The roots of human altruism may go back far further than had been realised, according to a new study showing that chimpanzees are also capable of helping others without any thought of personal reward.
Until now, most scientists have believed that altruism emerged only after the evolutionary line that produced humans split away from the one that gave rise to chimpanzees - around six million years ago.
However, new research shows that chimpanzees, too, are capable of altruistic acts and that this appears to be programmed into their genes.
In the study, young chimpanzees spontaneously and repeatedly helped humans who appeared to be struggling to reach sticks in the animals' enclosure.
It suggests that altruism may have been an element of the social life of the "primitive" apes that eventually led to both humans and chimpanzees.
"We thought we were very different from other animals including our primate relatives, but this is not the case. At least some altruism may have been present in the common ancestor to humans and chimpanzees," said Felix Warneken, a psychologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, who led the research.
Anthropologists have long recognised altruism as a vital element in allowing complex social groups to form. This raised the question of when it first evolved.
True altruism has always been seen as a uniquely human trait in that only people were thought capable of deliberately helping others, knowing there would be a cost to themselves.
Dr Warneken and his colleague Brian Hare carried out the study on 36 chimpanzees at the Ngamba Island Chimpanzee Sanctuary, in Uganda. The animals were not allowed to interact with any humans they knew or had received food from.
In the first experiment, the chimpanzee saw a person unsuccessfully reach through the cage bars for a stick on the other side, too far away for the person but within reach of the ape.
The chimpanzees spontaneously helped the reaching person regardless of whether this yielded a reward or not. When the chimpanzees could see the person making no effort to reach the object, they did not help.
A second experiment made it much harder to offer help, with the chimps forced to climb two metres to get the stick. There were no rewards but, again, the animals still helped.
A third piece of research looked at the apes' willingness to help each other. One chimp was made to watch as a second animal tried to get into a closed room containing food.
The only way it could get in was if the watching animal removed a chain on the door. In each case they did so.