Scientists have long linked the evolution of the human hand—unique for its lengthy opposable thumbs and dexterous fingers—to the rise of stone tools some 2.6 million years ago. These instruments, from primitive chunks of rock used as makeshift hammers to sharp stone flakes created by striking one stone against another and even small handaxes, are typically attributed to Homo habilis, an ancient human species nicknamed “handy man” in honor of its theorized role as the first toolmaker.
Early hominins practiced an array of tool-related activities, including hunting, foraging and cooking. But according to a new study from researchers at Chatham University and the University of Kent, not all of these activities were created equal. The team’s findings, newly published in the Journal of Human Evolution, suggest that a specific behavior—smashing animal bones to access their marrow—had an outsized effect on the development of early hand anatomy.
“These behaviors all involve different materials, different end goals, and different patterns of force and motion for the upper limb,” the researchers note in their study. “Therefore, it is unlikely that each behavior exerted equal influence on the evolution of the modern human hand.”
Bone marrow is a tasty, high-energy food. Early humans who had hands more suited to smashing open bones and acquiring the delicious snack might have been better equipped to survive in the harsh conditions of prehistory, and thus more likely to pass their genes—and dextrous hands—on to the next generation. To test that hypothesis, the team asked 39 volunteers to don a manual pressure sensor system called Pliance and demonstrate a bevy of Pleistocene-era activities, like cracking nuts, acquiring marrow with the aid of a hammerstone, and chipping away flint to shape tools known as flakes. Pliance, Science Alert’s Michelle Starr explains, is worn like a glove and allows researchers to determine the amount of pressure exerted on each finger during various activities.
Measurements varied across the board, but researchers found that the thumb, index and middle fingers always played a role of high importance. Behaviors requiring the most pressure were hammering bones for marrow and producing flint flakes. The behavior that required the least amount of pressure was nut-cracking. Tracy Kivell, a biological anthropology professor at Kent, said the team’s findings could explain why other primates are able to crack nuts without the benefit of a human-like hand.
Although modern humans and primates share the evolutionary benefit of opposable thumbs, the lengths of our fingers differ: Apes and monkeys have shorter thumbs and longer fingers ideally equipped for tree-swinging, while humans have elongated thumbs and shorter fingers designed for precision grasping. Interestingly enough, a 2015 study found that the hand of both species' common ancestor looked more like humans' than primates', suggesting that the human hand is more "primitive." (This doesn't mean that we're less intelligent than our primate counterparts, Science's Michael Balter notes—instead, it suggests that primates' hands evolved for life in the trees, while ours evolved in conjunction with neurological developments that enabled more advanced toolmaking.)
Earlier this month, a group of capuchins were spotted in Panama using stone tools to smash shellfish and other foods, the Washington Post’s Sarah Kaplan reports. In doing so, the monkeys became the first of their genus and the fourth of all nonhuman primates to enter the Stone Age, so to speak.
The new findings don’t exactly rewrite the story of human hand evolution. But the newfound emphasis on rich, high caloric-marrow draws attention to the variety of practices that contributed to today’s nimble fingers. Although the task of making stone tools certainly influenced the development of our ancestors’ hands, it was perhaps their voracious appetite for a fatty, hard-to-reach treat that made all the difference.