A new study indicates that the ability of the thumb similar to that of people today already existed about 2 million years ago, possibly in some of the first members of our genus Homo. The finding is the oldest evidence to date of an evolutionary transition to hands with powerful handles comparable to those of human toolmakers, which did not appear for more or less 1.7 million years.
Thumbs that allowed for a strong grip and improved ability to manipulate objects gave ancient Homo an evolutionary advantage or a hominid line closely related to hominid contemporaries, says a team led by Fotios Alexandros Karakostis and Katerina Harvati. The now extinct australopithecines made and used stone tools but lacked human skill like the thumb, thus limiting their tool-making ability, paleoanthropologists at Eberhard Karls University in Tübingen found in Germany.
The researchers digitally simulated how a key muscle influenced the movement of the thumb in 12 fossil hominids found previously, five 19th-century humans and five chimpanzees. Surprisingly, says Harvati, a pair of thumb fossils from about 2 million years ago in South Africa show agility and power just like modern human thumbs.
F.A. Karakostis et al / Current Biology 2021
Scientists disagree on whether the South African findings come from Homo or Paranthropus robustus, a species in a dead end of hominid evolution (SN: 4/2/20). But the dexterity of the thumb in those ancient fossils is comparable to that found in members of the Homo species that appeared after about 335,000 years ago, according to researchers Jan. 28 in Current Biology. This includes Neanderthals from Europe and the Middle East and a South African hominid named Homo naledi, which possessed an unusual mixture of skeletal traits (SN: 5/9/17).
In comparison, they conclude, Homo or P. robustus possessed the thumbs stronger than those of three species of Australopithecus several million years old, of which it was previously proposed that they have human hands (SN: 22/01/15).
“Australopithecus would probably be able to perform most hand movements (tool-related), but not as efficiently as the humans or other homo species we studied,” says Harvati. It suggests (SN: 11/6/09) that Australopithecus species tool repertoire has fallen closer to that of modern chimpanzees, which use branches to collect termites and rocks to crack nuts.
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Harvati’s team went beyond past efforts that focused only on the size and shape of the bones in the hands of ancient hominids. Using data from humans and chimpanzees on how the muscles and bones of the hands interact as they move, the researchers built a 3D digital model to recreate as a key muscle of the thumb – musculus oposens pollicis – attached to a bone at the base of the thumb and operated to bend the joint of the digit towards the palm and fingers.
These new patterns of functioning of the old thumbs underscore the slow evolution of hominid hands, says paleoanthropologist Matthew Tocheri of Lakehead University in Thunder Bay, Canada. Australopithecus manufactured and used stone tools some 3.3 million years ago (SN: 20/05/15). “But we didn’t see major changes in the thumb until about 2 million years ago, shortly after which stone artifacts became much more common in the African landscape,” he says.
Karakostis and Harvati’s old-fashioned 3-D thumb models represent a promising breakthrough, says paleoanthropologist Carol Ward of the University of Missouri at Columbia. But more work needs to be done to examine how other thumb muscles interacted with the musculus oposens pollicis to influence the functioning of that digit in different hominid species, he adds.
In a related discovery, Ward and colleagues – including Tocheri – reported in 2014 that an approximately 1.42 million-year-old hominid finger fossil from East Africa pointed to an early emergence of human manipulation skills.