Was the ancestor of all primates a solitary-living species? Did more social forms of primate societies evolve from this basic and simple society? Until now, the classic answer was yes. Using modern statistical analysis, including variations within species, researchers have now found that the ancestral primate social organization was most likely variable: most lived in pairs, and only 10 to 20% of individuals were solitary; living in pairs was likely ancient and caused by reproductive benefits, like access to partners and reduced competition within the sexes.
Ardipithecus ramidus, a hominid that lived in Africa more then 4 million years ago. Illustration by Arturo Asensio, via Quo.es.
For their study, University of Strasbourg researcher Charlotte Olivier and colleagues collected detailed information about the composition of social units in primate populations in the wild.
They built a detailed database that covered almost 500 populations from over 200 primate species. More than half of the primate species recorded in the database exhibited more than one form of social organization.
“The most common social organization was groups in which multiple females and multiple males lived together, for example chimpanzees or macaques, followed by groups with only one male and multiple females — such as in gorillas or langurs. But one-quarter of all species lived in pairs,” said Dr. Adrian Jaeggi, a researcher at the University of Zurich.
Taking into account several socioecological and life history variables such as body size, diet or habitat, the researchers calculated the probability of different forms of social organization, including for our ancestors who lived some 70 million years ago.
To reconstruct the ancestral state of primates, they relied on fossils, which showed that ancestral primates were relatively small-bodied and arboreal — factors that strongly correlate with pair-living.
“Our model shows that the ancestral social organization of primates was variable and that pair-living was by far the most likely form,” said Dr. Jordan Martin, a researcher at the University of Zurich.
“Only about 15% of our ancestors were solitary. Living in larger groups therefore only evolved later in the history of primates.”
In other words, the social structure of early primates was likely more similar to that of humans today than previously assumed.
“Many, but by no means all of us, live in pairs while also being a part of extended families and larger groups and societies,” Dr. Jaeggi said.
“However, pair-living among early primates did not equate to sexual monogamy or cooperative infant care.”
“It is more likely that a specific female and a specific male would be seen together for most of the time and share the same home range and sleeping site, which was more advantageous to them than solitary living,” said Dr. Carsten Schradin, a researcher at the University of Strasbourg.
“This enabled them to fend off competitors or keep each other warm, for example.”
The study was published in the Proceedings of the National Academy of Sciences.
Charlotte-Anaïs Olivier et al. 2023. Primate social organization evolved from a flexible pair-living ancestor. PNAS 121 (1): e2215401120; doi: 10.1073/pnas.2215401120
Source : Breaking Science News