Why Do Humans Have Thumbs?

There’s a never-ending stream of theories about Homo sapiens’ most important digit

Illustration by Harry Campbell

Of all the motions the hand can perform, perhaps none is so distinctively human as a punch in the nose. Other animals bite, claw, butt or stomp one another, but only the species that includes Muhammad Ali folds its hands into a fist to perform the quintessential act of intra-species male-on-male aggression.

David Carrier, an evolutionary biologist at the University of Utah, believes our key advantage is the dexterity and configuration of our thumb, which folds over the second and third fingers as a buttress, concentrating the striking power and protecting the delicate hand bones. (Crucially, male index fingers are short relative to the ring fingers, so they fit snugly behind the bulge of muscle at the thumb’s base; in women, the second and fourth fingers are typically the same length.) In a recent paper in the journal Biological Reviews, Carrier speculated that the bones of the masculine face may have co-evolved with the thumb to be able to withstand a punch. “It’s not settled,” he said in an interview, “but the evidence suggests that the male hand evolved to be a better club, while the female hand maximizes dexterity.”

Carrier’s controversial hypothesis is part of a reassessment of the human thumb by anthropologists, who for a long time focused on its role in activities like picking up a grape. The two-finger precision grip was important in evolution, says physical anthropologist Mary Marzke of Arizona State University. “But if you think about it, you don’t really use it that much. Even surgeons don’t.” Marzke has researched other grips, especially the “cupping,” or “wraparound,” grip, work followed by Alastair Key of the University of Kent, who used sensors to measure the forces on the digits of knappers as they chipped at rocks to replicate primitive tools. His recent paper in the Journal of Human Evolution suggests that an overlooked factor in thumb evolution is the nondominant hand—the one that holds the rock while the dominant hand shapes it into a stone tool.

A rock for throwing is a kind of tool, of course, and the cupping grip a way of using it. “I may throw like a girl,” says Suzanne Kemmer of Rice University, “but I throw better than any chimpanzee.” Kemmer, a linguist, thinks that by enabling fine motor skills the thumb promoted the development of the brain. Take away the thumb, she says, and Facebook would need a different icon for “Like,” you couldn’t thumb your nose at anyone, and umpires would have to find a less satisfactory gesture for throwing players off the field. So never take your thumbs for granted—especiallynotwhenyou’retyping.

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