Early humans may have evolved as prey animals rather than as predators, suggest the remains of our prehistoric primate ancestors that were devoured by hungry birds and carnivorous mammals.
The discovery of multiple de-fleshed, chomped and gnawed bones from the extinct primates, which lived 16 to 20 million years ago on Rusinga Island, Kenya, was announced today at the Society of Vertebrate Paleontology's 70th Anniversary Meeting in Pittsburgh.
At least one of the devoured primates, an early ape called Proconsul, is thought to have been an ancestor to both modern humans and chimpanzees. It, and other primates on the island, were also apparently good eats for numerous predators.
"I have observed multiple tooth pits and probable beak marks on these fossil primates, which are direct evidence for creodonts and raptors consuming these primates," researcher Kirsten Jenkins told Discovery News.
Creodonts were ancient carnivorous mammals that filled a niche similar to that of modern carnivores, but are unrelated to today's meat eaters, she explained. The Rusinga Island creodonts that fed on our primate ancestors were likely wolf-sized.
"There is one site on Rusinga Island with multiple Proconsul individuals all together and these are covered in tooth pits," added Jenkins, a University of Minnesota anthropologist. "This kind of site was likely a creodont den or location where prey could be easily acquired."
Analysis of tooth pits, de-fleshing marks, bone breakage patterns, gnawing and other damage to the primate bones indicate that raptors were also hunting down these distant relatives of humans.
"Primatologists have observed large raptors taking monkeys from trees," Jenkins said. "When a raptor approaches a group of monkeys, those monkeys will make alarm calls to warn their group and attempt to retreat to lower branches. The primates on Rusinga had monkey-like postcrania and likely had very similar locomotor behavior."
The study presents the first evidence of raptor predation on fossil primates from Rusinga, which was part of the side of a large volcano 20 million years ago.
Multiple ash layers suggest that eruptions killed countless animals from time to time. But when the volcano was inactive, the site supported a wooded area.
Jenkins is not certain what selective pressures predators placed on these very early primate ancestors to humans, but she said they "can affect behavior, group structure, body size and ontogeny (the life cycle of a single organism)."
Robert Sussman, professor of physical anthropology at Washington University in St. Louis, has long argued that primates, including early humans, evolved not as hunters but as prey of many predators, including wild dogs and cats, hyenas, eagles and crocodiles.
"Despite popular theories posed in research papers and popular literature, early man was not an aggressive killer," said Sussman, author of the book "Man the Hunted: Primates, Predators and Human Evolution." "Our intelligence, cooperation and many other features we have as modern humans developed from our attempts to out-smart the predator."
He added that the idea of man as hunter "developed from a basic Judeo-Christian ideology of man being inherently evil, aggressive and a natural killer."
"In fact, when you really examine the fossil and living non-human primate evidence, that is just not the case," he explained.
Jenkins and her colleagues continue to excavate at Rusinga and nearby Mfangano islands, hoping to find more fossils -- especially those from birds -- so that the scientists can identify the species that were hunting the prehistoric primates.