What family constellations did our early ancestors live in? Researchers have now found evidence of this with the help of a comparative behavioral study. To do this, they evaluated the social relationships of more than 200 modern primate species and, on this basis, drew conclusions about the lifestyle of our earliest ancestors. Accordingly, the first primates probably lived together as pairs 70 million years ago. These results contradict previous assumptions that the first primates were solitary animals.
Today’s primates, which include us humans, are considered very social creatures. Monkeys and great apes often live in groups and maintain complex relationships. “However, it is often assumed that the ancestor of all primates was a solitary animal and that other forms of social organization only developed later,” explains a team led by Charlotte-Anaïs Olivier from the University of Strasbourg in France. “Numerous studies have suggested that the ancestor of primates was small, nocturnal, and lived in trees.” These characteristics also apply to some lemur species living today. These, in turn, were believed to be mostly solitary creatures – and this assumption was transferred to the ancestor of all primates.
More social than expected
“Numerous recent field studies have now shown that lemurs are significantly more social than expected and often live together in pairs,” report Olivier and her team. To more reliably estimate how social the first primates likely were, the research team compiled a database containing information about the social lives of more than 200 primate species. To do this, they included field studies on almost 500 wild populations.
They found that most primate species have diverse forms of social organization. “The most common were groups in which several females and several males lived together, such as chimpanzees or macaques. The second most common were groups with just one male and several females – like gorillas or langurs,” reports co-author Adrian Jäggi from the University of Zurich. “A quarter of all species also organized themselves in pairs.”
Eat individually, sleep together
Olivier and her team did not classify many species that were described as solitary in previous studies as solitary. “In previous studies, species were described as living solitary lives when the individuals search for food alone,” explain the researchers. “However, our database shows that many of these populations live together as pairs or groups outside of foraging and share common sleeping places.” To define them as a couple, it was sufficient for the research team if two individuals shared a territory and often slept together – even if they did not sleep together raised offspring or lived sexually monogamous lives.
Using statistical methods, the researchers established connections between a species’ social life and other characteristics such as body size, diet, activity patterns and habitat. From this, they created a statistical model that predicts, for each combination of these characteristics, which form of social organization is most likely under these conditions. They also applied this model to the last common ancestor of all primates. In accordance with fossil finds and previous studies, they assumed that it was a small, nocturnal tree-dweller.
Couple relationship as an original form
“Our model shows that living as a couple was by far the most likely form of social organization among our last common ancestors,” explains Olivier’s colleague Jordan Martin. “Life in larger groups probably only arose later in the history of primates.” This means that the social structure of the first primates may have been more similar to today’s humans than previously thought. “We also often, but not always, live as couples and are at the same time embedded in large families and larger groups and societies,” says Jäggi. “However, we cannot make any statements about the exact nature of the couple’s life at the time. Whether two individuals develop an emotional couple relationship, live sexually monogamously, or raise offspring together varies greatly, even among primate couples today.”
Source: Charlotte-Anaïs Olivier (University of Strasbourg, France) et al., Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2215401120