Romantic heterosexual relationships in humans may have evolved from same-sex pairings in a common ancestor of humans and chimpanzees, according to a novel hypothesis by a researcher at The University of Texas at Austin.

The prevailing explanation of heterosexual pair bonding and romantic love in humans is that it evolved from the mother-infant bond present in many mammals. In a paper published in the journal Evolutionary Anthropology, anthropology professor Aaron Sandel cites primate research鈥攊ncluding his own decade-long studies of chimpanzees in Kibale National Park, Uganda鈥攖o propose instead that this behavior evolved in humans from same-sex pair bonding already present in a common ancestor of humans and chimpanzees.

A pair bond is a cooperative relationship between nonrelated adults that remains stable over time and includes an emotional connection, rather than being merely transactional. Chimpanzees, humans' closest relative, do not pair bond with their mates, but adult males of the species form same-sex bonds lasting as long as 13 years.

"Part of the evolutionary puzzle is that our closest living relatives, the great apes, including chimpanzees and bonobos, do not form enduring bonds with their mates," Sandel said. "So biological anthropologists have assumed that whatever led to pair bonds in humans must have something to do with other uniquely human traits, such as walking upright, or having infants with huge brains, or hunting, or making fire. But what if pair bonds do occur in some of our ape relatives, and we just overlooked them?"

Several indicators of emotional connection can be observed in same-sex relationships between male chimpanzees, including evidence of reduced stress levels, behaviors specific to interactions with that partner, and possibly jealousy if a partner engages in grooming activities with individuals outside the pair.

Previous research may have failed to categorize such behaviors as indicative of pair bonding, Sandel said, because chimpanzees also engage in social behaviors with non-partners in their larger social groups, similar to how humans form friendships with both their mates and others in their communities.

Sandel proposes that same-sex attachments such as those seen among chimpanzees existed in our species prior to the emergence of opposite-sex pair bonds with mates.

"This raises the possibility that romantic love in humans finds its origin in same-sex friendships of apes," Sandel said. "And even in our own lives, we may have underestimated the importance of same-sex bonds and friendship."

More information: Aaron A. Sandel, Male鈥搈ale relationships in chimpanzees and the evolution of human pair bonds, Evolutionary Anthropology: Issues, News, and Reviews (2023). DOI: 10.1002/evan.21986

Provided by University of Texas at Austin