex or our own, is driven only partly by our evolutionary heritage. It is also determined by our life experiences, values, and goals. Whatever their style, people more often than not choose mates who resemble them. Any new style can catch on—if enough potential dating and mating partners find it attractive. CHAPTER 4 THE ORIGIN OF HUMAN RACES IMAGINE YOU WERE INTRODUCED TO THREE people: one each from Nigeria, Japan, and Sweden. You would probably have no trouble deciding at a glance which person came from which country. You’d see differences in skin color, the color and shape of eyes, the color and texture of hair, and general body size and shape. These differences would point to different continents of origin: Africa for the Nigerian, Asia for the Japanese, and Europe for the Swede. A trained anthropologist might be able to do better, placing each person in the right part of his or her country. Human features vary with geography, creating racial variations. Since scientists have answered so many highly technical questions about obscure species of plants and animals, you might expect them to have answered one of the most obvious questions about us: Why do people from different areas look different? Our understanding of how humans came to differ from other animals would be incomplete if we did not consider how, in the process, human populations became visibly different. The subject of human races, however, is an explosive one. British scientist Charles Darwin shied away from it in his 1859 book On the Origin of Species, which introduced the world to the ideas that form the foundation of modern biology. Even today, few scientists dare to study racial origins, for fear they will be called racists just for being interested in the subject. Another reason we don’t understand the meaning of racial variations is that it’s an unexpectedly difficult problem. Darwin’s own theory was that human races originated because of sexual selection—in other words, the mating choices people make. To this day, that theory is controversial. Modern biologists usually argue that racial origins came about through a different process, called natural selection. Yet they can’t even agree how natural selection might have led to features such as dark skin in the tropics, to take just one example of racial variation. In this chapter we’ll look at the two forces that have been suggested as the source of racial variations: natural selection and sexual selection. You’ll see that I think natural selection played only a secondary role, and that sexual selection has been the primary shaper of human racial variation. Visible Variations Racial variation is not confined to humans. Most animal and plant species that are distributed over wide areas, including gorillas and common chimpanzees, also vary geographically. This means that populations of the same species living in different parts of the species’ range have recognizable, visible differences. How can we decide whether two recognizably different populations, from different places, belong to the same species? If no interbreeding occurs naturally when they come into contact, they are separate species. If interbreeding does occur, they belong to the same species, but because of their differences, they can be considered separate races (also known as subspecies) within that species. All gorillas, for example, belong to a single species. Within that species are three races, or subspecies, of gorillas: mountain, eastern lowland, and western lowland. They are set apart by visible differences in body size, hair length, and hair color. Closely related species that occasionally interbreed in captivity, such as tigers and lions, remain separate species because they do not interbreed in the wild. All humans, in contrast, belong to the same species, because some interbreeding has taken place whenever any two human populations have come into contact. Racial variation has characterized humans for at least the past several thousand years, and possibly much longer. Writing around 450 BC, the Greek historian Herodotus described black-skinned Ethiopians from Africa and a redhaired, blue-eyed tribe from Russia. Ancient paintings, mummies from Egypt and Peru, and bodies preserved in European peat bogs show that several thousand years ago, people differed in their hair and faces as much as they do today. Fossils show that by at least ten thousand years ago, skulls of people from various parts of the world were different, in ways similar to the racial variations in skull shape that anthropologists see in people today. Does Natural Selection Explain Skin Color? Now let’s turn to the question of how our visible geographic differences originated. One argument is that they are the result of natural selection, a mechanism that drives evolution, the pattern of change in life forms over time, as new species develop from earlier ones. Natural selection simply means that genetic traits that help a plant or animal survive get passed on to that organism’s offspring. New traits arise, or existing traits change, when changes occur in genes because of random mutations. Genetic changes are just as likely to harm an individual plant or animal, or to have no effect on it, as they are to help it. But if the mutation does help the organism—if it leads to a slightly longer bill, say, that lets a bird pick more beetles out of tree bark—that organism is likely to live longer than others of its species, which means that it can have more offspring. Those offspring will inherit its genetic program, including the new mutation. They will have the survival advantage that the mutation brought, and their offspring will spread it more widely. In time, the mutation becomes established in a population, resulting in a new subspecies or even a whole new species. Natural selection accounts for many differences between species, such as the fact that lions have paws with claws while we have grasping fingers. Natural selection also explains some racial or geographic variation within species. For example, Arctic weasels that live