Human social organisation may have evolved far earlier than previously thought, a study on the behaviour of gorillas suggests.
Western lowland gorillas have more intricate social structures than once thought and forge lifetime bonds between distant relations.
They also develop social tiers with striking parallels to human societies, researchers say.
The study suggests the origins of human social systems stretch back to the ancestor shared with gorillas, rather than from the social brain of hominins after the split from other primates.
Published in the journal Proceedings of the Royal Society B, the study used more than six years of data from two research sites in the Democratic Republic of Congo, where scientists documented the social exchanges of hundreds of western lowland gorillas.
Dr Robin Morrison, a biological anthropologist from the University of Cambridge, and the research team observed that gorillas live in small family units – a dominant male and several females with offspring – or as solitary male bachelors.
They analysed the frequency and length of associations.
According to the data, there was a tier of regular interaction – an average of 13 gorillas – that maps closely to dispersed extended family in traditional human societies, for example aunts, grandparents and cousins.
A further tier of association involved an average of 39 gorillas, similar to an “aggregated group” that spends time together without necessarily being closely related.
“An analogy to early human populations might be a tribe or small settlement, like a village,” said Dr Morrison.
Where silverbacks were half-siblings they were more likely to be in the same tribe.
But more than 80% of the close associations detected were between more distantly related – or even apparently unrelated – silverbacks, the study says.
Dr Morrison commented: “Females spend time in multiple groups throughout their lives, making it possible for males not closely related to grow up in the same natal group, similar to step-brothers.
“The bonds that form may lead to these associations we see as adults.
“If we think of these associations in a human-centric way, the time spent in each other’s company might be analogous to an old friendship.”
Researchers uncovered hints of an even higher social tier of periodic aggregations, similar to an annual gathering or festival based around “fruiting events”.
Although they said these are too infrequent to detect with certainty from this study’s data.
Dr Morrison argues that sporadic fruiting schedules of the gorillas’ preferred foods may be one reason why they – and consequently maybe we – evolved this “hierarchical social modularity”.
She said: “Western gorillas often move many kilometres a day to feed from a diverse range of plants that rarely and unpredictably produce fruit.
“This food is easier to find if they collaborate when foraging.”
A small number of mammal species that also rely on idiosyncratic food sources have a similar social structure to humans.
They also all have spatial memory centres in their brain to rival those of humans.
Before now, the species on this short list were evolutionarily distant from humans.
Chimpanzees, the relatives to humans, live in small territorial groups with fluctuating alliances that are highly aggressive with neighbours.
One theory for human society is that it required the evolution of a particularly large and sophisticated “social brain” unique to the hominin lineage.
However, the study suggests the addition of gorillas indicates the simplest explanation may be that human social complexity evolved much earlier, and is instead merely absent from the chimpanzee lineage.
Dr Morrison said: “While primate societies vary a lot between species, we can now see an underlying structure in gorillas that was likely present before our species diverged, one that fits surprisingly well as a model for human social evolution.
“Our findings provide yet more evidence that these endangered animals are deeply intelligent and sophisticated, and that we humans are perhaps not quite as special as we might like to think.”
Researchers used statistical algorithms to reveal patterns of interaction between family groups and individuals in the datasets.