"On the Imperfection of the Geological Record" is the ninth chapter of both On the Origin of Species by Means of Natural Selection by Charles Darwin and Darwin's Ghost by Steve Jones. Beginning with this chapter, Darwin and Jones address the topic of geology. Like many of the chapters in Jones’ and Darwin’s books, one version of this chapter is more interesting than the other. In this case, it is the version in Jones’ Darwin’s Ghost.
Right near the beginning of his chapter, Jones points out that "[t]he fossil record—in defiance of Darwin’s whole idea of gradual change—often makes great leaps from one form to the next." How can evolution make sense if "many species appear without warning, persist in fixed form, and disappear, leaving no descendants?" The answer? The fossil record is much less reliable and less complete than one might think. Bones are destroyed, washed away, and, given time, decompose. Few create fossils before they go.
The length of natural history is also much longer than anyone has known until recently. Archbishop Ussher’s famous estimate for the Earth’s origin was 4004 B.C. Jones also writes that "French naturalist George Cuvier noted that the five-thousand-year-old ibis mummies brought back by Napoleon from Egypt were identical to their fellows alive today. That, he felt, showed that animals could not change, as five millennia seemed to him to encompass most of the Earth’s history." It wasn’t until 1788, with discoveries by geologist James Hutton, that it became clear that the Earth is very old. Even then, no estimate existed of its age. Further research gives an age of about 4.5 billion years.
In these vast periods of time, a lot can happen to the fossil record. Most of this change can be blamed on water. Over even a hundred years, the force of rain was enough to wipe the hieroglyphs off a obelisk brought to New York from Thebes, where it had stood for three thousand years, intact in the dry weather. In a little over a thousand years, since having its original limestone surface removed, the Great Pyramid has lost sixty thousand cubic yards of material. In several million years, the Indian face of the Himalayas has been gashed by valleys and strange formations of rock, while Tibet, on the other side of the mountains, is dry and flat. Without rain, it has changed far less over time.
Jones gives another example of the power of water. For thirty years, the Glen Canyon Dam dammed the Colorado River. When it was opened for a week in March and April of 1996, the river rose by twenty feet, but, more dramatically, thousands of bones ("any of which might in time have become a fossil," Jones writes) were moved and resettled in the Grand Canyon. "In the Grand Canyon’s long history," Jones writes, "such deluges must have happened uncounted times."
Jones writes that "[I]t was explained to Darwin in South America that the bones of giant sloths deep below the surface proved how, in an earlier age, they lived in holes." The truth, Jones continues, is simpler. Given long enough, the land flows, moves, and changes. Things that are in it, like fossils, are moved as well. In an experiment at the East Fort River in Wyoming, cow bones were marked and scattered in the 1970s. Only ten years later, many of them were buried deep in sediment, mingled with bones thousands of years old. Living creatures can also confuse our understanding of the fossil record. For instance, we expect to find the shells of sea snails in the sea, but they are sometimes found mixed with the bones of land mammals and birds. "The snails," explains Jones, "were picked up by ancient seabirds who dived for food and left the fragments around their nests."
The effect of all these problems is that there are gaps in the fossil record. The passenger pigeon, which once flourished in America, died out in 1914. "At the time of the Mayflower," Jones points out, "nine billion were alive—more than all the birds of America today." No fossilized passenger pigeon bones have ever been found. The only records that exist of it are those made by people who saw them. Krill, a tiny sea creature eaten in vast quantities by some species of whales, sometimes swim in shoals consisting of a hundred million tons of the animals, yet only one krill fossil has ever been found. It was eaten by a fish and fossilized within it. How many other species are missing entirely from the fossil record?
In this chapter, both Darwin and Jones address "the sudden appearance of whole groups of Allied Species." In fossil formations, it often appears that many related species have begun existence at the same time. Jones explains that, again, this appearance is due to our overestimating the perfection of the geological record. A species or group of species that appears to emerge suddenly can, on further investigation, turn out to be descended from other species that are less different from previously known ones. For instance, Archaeopteryx, an early, dinosaur-like bird, seemed to be separated from both dinosaurs and birds by gigantic gaps. It was not until the 1990s that intermediate species were found. The reason it took so long is that so few fossils of these ancient species exist.
The main point made by both Darwin and Jones in this chapter of Origin of Species is that the fossil record is far more complicated, more varied, and more incomplete than anyone ever imagined. In Darwin’s words, "I look at the natural geological record, as a history of the world imperfectly kept, and written in a changing dialect; of this history we possess the last volume alone, relating only to two or three countries. Of this volume, only here and there a short chapter has been preserved; and of each page, only here and there a few lines." This is the only record we have of most of the history of life on Earth.
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