Thumb dexterity comparable to that of individuals today already existed around 2 million years earlier, perhaps in a few of the earliest members of our own genus Homo, a new research study suggests. The finding is the earliest evidence to date of an evolutionary transition to hands with effective grips comparable to those of human toolmakers, who didn’t stand for roughly another 1.7 million years.

Thumbs that made it possible for a forceful grip and enhanced the capability to control things provided ancient Homo or a closely associated hominid line an evolutionary advantage over hominid contemporaries, says a team led by Fotios Alexandros Karakostis and Katerina Harvati. Now-extinct Australopithecus made and used stone tools but did not have humanlike thumb mastery, therefore restricting its toolmaking capability, the paleoanthropologists, from Eberhard Karls University of Tübingen in Germany, found.

The scientists digitally simulated how an essential muscle influenced thumb motion in 12 formerly found fossil hominids, five 19 th century humans and five chimpanzees. Surprisingly, Harvati states, a set of approximately 2-million-year-old thumb fossils from South Africa show agility and power on a par with modern human thumbs.

thumb muscle illustrated
A vital thumb muscle (illustrated) most likely worked with fossil thumb bones to produce humanlike gripping power surprisingly early in the evolution of our genus. F.A. Karakostis et al/ Existing Biology 2021

Researchers disagree about whether the South African discovers come from early Homo or Paranthropus robustus, a types on a dead-end branch of hominid development ( SN: 4/2/20). The thumb mastery in those ancient fossils is similar to that discovered in members of Homo types that appeared after around 335,000 years earlier, the researchers report January 28 in Current Biology

By contrast, they conclude, Homo or P. robustus possessed thumbs that were more powerful than those of three several-million-year-old Australopithecus types, two of which have formerly been proposed to have humanlike hands ( SN: 1/22/15).

Australopithecus would probably be able to carry out most [tool-related] hand movements, however not as effectively as human beings or other Homo species we studied,” Harvati states. The tool-wielding repertoire of Australopithecus types fell better to that of contemporary chimpanzees, who use branches to collect termites and rocks to split nuts, she suggests ( SN: 11/ 6/09).

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Harvati’s team went beyond past efforts that focused only on the size and shape of ancient hominids’ hand bones. Using information from people and chimpanzees on how hand muscles and bones engage while moving, the researchers built a digital, 3-D design to re-create how a crucial thumb muscle– musculus opponens pollicis– connected to a bone at the base of the thumb and operated to bend the digit’s joint toward the palm and fingers.

These new designs of how ancient thumbs worked highlight the sluggishness of hominid hand evolution, states paleoanthropologist Matthew Tocheri of Lakehead University in Thunder Bay, Canada. Australopithecus made and used stone tools as early as around 3.3 million years back ( SN: 5/20/15). “However we don’t see significant modifications to the thumb up until around 2 million years ago, right after which stone artifacts become much more typical across the African landscape,” he states.

Karakostis and Harvati’s 3-D designs of ancient thumb dexterity represent a promising advance, states paleoanthropologist Carol Ward of the University of Missouri in Columbia. Even more work needs to analyze how other thumb muscles communicated with musculus opponens pollicis to affect how that digit worked in different hominid species, she adds.

In an associated finding, Ward and her associates– including Tocheri– reported in 2014 that an approximately 1.42- million-year-old hominid finger fossil from East Africa indicated an early emergence of humanlike control abilities.


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