[LINK] "Humans have more primitive hands than chimpanzees"

[rfmcdonald]

Science does its readers a service in reporting new findings suggesting that chimpanzees cannot be taken to represent the original state of proto-humans, that this species also evolved over time. Michael Balter writes.

Humans and chimps diverged from a common ancestor perhaps about 7 million years ago, and their hands now look very different. We have a relatively long thumb and shorter fingers, which allows us to touch our thumbs to any point along our fingers and thus easily grasp objects. Chimps, on the other hand, have much longer fingers and shorter thumbs, perfect for swinging in trees but much less handy for precision grasping. For decades the dominant view among researchers was that the common ancestor of chimps and humans had chimplike hands, and that the human hand changed in response to the pressures of natural selection to make us better toolmakers.

But recently some researchers have begun to challenge the idea that the human hand fundamentally changed its proportions after the evolutionary split with chimps. The earliest humanmade stone tools are thought to date back 3.3 million years, but new evidence has emerged that some of the earliest members of the human line—such as the 4.4-million-year-old Ardipithecus ramidus (“Ardi”)—had hands that resembled those of modern humans rather than chimps, even though it did not make tools. And back in 2010, a team led by paleoanthropologist Sergio Almécija, now at George Washington University in Washington, D.C., began arguing that even earlier human relatives, dating to 6 million years ago—very soon after the human-chimp evolutionary split—already had humanlike hands as well. This even included the ability to press the thumb against the fingers with considerable force, a key aspect of precision gripping.

To get a grasp on what early hands really looked like, Almécija and his colleagues analyzed the thumb and finger proportions of a large number of living apes and monkeys, including modern humans. They then compared these to the hands of several extinct species of apes and early humans, including Ardi, the Neandertals, and the 2-million-year-old Australopithecus sediba from South Africa, which its discoverers controversially think might be a direct ancestor of humans. The sample also included the 25-million-year-old fossil ape known as Proconsul.

The team crunched the measurements from all these samples using sophisticated statistical methods designed to determine the course of hand evolution over time. The researchers found that the hand of the common ancestor of chimps and humans, and perhaps also earlier ape ancestors, had a relatively long thumb and shorter fingers, similar to that of humans today. (Gorillas, which spend most of their time on the ground and not in trees, have similarly shaped hands.) Thus, the human hand retains these more “primitive” proportions, whereas the elongated fingers and shorter thumbs of chimps, as well as orangutans, represent a more specialized and “derived” form ideal for life in the trees, the team reports today in Nature Communications.