The popular view of our ancient ancestors as
hunters who conquered all in their way is wrong, researchers have told
a major US science conference.
Instead, they argue, early humans were on the menu for predatory beasts.
This may have driven humans to evolve increased levels of co-operation, according to their theory.
James Rilling, at Emory University in Atlanta, US, has
been using brain imaging techniques to investigate the biological
mechanisms behind co-operation.
He has imaged the brains of people playing a game under
experimental conditions that involved choosing between co-operation and
From the parts of the brain that were activated during
the game, he found that mutual co-operation is rewarding; people
reacted negatively when partners did not co-operate.
Dr Rilling also discovered that his subjects seemed to
have enhanced memory for those people that did not reciprocate in the
Man ‘the hunted’
By contrast, our closest relatives – chimpanzees – have
been shown not to come to the aid of others, even when it would pose no
cost to themselves.
"Our intelligence, co-operation and many other features
we have as modern humans developed from our attempts to out-smart the
predator," said Robert Sussman of Washington University in St Louis.
According to the theory espoused by Professor Sussman,
early humans evolved not as hunters but as prey for animals such as
wild dogs, cats, hyenas, eagles and crocodiles.
He points to the example of one ape-like species thought to be ancestral to humans, Australopithecus afarensis.
A. afarensis was what is known as an "edge species"; it could live in trees and on the ground, and could take advantage of both.
"Primates that are edge species, even today, are basically prey species, not predators," Professor Sussman explained.
Dr Agustin Fuentes at the University of Notre Dame agrees with the predation hypothesis.
He believes early humans were subject to several evolutionary pressures, including predation.
But he also thinks they were expending more energy at this time and that child-rearing became more demanding.
All these factors contributed to an emergence of sociable behaviour in hominids that made them harder targets for predators.
Dr Fuentes points to fossil evidence of predation in two different groups of humanlike species: Australopithecus and Paranthropus.
The latter group, it appears, could not adapt to
pressures such as predation, and became extinct between one and 1.2
million years ago.
The scientists outlined their work at the American Association for the Advancement of Science annual meeting in St Louis, US.
By Paul Rincon