Sussman is the co-author of a 2005 book, Man the Hunted: Primates, Predators, and Human Evolution.
In the book, Sussman and Donna L. Hart, a University of Missouri-St. Louis anthropologist, first argued that early humans evolved not as hunters but as prey.
The book title harks back to a 1966 symposium, "Man the Hunter," held at the University of Chicago and a 1968 book with the same title.
Both the symposium and the 1968 book represented what was then cutting edge research into the planet's living hunter-gatherer societies. Many anthropologists would study these cultures' traditional lifestyles to gain insight into early human behaviors.
Some of the most celebrated research in support of the view of humans as warriors had come from Napoleon Chagnon, an anthropologist now retired from the University of California, Santa Barbara (UCSB).
Chagnon studied warfare and other attributes of the Yanomami, or Yanomamo, tribes of the Amazon Basin. His 1968 book on the tribe sold a million copies and became required reading in many anthropology classrooms.
This year Douglas Fry, a researcher affiliated with both Åbo Akademi University in Finland and the University of Arizona in Tucson, published a book called The Human Potential for Peace, which refutes some of Chagnon's key findings.
Fry writes that early studies defining humans by their capacity for killing are flawed. There's just as much evidence, he says, that humans had an established track record in peaceful conflict resolution.
Specifically, Fry's new book pokes holes in Chagnon's assertion that Yanomami men who were efficient warriors had more children.
Fry says a reanalysis of the data reveals that Chagnon failed to control for age differences. Fry concludes that it was actually older tribal members, not necessarily the best warriors, who had achieved greater success at reproduction.
And that, Fry says, can be expected in any culture, regardless of a propensity for violence.
What Do the Fossils Say?
Instead of studying living traditional cultures, as Chagnon did, Washington University's Sussman decided to base his research for Man the Hunted on a hard look at the fossil record.
"I have always, since my early days in anthropology, thought the hunting hypothesis was based on little actual evidence from the fossils," Sussman said.
Sussman found that our ancestors from three or four million years ago, Australopithecus afarensis, had small teeth, lacked tools, and were about three feet (one meter) tall.
Lacking size or weapons, this early human species most likely used brains, agility, and social skills to escape from predators, the anthropologist says.
At that time, he says, A. afarensis suffered the same predation rates as many other primate speciesabout 6 percent.
But about two million years ago there was a shift in the record. Somehow predation rates on other species suddenly went up while rates on human ancestors declined.
Another group of primates with humanlike attributes, the genus Paranthropus, went extinct by about one million years agothe same time our predecessor, Homo erectus, was expanding across Africa and Eurasia.
All the Angles
Several other researchers presented in St. Louis their work exploring various genetic, hormonal, and psychiatric explanations for early humans' success.
James K. Rilling directs the Laboratory for Darwinian Neuroscience at Emory University in Atlanta. His brain-imaging studies have revealed a potential connection between the act of cooperating and the brain's reward centers.
If prehistoric humans got instant gratification from cooperating, he says, that may have aided group survival.
And Charles Snowdon, a psychologist and zoologist at the University of Wisconsin in Madison, pointed out that expectant monkey fathers gain weight and take on hormonal changes along with their pregnant partners.
The study offers evidence that these primates evolved to be good fathers, an important attribute for protecting young from predators.
Snowdon's endocrine studies have also shown that the likelihood that male primates will dally with new females decreases when the male already has a mateand still more when the pair is raising offspring.
It's possible a similar system of mate fidelity aided the group cohesion needed to minimize predation in early humans, he said.
The University of Arizona's Fry says the notion that early humans relied on cooperation changes more than the widespread image of a club-toting early human in a warlike stance.
He believes it has implications for today's human interactions.
"Many of us Westerners share a view of human nature that humans are naturally warlike," Fry said. "This view helps perpetuate a self-fulfilling prophecy."
Changing our perspective to match the anthropological record, he said, "opens new possibilities in today's world."
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