Humans are the only primates with a chin, a feature that has puzzled biologists. A new analysis suggests it emerged not for a specific purpose but as a side effect of other evolutionary changes. Researchers examined hundreds of ape skulls to reach this conclusion.
Biologists have long debated the evolution of the human chin, a bony projection on the lower jaw that extends beyond the front teeth. Unlike other primates or even extinct human species, this trait defines Homo sapiens. Previous theories proposed it might ease chewing strain, aid speech, or result from sexual selection. Others suspected it had no direct function, arising incidentally from broader skull changes.
Noreen von Cramon-Taubadel at the University at Buffalo in New York state led a study challenging the idea that every unique feature evolves purposefully. "There has been a tendency to assume that every feature that differs significantly between species has been shaped by natural selection for a specific purpose, but this ‘purposeful’ view of evolution is inaccurate," she said. "Evolution is often messier and less directed than people expect or assume."
The team analyzed 532 skulls from humans and 14 ape species, including chimpanzees, bonobos, gorillas, orangutans, and gibbons. They measured 46 anatomical distances, focusing on chin-related areas, and mapped these onto an evolutionary tree. Using a genetic model, they estimated the last common ape ancestor's head shape and tested for selection versus random drift.
Results showed three chin traits under direct selection, while six others were by-products of unrelated adaptations. As ancestors adopted upright postures, skull bases flexed, faces tucked under larger brains, and smaller teeth reduced jaw size. This caused the upper jaw to recede, leaving the lower jaw to project forward.
"This unique feature appears to have emerged as a consequence of humans evolving an upright posture, larger heads and smaller teeth," von Cramon-Taubadel explained, noting how selection in one body area affects others.
Alessio Veneziano at the French National Museum of Natural History in Paris called the chin a "textbook example" of non-adaptation. James DiFrisco at the Francis Crick Institute in London emphasized the skull and jaw's integration: "Just because an observable feature like the chin looks like a distinct ‘thing’ doesn’t mean it actually evolves as an independent unit."
The findings, published in PLOS One (DOI: 10.1371/journal.pone.0340278), illustrate evolution's interconnected nature, with traits like the chin as spandrels—unintended outcomes of other selections.
