“It has been commonly assumed that social behaviour became more complex over evolutionary time, moving from individuals living alone to pairs, then harems, and finally large groups – in other words that groups got bigger and bigger” says Dr Quentin Atkinson from The University of Auckland, one of the authors of the study published in Nature today.
“In fact, we found that the first transition in the primate family tree was straight from solitary to group living, and any change to pair or harem social structures occurred later. Amongst early primates living in groups, we also found that the groups became more stable over time.”
“One of the main findings was that the transition from solitary living to large groups occurred alongside the change from nocturnal to daytime activity.” The researchers concluded that the protection from predators afforded by group living is what allowed our primate ancestors to begin exploring the world during the daytime.
Humans, together with our closest relatives the chimpanzees, gorillas and orang-utans, are classified as living in groups, although gorillas can at times switch to harems made up of one male and several females.
Once the transition from individual to group living took place – 52 million years ago in the ancestral line that gave rise to humans, and later in another branch of the primate family tree – no shift back to solitary behaviour ever occurred. Primate ancestors that subsequently began living in pairs did not switch back to group living, whereas those that began living in harems could transition back and forth with large groups. There was never a transition directly from pair to harem living or vice versa.
The Nature study looked at the behaviour of present-day primates and, based on their evolutionary relationships to one another, worked out the changes in social behaviour that must have occurred at various points in the family tree to produce the distribution of behaviour seen today.
“It’s a bit like if you have red hair in your family and want to know which ancestor it came from,” explains Dr Atkinson. “One way of doing that would be to look at all your relatives living today to see which ones have red hair and trace back through that lineage to work out when red hair must have arisen – how many generations ago and where. The work that we have done is analogous to that, but rather than looking at a family tree of people we’re looking at a family tree of species.”
“There’s a lot of interest in the primate family tree because they are our closest relatives. Some of the most interesting questions we can ask about primates relate to their social behaviour, because it’s social behaviour more than anything else that differentiates us from other species.”
“Unfortunately behaviour doesn’t fossilise so it’s more difficult to study than an anatomical characteristic like skull shape, and we have to learn about its evolution indirectly. I like to think of what we do as bringing fossils to life, by working out how they must have behaved.”
The study came about from a collaboration formed when Dr Atkinson worked in Oxford with colleagues Dr Susanne Shultz and Christopher Opie. Following on from their Nature paper the scientists plan to study social evolution in other species and other aspects of behaviour in primates and other mammals.