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[How Humans Became Social] "Researchers have long believed that it was a gradual process, evolving from couples to clans to larger communities. A new analysis, however, indicates that primate societies expanded in a burst, most likely because there was safety in numbers."

[Study reveals clues to how humans became sociable] "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."