Our ancestors swung from tree to tree

Forum Science Our ancestors swung from tree to tree

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    Our ancestors swung from tree to tree

    More monkey-like than expected: The last common ancestor of humans and chimpanzees probably moved from tree to tree, unlike previously assumed. This is suggested by new analyzes of the hand anatomy of the 4.4 million year old hominid Ardipithecus ramidus. The results challenge previous studies that suggest that “Ardi” already had human-like hands.

    Ardipithecus ramidus is considered to be the oldest known human ancestor. The fossil of a female, which was first described in 2009 and baptized under the name “Ardi” and who lived in Ethiopia 4.4 million years ago – around 1.2 million years before the Australopithecus woman “Lucy”, became famous. This means that “Ardi” is closest to the last common ancestor of humans and chimpanzees.

    The fossil also caused a stir because the researchers assumed that “Ardi”, unlike great apes, did not walk on the knuckles, but on the palms of the hands. According to the studies at the time, the hands were ideally suited for climbing, but less so for shackling and swinging.

    Comparison with other human and ape species

    A team led by Thomas Prang from Texas A&M University is now contradicting this assessment on the basis of new anatomical analyzes. “Our results show that the hand of Ardipithecus ramidus shows clear similarities to great apes despite subtle differences,” the researchers report.

    Typical adaptations for pendulous locomotion include long, crooked finger bones and a small, less dexterous thumb. According to previous studies, these characteristics appeared to be missing in “Ardi”. But Prang and colleagues come to a different conclusion. For their study, they analyzed the finger and metacarpal bones of the famous fossil and compared them with the hand anatomy of other living and extinct human and ape species with the help of statistical software.

    Hands in hand

    The comparative analyzes showed: The length proportions and the shape of the finger and hand bones of Ardipithecus are more similar to those found in monkeys that are hanging around. In addition, the curvature of the bones suggests that “Ardi” walked on the ankles instead of the palms. The feet, on the other hand, were apparently already adapted to moving on the ground.

    “The presence of a hand adapted to the dangling and a terrestrially adapted foot in Ardipithecus ramidus is remarkable because this combination is only observed in ankle-walking great apes living today,” said the researchers. “The knuckle-walking hand position is a compromise that allows large, walking monkeys to spend a significant amount of their time on the floor.”

    “Ardi” therefore moved in a similar way to today’s great apes: They swung from tree to tree, but could also walk on the ground, typically moving on all fours. Her hands were less generalized than previously thought. Larger evolutionary shifts towards hands that are adapted for tool use therefore only took place later.

    Conclusions about the common ancestor

    According to the researchers, the findings on “Ardi” also indicate that the last common ancestor of humans and great apes was much more ape-like than expected. “Taken together, these results falsify the hypothesis that hominins evolved from an ancestor with a generalized hand who had no adaptations to hang,” conclude Prang and colleagues.

    Further research and fossil evidence are required to better understand the developments that eventually led to the human hand. (Science Advances, 2021, doi: 10.1126 / sciadv.abf2474)

    Source: AAAS/ Science Advances

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Forum Science Our ancestors swung from tree to tree