The human thumb is a nimble wonder, allowing us to make tools, sew clothing, and open pickle jars. But just how and when this unique digit evolved has long been a mystery. Now, a new study modeling muscle in fossilized thumbs suggests about 2 million years ago, our ancient ancestors evolved a uniquely dexterous appendage while our other close relatives remained … all thumbs.
It’s a “thorough, robust analysis,” says Tracy Kivell, a paleoanthropologist at the University of Kent who was not involved with the work. But she and others caution that the research is too preliminary to provide a true smoking gun.
Figuring out how ancient thumbs worked isn’t easy. Fossils don’t preserve muscles, so most previous attempts to estimate ancient dexterity relied on how closely our ancient relatives’ hand bones resembled our own. Hand bones are also small and relatively rare in the fossil record. But resemblance can be deceiving: Depending on how the muscles are connected, some species with similar bone anatomy might have very different grip strengths, and vice versa.
To analyze ancient thumbs on their own terms, paleoanthropologists at the University of Tübingen digitized the fossil thumb bones from a variety of ancient hominins, a group that includes all species in our own genus, Homo, as well as other very closely related species. The researchers looked at bones from two early modern humans and four Neanderthals from the past 100,000 years, and the diminutive, cave-dwelling H. naledi (from about 250,000 to 300,000 years ago). They also looked at a sister genus to Homo called the Australopithecines, which included Australopithecus afarensis, A. africanus, and A. sediba.
The researchers then used 3D computer software to digitally reconstruct on the fossils a muscle known as the opponens pollicis—which allows your thumb to flex inward—to its attachment site at the base of the palm. They simulated the approximate force the muscle could exert, with more force equating a better, more precise grip, for example, when holding steady a needle and thread or swinging a hammer.
To validate their model, the scientists applied the same approach to the thumb bones of modern humans and chimpanzees. They found that their model’s force estimates matched up with the known capabilities of the two species.
All the members of our genus Homo they investigated had basically the same thumb grip strength as modern humans, the researchers report today in Current Biology.
The team also found modern-looking thumb movement in two hominin specimens from the Swartkrans site in South Africa, which have been dated to about 2 million years ago. Because their skeletons are so incomplete, no one has been certain which species or genus they belonged to. The authors argue their humanlike thumb is good evidence they may be members of Homo, though they admit the jury is still out. Either way, the Swartkrans fossils represent the earliest known humanlike thumbs in the fossil record, the authors say.
But the other human relatives in the study, the Australopithecines, had much weaker thumbs, the team found, closer to those of modern chimpanzees. That’s a bit surprising in the case of A. sediba—which, like the Swartkrans fossils, dates to about 2 million years ago. Its humanlike hand proportions caused many to believe it may have possessed humanlike dexterity. “Even though Australopithecines, including A. sediba, may have exhibited tool-related behaviors, they had not yet developed a humanlike level of efficiency,” the authors tell Science.
Overall, the work suggests the modern human thumb arose about 2 million years ago in the Homo genus, the researchers conclude. This may have allowed ancient humans to get better and better at making stone tools, ultimately surpassing the other hominins.
Evie Vereecke, an anthropologist and anatomist at KU Leuven, praises the authors’ approach. But she says the findings should be treated with caution. “We know that dexterity is not only due to one muscle.”
Laurent Vigouroux, a biomechanics researcher at Aix-Marseille University who studies the mechanics of the human grip, agrees. He notes there are more than 10 different muscles that contribute to thumb movement, and it’s possible that weaker opponens pollicis in some species may have been compensated for by some other muscle or muscles.
Still, both Vigouroux and Vereecke say the study’s basic approach is likely to be useful for the field—giving anthropologists a kind of common language for analyzing the muscular characteristics of fossils. “Not long ago, everyone who found fossilized remains had their own interpretation” of the long-gone musculature, Vereecke says. “This could help bring them onto the same page.”
sciencemag.org, 28 January 2021