in areas covered by winter snow change color from brown in summer to white in winter, while weasels that live farther south stay brown all year. That racial difference improves the weasels’ survival. White weasels would be glaringly easy to see against a brown background, but against snow they are camouflaged and less obvious to their prey. Natural selection surely explains some geographic variation in humans. For example, many Africans but no Swedes have the sickle cell hemoglobin gene, because that gene protects against malaria, a tropical disease that occurs in Africa but not in Sweden. Other localized human traits that must have evolved through natural selection include the big chests of Indians in the Andes Mountains of South America (good for getting oxygen from thin air at high altitudes) and the compact shapes of Eskimos (good for saving heat in a cold climate). Can natural selection explain the racial differences that we think of first—skin color and eye color? If so, we might expect that the same trait, such as blue eyes, would reappear in parts of the world with similar climates, and that scientists would agree on what the trait is good for. Skin color should be the easiest trait to understand. Our skins run the spectrum from various shades of black, brown, copper, and yellowish to pink with or without freckles. The usual story to explain this variation by natural selection goes like this: People from sunny Africa have black skin. So do people from other sunny areas, such as southern India and New Guinea. Skin gets paler as you move north or south away from the equator, until you reach northern Europe, where you find the palest skin of all. Obviously, dark skin evolved in people who were exposed to a lot of sunlight, because dark skin protects against sunburn and skin cancer. Doesn’t that make sense? In some parts of Australia, more than three-quarters of Aboriginal children have light blond hair, although it sometimes turns brown as they grow up. The other population in which blond hair is common is pale-skinned northern Europeans. Unfortunately, it’s not so simple. To begin with, sunburn and skin cancer cause very few deaths compared with infectious diseases. This means that sunburn and skin cancer would not be powerful pressures for natural selection. At least eight other theories have been suggested to explain why natural selection could have favored dark skin in the tropics and pale skin in the north. Among these theories: dark skin provides camouflage in the jungle, and pale skin is less sensitive to frostbite. But the biggest objection to any of these theories is that the link between dark skin and sunny climates is not perfect. Native peoples evolved dark skin in some areas with relatively little sunlight, such as the forested Australian island of Tasmania, while in sunny areas of tropical Southeast Asia, skin color is only medium. No American Indians have black skin, even in the sunniest regions. In the Solomon Islands of the Pacific, jet-black people and lighter-skinned people live on islands that share the same climate conditions. Anthropologists argue that some light-skinned tropical people migrated to their sunny regions too recently to have evolved dark skin. The ancestors of American Indians may have reached the Americas just eleven thousand years ago—perhaps not long enough to evolve black skin in the hot American tropics. To support the climate theory of skin color, anthropologists also point out that Scandinavians, who have pale skin, live in the cold, dark, foggy North. The problem is that Scandinavians have been in northern Europe for only four or five thousand years—even less time than American Indians have been in the Amazon. Either Scandinavians acquired their pale skin long ago in some other place, or they evolved it in less than half the time that American Indians have lived in the Amazon without evolving black skin. If the link between skin color and climate seems weak, there appears to be no link at all between climate and the color of hair or eyes. Blond hair is common in cold, wet, Scandinavia and also among Aborigines of the hot, dry center of Australia. What do these two areas have in common, and how could blond hair help both Swedes and Aborigines to survive? Sexual Selection and Physical Appearances When Darwin considered the problem of human geographic variations, he decided that natural selection had nothing to do with it. He came up with a theory he preferred: sexual selection. Darwin had noticed that many animals have features with no obvious survival value, such as the long, colorful tails of male peacocks and the dark, shaggy manes of male lions. These features do help animals get mates, however, either by attracting individuals of the opposite sex or by intimidating rivals. Male peacocks and lions that are especially successful at attracting females and scaring off rivals will leave more descendants than other males. Their genes will get passed on, and those genes will spread through a population because of sexual selection, or mating preferences, not because of natural selection. The same argument applies to female traits as well. For sexual selection to work, evolution must produce two changes at the same time. One sex must evolve a trait, and the other sex must evolve a liking for that trait. Male peacocks could hardly afford to flash their fancy tails if the sight revolted females and drove them away. As long as one sex has it and the other sex likes it, sexual selection could lead to just about any trait, if that trait doesn’t interfere too much with survival. Could human variations such as skin color be the result of sexual preferences that just happen to vary from place to place? Darwin believed the answer was yes. He noted that people in different parts of the world define beauty in terms of what is familiar to them. Individuals on the Pacific island of Fiji, or among the Bushmen of southern Africa, or in Iceland grow up learning the local standards of beauty. These standards tend to live on in each population because indivi