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Nature

A Chimpanzee Raised Like A Human Sheds Light On Early Human Mobility

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Stephen Luntz

Freelance Writer

clockMay 11 2020, 20:00 UTC
suzy on stairs

Suzy the chimpanzee was brought up wearing clothes, walking on two feet, and not climbing trees. Out of this cruelty, scientists have learned something important about the shape of our ancestor's feet and hands. Wallace et al./PNAS from Gertrude Lintz's autobiography

If you're unhappy about the conditions in which the big cats were kept in "Tiger King", consider this: In the 1930s, exotic animal collector Gertrude Lintz raised chimpanzees as if they were human. The cruelty of depriving them of the opportunity to do normal chimp things can't be undone, but scientists have learned from what occurred to improve our understanding of one of the developments that shaped humanity.

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Primates that live in trees have curved bones in their hands and feet for better grip. The same features can be seen in fossils of human ancestors. However, scientists have debated whether these curves are genetic or a response to climbing from an early age, which would influence how we interpret the presence of similar features in fossils from our ancestors.

To answer the question, however, we either need to have humans spend their life climbing trees from childhood or deprive members of tree-dwelling species of the chance to climb. Even if there were scientists cruel enough to propose such an experiment today, they'd be unlikely to get it past an ethics committee. However, there is no problem with examining the bones of an animal mistreated before our standards improved.

Dr Ian Wallace of the University of New Mexico learned of a chimpanzee named Suzy who was captured in Africa as an infant and brought to America, where Lintz bought her. Along with more than a dozen other chimpanzees, Lintz attempted to raise Suzy like a human child, keeping her indoors and making her wear clothes, eat with cutlery, and walk only on her hind legs, including up and down stairs. Opportunities to climb were strictly limited.

Having obtained Suzy's bones, Wallace was able to measure their curvature. In Proceedings of the National Academy of Sciences, Wallace points out that chimpanzees less than 5 years old spend around 70 percent of their time in trees. During this development phase, their phalanges (finger and toe bones) become curved to help them grip.

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“Most researchers believe that phalangeal curvature is a phenotypically plastic trait that develops throughout life in response to mechanical loads experienced by hands and feet during arboreal locomotion," Wallace writes in the paper. Nevertheless, Wallace and co-authors find Suzy's phalanges are as curved as wild chimpanzees, unlike our straight ones, suggesting the trait is genetic.

The human voluntary descent from trees would have been far more gradual than Suzy's forced one. Still, the findings show phalangeal curvature could have persisted after our ancestors' climbing reduced. Consequently, to determine the lifestyle of an Australopithecus or early human, looking at bone curvature may not be enough.

The authors acknowledge the dangers of relying on a sample of one but haven't located the bones of any other chimpanzees raised in this way.


Nature

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