In some ways, human hands may be more primitive than those of chimpanzees, researchers from Stony Brook University report in a new study published Wednesday in the journal Nature.
Study authors Sergio Almecija, Jeroen Smaers, and William Jungers explained in a press release that the proportions of the human hand have changed little from those found in the last common ancestor (LCA) of chimps and people, and that the structure of the modern human hand is largely primitive in nature instead of the result of selective pressure associated with stone tools.
Furthermore, they explain that one of the most distinctive features of the human hand compared to chimpanzees and other apes is a long thumb in relation to the fingers. This trait is often said to be one of the main reasons for the success of the species, but there are competing theories of how the human hand actually evolved over time.
The Stony Brook team measured the hand proportions of modern humans, living and fossil apes, and fossils of human ancestors such as Ardipithecus ramidus, and Australopithecus sediba to find out how the extremity evolved. They found a recent, convergent evolution of finger elongation in chimps and orangutans, and little change between humans, their ancestors, and gorillas.
Cognitive changes, not hand evolution, linked to tool use
“Many scholars assumed that chimps represent the primitive condition from which humans evolved, but this idea has not actually been ever tested,” Almecija explained to redOrbit via email. “Our results show that whereas humans fall within ‘the norm,’ which is also the case in gorillas, chimpanzees and orangutans are the ‘weird’ great ape species.”
“By this I mean that they depart from the rule,” he added. Based on their data, they found that the hands of chimpanzees and orangutans likely evolved as “an adaptation for below-branch arboreal locomotion,” which is “biomechanically more efficient” in creatures that have “hook-like hands with long digits and short thumbs.” Comparatively, human hand proportions have “changed very little (in terms of relative length) since the primitive condition.”
The research by Almecija and his colleagues supports the hypothesis that the long thumb-to-fingers ratio of the human hand evolved at the same time as other more dexterous anthropoids, while also challenging the assumption that a chimp-like hand served as the starting point of the human-chimpanzee LCA.
But that doesn’t mean that human hands have not evolved too. In fact, Almecija told redOrbit that in “human hands have evolved… more than chimps in other aspects (e.g., thumb and joint robusticity), but in terms of digital and thumb lengths, chimpanzees have evolved more.” The findings also indicate that “the most relevant evolutionary change relating to stone tool culture was probably neurological (i.e., enhanced cognition), not hand morphology.”
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