Human hands more primitive than chimp hands

Human hands more primitive than chimp hands
Wednesday, 15 July 2015 Jennifer Viegas
Discovery News

The development of an opposable thumb that enables humans to grip and manipulate objects is widely believed to give us an evolutionary edge.

But new research finds that human hands are more primitive than those of our closest primate ancestors chimpanzees.

The study, published in the the journal Nature Communications, found that human hand proportions have changed little from those of the last common ancestor of chimps and humans, while the hands of chimps and orangutans have evolved quite a bit.

"The findings suggest that the structure of the modern human hand is largely primitive in nature, rather than, as some believe, the result of more recent changes necessary for stone tool-making," says Kurtis Hiatt, a spokesperson for The George Washington University.

The researchers, led by Sergio Almécija, a scientist in the university's Center for the Advanced Study of Human Paleobiology, analysed the hands of humans, chimps and orangutans, as well as the remains of hands for early apes like Proconsul heseloni and the hands of human ancestors, such as Ardipithecus ramidus and Australopithecus sediba.

Almécija and colleagues, Jeroen Smaers and William Jungers from Stony Brook University, discovered that human hands today are not that different from those of the early human ancestors.

"Human hands are marked by a relatively long thumb when compared to the length of their four other fingers — a trait that is often cited as one of the reasons for the success of our species because it facilitates a 'pad-to-pad precision grip,'" says Hiatt.

Conversely, chimp hands are much longer and narrower. Since the thumb is not as long, it just meets up with the palm, while the chimp's other four fingers extend upward. As a result, chimps and orangutans do not have opposable thumbs as we do.

Gorillas also appear to have inherited our more primitive hand structure. Like human hands, gorilla hands have five fingers, including an opposable thumb. Gorilla feet are similar to ours too. Each gorilla foot has five toes, but their big toe is opposable and can move much more flexibly than ours can.

Almécija and his colleagues suspect that all living primates survived a late Miocene (12 to 5 million years ago) extinction event by specialising to exist in certain habitats. While chimps and orangutans became tree-climbing specialists, humans evolved to become more terrestrial.

Gorillas did too.

While we tend to think that gorillas spend much of their time hanging around in trees, the truth is that they only spend about 5 to 20 percent of their time in trees. Even then, the tree scaling is just to escape threats or to forage for food.

The new study challenges the assumption that the evolution of a more "sophisticated" hand in humans first appeared in the common ancestor of chimps and our species.

Our hands, however useful, may instead represent a very primitive anatomical structure that's been around for millions of years.
hands
Friday, 23 January 2015 Jennifer Viegas
Discovery News

The researchers analysed the internal structure of hand bones from a range of species and found that the hands of A. aftricanus resemble those of modern humans (TL Kivell)
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Some of the earliest, and still very ape-looking members of our family tree were the first to process foods, make handcrafted stone tools, and do relatively sophisticated tasks, suggests a new study.

The study, published in the latest issue of the journal Science challenges the long-standing assumption that Homo habilis, aka. 'Handy Man', was the first crafter of stone tools around 2.4 million years ago.

"Instead, I think our findings show that the traditional view that stone tool use was something that only members of our own genus Homo were capable of is outdated," says senior author Dr Tracy Kivell of the University of Kent .

Stone tool usage "goes back much earlier -- long before the appearance of Homo -- than we originally thought," she adds.

Kivell, lead author Dr Matthew Skinner, and their colleagues came to that conclusion after analysing the internal bone structure from Australopithecus hands from the Pliocene Epoch, approximately 5.3 - 2.6 million years ago.

Australopithecus individuals had a mixture of ape and human features. They sported foot and leg bones suited for walking upright, but had long arms appropriate for tree climbing.

The researchers found that Australopithecus africanus had human-like hands that were capable of precision grips, such as squeezing small objects. These early members of the human family tree also had an opposable thumb. The hand features likely evolved from even earlier human ancestors that possessed long fingers and short thumbs, which facilitate manoeuvring in trees.

Prior studies have found that both making and utilising stone tools by hand requires forceful, precision-pinch grips. Since A. africanus could use its hands in such a way, logic holds that they were indeed making stone tools, but where are the tools?

That is now a big mystery, because the earliest known stone tools date to after A. africanus lived.

First stone tools
"The first recognisable stone tools consist of stone pebbles and simple flakes and date to about 2.5 million years ago from Ethiopia," says Skinner, who is a paleoanthropologist at the University of Kent and the Max Planck Institute for Evolutionary Anthropology.

"This is also when we start to find animal bones with cut-marks made from stone tools."

"However," he adds, "there is some evidence for these type of cut-marks at 3.4 million years ago, a time period only associated with Australopiths."

There are challenges to finding very early stone tools, he explains, because "they are likely very simple flakes that were originally scattered across the landscape."

Dr Matthew Tocheri, who is Canada Research Chair in Human Origins at Lakehead University, says the new study makes a convincing case that "Australopiths were not only capable of using their hands in more human-like ways than living great apes, but also that they actually used their hands in more human-like ways. That's why the bone (in A. africanus) has remodeled in response to that more human-like hand use."

He suspects that the lack of stone tools in the archaeological record prior to 2.5 million years ago suggests that the tools were not as adaptively important to A. africanus as they were to later humans.

Dr Brian Richmond, a curator in the Division of Anthropology at the American Museum of Natural History says "With this study, we finally have evidence of what we long suspected--Australopiths used their human-like hand proportions to handle objects in human-like ways."

He says that the study raises many new questions, and not just about the missing tools. For example, he wonders if all Australopiths had human-like hands, or if it was just A. africanus. Also, he is curious if the bones provide evidence for frequent, or infrequent but high-intensity, hand activity.

The answers to these and other related questions could help to resolve when and how the first stone tools were made, who exactly made them, and what they might have been used for millions of years ago. They could also shed direct light on the evolution of our species, which has relied so much on tools -- from prehistoric stone hammers to present day high tech computers -- since the earliest days of our existence.