Your Amazing Thumb Is About 2 Million Years Old | Science

By Michael PriceJanuary 28, 2021 at 11:20 am

The human thumb is an agile wonder, which allows us to make tools, sew clothes and open jars of pickles. But how and when this single number evolved has long been a mystery. Now, a new study modeling the muscle in fossilized thumbs suggests that about 2 million years ago our ancient ancestors developed a particularly neat appendage while our other close relatives remained… every inch.

This is a “deep and robust analysis,” says Tracy Kivell, a paleoanthropologist at the University of Kent who was not involved in the work. But she and others warn that the research is too preliminary to provide a real smoking gun.

Understanding how the old thumbs worked is not easy. Fossils do not preserve muscle, so most previous attempts to estimate ancient dexterity have relied on the resemblance of the bones of our ancient parents’ hands to ours. Hand bones are also small and relatively rare in the fossil record. But the resemblance can be deceptive: depending on how the muscles are connected, some species with similar bone anatomy can have very different grip strengths, and vice versa.

To analyze ancient thumbs on their own terms, paleoanthropologists at the University of Tübingen scanned the fossil thumb bones of a variety of ancient hominids, a group that includes all species of our own genus, Homo, as well as other closely related species. Researchers examined the bones of two early modern humans and four Neanderthals from the past 100,000 years, as well as the tiny cave dwelling H. naledi (about 250,000 to 300,000 years ago). They also looked at a sister genre for Homo called the Australopithecines, which included Australopithecus afarensis, A. africanus, and A. sediba.

The researchers then used 3D computer software to digitally reconstruct a muscle called adversens pollicis – which allows your thumb to flex inward – on the fossil record – to its attachment site at the base of the palm. They simulated the approximate force the muscle could exert, with more force equating to a better, more precise grip, for example, by holding a needle and thread firmly 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 strength estimates matched the known abilities of both species.

All members of our gender Homo they studied had basically the same thumb grip strength as modern humans, researchers today report in Current biology.

The team also found modern-looking thumb movement in two hominid specimens from the Swartkrans site in South Africa, dated to around 2 million years ago. Because their skeletons are so incomplete, no one has been certain of the species or genus to which they belong. The authors argue that their human-like thumb is good evidence that they can be members of Homo, although they admit that the jury is still out. Either way, the Swartkrans fossils represent the earliest known human inches in the fossil record, say the authors.

But the other human relatives in the study, the Australopithecines, had much smaller inches, according to the team, closer to those of modern chimpanzees. It’s a bit surprising in the case of A. sediba—Which, like the fossils of Swartkrans, dates back about 2 million years. His human proportions have made many believe he possesses human dexterity. “Although Australopithecines, including A. sediba, may have exhibited tool-related behaviors, they had not yet developed a level of efficiency similar to that of humans, ”say the authors Science.

Overall, the work suggests that the modern human thumb appeared around 2 million years ago in the Homo kind, the researchers conclude. This may have allowed ancient humans to become increasingly better at crafting stone tools, ultimately overtaking other hominins.

Evie Vereecke, anthropologist and anatomist at KU Leuven, praises the authors’ approach. But she says the results should be treated with caution. “We know that dexterity is not just due to a muscle.”

Laurent Vigouroux, a biomechanics researcher at Aix-Marseille University who studies the mechanics of the human grip, agrees. He notes that there are more than 10 different muscles that contribute to the movement of the thumb, and it is possible that weaker opponents in some species were outweighed by other muscles or muscles.

Still, Vigouroux and Vereecke say the study’s basic approach is likely to be useful for the field – giving anthropologists some sort of common language for analyzing muscle characteristics in fossils. “Not that long ago, everyone who found fossilized remains had their own interpretation” of long-lost musculature, says Vereecke. “It could get them on the same page.”