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Are human hands really more primitive than chimp hands?

Human hands have changed very little over the past 6 million years, says a new study, while chimps and orangutan hand structures have evolved notably.

An adult male chimpanzee demonstrates bipedal transport of nuts and stone tools at Bossou's outdoor laboratory. He first gathers up a whole pile of 20 Coula edulis nuts, using both hands and mouth, then walks bipedally to a set of stone tools provided, where he selects a stone anvil and a stone hammer.. He then continues to walk bipedally, moving to different spot, where he stops to process the nuts and to consume their kernels. During the final stage of transport, he uses three limbs and his mouth to carry the nuts and stone tools.

When humans think about primate evolution, we tend to picture ourselves at the pinnacle. But could we be overplaying our hands?

New research suggests that human hands may actually be more primitive than the hands of other dexterous primates, like chimpanzees. The study was led by Sergio Almécija, a paleoanthropologist at Stony Brook University, and published Tuesday in Nature Communications.

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Paleoanthropological studies tend to lean on the notion that human ancestors were originally monkey-like, slowly losing those traits through evolutionary time. But in some ways, that may not be entirely true.

"Contrarily to most studies in the field of human evolution, we did not assume that the last common ancestor of humans and chimpanzees was like a chimpanzee," Dr. Almécija says in a phone interview. "Instead, we tested that assumption by incorporating actual morphological and phylogenetic information in a large sample of primate species."

Almécija and his colleagues found that the hands of our distant ancestors were actually very similar to our own. By contrast, chimpanzee hands have changed significantly "since their last common ancestor, around 6 million years ago," Almécija says. Most strikingly, their fingers have become much longer. They share hand structure with orangutans, who probably evolved them independently.

"Digital lengthening in chimpanzees and orangutans probably relates to specialized, below-branch locomotion in large bodied apes," says Almécija.

"Humans have just subtly changed our hand proportions – a little bit of thumb lengthening, a little bit of digital reduction – to improve our 'precision grips.' ”

Almécija says that this new finding could shift how paleoanthropologists approach human evolution.

"The bigger implication of our study is that any evolutionary model of human hand evolution assuming a chimpanzee-like ancestor will likely be flawed from the beginning," Almécija says.

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"Our results indicate that our overall hand proportions haven’t changed that much – they have been inherited from a last common ancestor that was, in this respect, more similar to a human than to a chimpanzee," Almécija adds. "Thus, when the humans first started to produce stone tools systematically, their hands were pretty much like ours today."