Researchers have used 3D modeling software to reconstruct a critical thumb muscle, the opponents pollicis (pictured here), in the fossils of ancient hominins.
Katerina Harvati; Alexandros Karakostis; Daniel Haeufle
By Michael Price
The human thumb is a quick wonder, allowing us to make tools, sew clothes, and open picket bottles. But how and when this unique figure developed has long been a mystery. Now, a new study modeling the muscles in petrified thumbs indicates about 2 million years ago, our ancient ancestors developed a unique dexterous appendage while our other relatives remained … all thumbs up.
This is a ‘thorough, 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 offer a real smoking weapon.
It is not easy to find out how ancient thumbs worked. Fossils do not preserve muscle, and most previous attempts to estimate ancient dexterity depended on how close our ancient family members’ hand bones looked to our own. Hand bones are also small and relatively rare in the fossil record. But similarity can be misleading: depending on how the muscles are connected, some types 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 digitized the fossil thumbs 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 the bones of two early modern humans and four Neanderthals from the past 100,000 years, and the small, cave-dwelling H. naledi (from about 250,000 to 300,000 years ago). They also looked at a sister species Homo called the Australopithecines, which includes Australopithecus afarensis, A. africanus, en A. sediba.
The researchers then used 3D computer software to digitally reconstruct a muscle known as the opposites pollicis – which causes your thumb to bend inward – to the attachment site at the bottom of the palm. They simulate the approximate force that the muscle can exert, with more force comparing to a better, more precise grip, for example when holding 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 the power estimates of their model matched the known capabilities of the two species.
All the members of our generation Homo their investigation has basically the same grip strength as modern humans, the researchers report today Current biology.
The team also found thumb movements that look modern in two hominin samples from the Swartkrans website in South Africa, which were dated about 2 million years ago. Because their skeletons are so incomplete, no one was sure what species or species they belonged to. According to the authors, their human thumb is a good proof that they can be members Homo, although they acknowledge that the jury is not yet available. Either way, the Swartkrans fossils are the earliest human thumbs up in the fossil record, the authors say.
But the other family members in the study, the Australopithecines, had much weaker thumbs, the team found closer to that of modern chimpanzees. This is a bit surprising in the case of A. sediba—Which, like the Swartkrans fossils, date to about 2 million years ago. The human hand relations have led many to believe that it may have human dexterity. Although Australopithecines, including A. sediba, which may have exhibited instrument-related behavior, they have not yet developed a human level of efficiency, ‘the authors say Science.
In general, the work indicates that the modern human thumb originated in the United States about 2 million years ago Homo genus, the researchers conclude. This may have made ancient people better and better at making stone tools and eventually surpassing the other hominins.
Evie Vereecke, an anthropologist and anatomist at the KU Leuven, praises the authors’ approach. But she says the findings need to be handled with caution. “We know that agility is not just about one muscle.”
Laurent Vigouroux, a biomechanics researcher at Aix-Marseille University studying the mechanics of the human grip, agrees. He notes that there are more than ten different muscles that contribute to thumb movement, and it is possible that weaker opponents pollicis in some species have been compensated by another muscle or muscles.
Yet both Vigouroux and Vereecke say the basic approach of the study is likely to be useful to the field – giving anthropologists a kind of general language to analyze the muscle characteristics of fossils. “Not long ago, everyone who found fossilized remains had their own interpretation” of the muscle that is long gone, Vereecke says. “It might help to get them on the same page.”