2-Million-Year-Old Fossils Reveal What's Up with Ancient Thumbs

Fossil hands help solve the puzzle of when humans gained manual dexterity.
X-ray of two hands
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Charles Q. Choi, Contributor

(Inside Science) -- Ancient thumbs now suggest human-like manual dexterity may have begun emerging by about 2 million years ago, shedding new light on previous research concerning the rise of advanced tool use, a new study finds. 

The ability to make and use complex tools depends not just on the extraordinarily powerful human brain, but also on the dexterity of the human hand. However, there has been a great deal of conflicting evidence surrounding when complex tools developed, when advanced manual dexterity arose, and in which group it first evolved -- the genus Homo, which is the human lineage, or the genus Australopithecus, which is the most likely direct ancestor of the human lineage.

The earliest known stone tools may date back roughly 3.3 million years, predating Homo by about 500,000 years. Thus, shedding light on the origins of manual dexterity "is crucial for our understanding of human evolution," said study lead author Alexandros Karakostis, a hand biomechanics expert at the Eberhard Karls University of Tübingen in Germany.

Previous research into the evolution of manual dexterity typically just compared fossils with modern human bones. In the new study, Karakostis and his colleagues developed computer simulations of the mechanics of ancient hands that for the first time took into account soft tissue working in concert with bone anatomy.

The scientists first analyzed 3D scans of bones from a number of extinct hominins -- those relatives of modern humans dating from after our ancestors split from those of chimpanzees. They focused on the thumb, the key element behind precise, strong grips.

"If you look at chimpanzee hands, they have really long palms and really, really long fingers, but the thumb seems comically short compared to ours, and weak," said paleoanthropologist Osbjorn Pearson at the University of New Mexico, who did not take part in this work. "At some point in our evolution, the four fingers shrank and so did the palm, but the thumb got really big compared to chimps'."

The researchers next overlaid these bones with virtual muscles, using both humanlike and chimplike models for muscle sizes. Finally, they estimated how capable ancient thumbs were of opposition -- that is, coming in contact with the other fingers and the palm -- from details such as the size and shape of thumb bones versus bones of other fingers, the locations where muscles likely attached, and the range and direction of motions allowed by joints.

In a set of hand bones from Swartkrans cave in South Africa, the scientists found a surprisingly high level of thumb dexterity, similar to that of modern humans. The bones from Swartkrans may have belonged either to early Homo or extinct hominins known as Paranthropus, and they date to roughly 2 million years ago.

These were the oldest bones in the study to show this increased dexterity. They came from a time when important hallmarks in human evolution were gradually developing, "including the emergence of the larger-brained species Homo erectus, dispersal of human species out of Africa, more systematic tool use, and overall increased cultural complexity," Karakostis said.

The dexterity of the Swartkrans cave fossils was shared with all later members of Homo, including Homo naledi, despite the fact that this small-brained, 300,000-year-old species has not yet been associated with any stone tools.

These findings regarding the Homo fossils highlight "the crucial adaptive role that this biomechanical advantage likely played in the course of the biological and cultural evolution of our genus," said study senior author Katerina Harvati, a paleoanthropologist at the Eberhard Karls University of Tübingen in Germany. "This advantage may have played a crucial role in the subsequent gradual development of complex culture that characterized our lineage."

In contrast, Australopithecus thumbs appeared to be consistently less dexterous. This included Australopithecus sediba, which, like the Swartkrans bones, dates to roughly 2 million years ago. These new findings may contradict prior work suggesting A. sediba's thumb proportions reflected humanlike tool use. 

However, it remains an open question whether australopiths might have actually possessed better grips than suggested by this new study. For example, although their thumb bones are relatively slender compared to those of humans, "perhaps their internal bone structure was thicker to help cope with larger forces," said paleoanthropologist Tracy Kivell at the University of Kent in England, who was not involved in this research. "Or did they use different behavioral solutions -- for example, slightly different ways of grasping a tool? There are a lot of exciting questions to still address."

The researchers now plan to investigate how manual dexterity in other groups, such as Neanderthals, might have differed from that of modern humans, "and how that might be reflected in the cultural remains they left behind," Harvati said. "We are also really interested in examining early hominins, such as the Australopithecus species, more closely to try to reconstruct their activity patterns and possible tool-related behaviors."

The scientists detailed their findings online Jan. 28 in the journal Current Biology.

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Charles Q. Choi is a science reporter who has written for Scientific American, The New York Times, Wired, Science, Nature, and National Geographic News, among others.