How Dexterous Thumbs May Have Helped Shape Evolution Two Million Years Ago

So much of modern-day life revolves around using opposable thumbs, from holding a hammer to build a home to ordering food delivery on our smartphones. But for our ancestors, the uses were much simpler. Strong and nimble thumbs meant that they could better create and wield tools, stones and bones for killing large animals for food. Because developing dexterous, opposable thumbs pushed our ancestors to make and use tools, eat more meat and grow bigger brains, scientists have long wondered if such thumbs began only with our own genus, Homo, or among some earlier species.

Now a new study combines the ancient evidence of fossil fingers and thumbs with cutting-edge computer muscle modeling to conclude that South African hominins boasted flexible, capable thumbs much like ours as far back as two million years ago. “It is remarkable that such a level of thumb dexterity, similar to that of people living today, would be observed in hominins alive two million years ago,” says Katerina Harvati, a paleoanthropologist at the Senckenberg Center for Human Evolution and Paleoenvironment, University of Tübingen (Germany).

That time period is a notable one as it came before major evolutionary events, including the rise of the large-brained Homo erectus 1.9 million years ago, that species’s dispersal outside Africa 1.8 million years ago and the replacement of primitive stone tools by sophisticated Acheulean handaxes about 1.76 million years ago. “We believe that [thumb dexterity] constituted a crucial evolutionary advantage which likely enabled the subsequent gradual development of complex culture in our lineage,” says Harvati, co-author of the new study published in Current Biology.

Shorter thumbs and longer fingers are helpful for climbing. But as our ancestors forsook life in the trees, and increasingly began to make and manipulate objects, shorter fingers and longer opposable thumbs would have produced a hand assembly that got better and better at grasping. Over time natural selection could have refined these anatomical changes based on the many ways humans used their hands and which of those proved most rewarding, like smashing animal bones to collect their high energy marrow.

Tracy Kivell, a paleoanthropologist not involved in the study who specializes in the morphology of primate hands at the University of Kent and Max Planck Institute for Evolutionary Anthropology, notes that many primates with different hands are capable of precise and powerful grips. But humans excel at precision grips that match the pad of the thumb to the pads of the fingers—and for those a powerful thumb is essential. “Experimental studies have shown that humans use forceful precision grips when they make and use stone tools, so it's often thought that this ability in humans evolved in response to tool use,” Kivell says.

Anthropologists have spent a lot to time comparing hand, finger and thumb fragments left behind by species across many branches of the human family tree over millions of years to see where and when this ability developed. Such straightforward comparisons of thumb and finger shapes and their similarity to our own are useful but they don’t tell the whole story, because in nature, different shapes and forms sometimes perform in similar ways.

So Harvati and colleagues put some digital flesh on varied bones. The team compared fossilized hands and thumbs of a suite of species ranging from Australopithecus afarensis, at nearly four million years old, to early Homo sapiens, to Homo naledi to modern chimpanzees and humans. Among those specimens were some intriguing two-million-year-old hand bones from South Africa’s Swartkrans cave, that may be either early Homo or Australopithecus robustus. They then used a biomechanics model to recreate how muscles would have been able to manipulate those various thumbs, providing a look at how they once functioned.

The team first tested the accuracy of the models by using them on living humans and chimps with known muscle parameters. The model’s calculations closely matched experimental study results for those species, which gave the authors confidence that their approach could reliably reconstruct dexterity in the fossilized hands.

Kivell says that musculoskeletal modeling isn’t often used in paleontology simply because muscle data isn’t part of the fossil record. It must come from living species. “They used human and chimpanzee muscle data to offer two potential 'extremes' for what muscle size and force might have been in fossil hominins,” Kivell says. “Although there are a lot of caveats and 'unknowns', I think it's still an informative approach to use.”

The study found that two million years ago the inhabitants of Swartkrans cave in South Africa, had surprisingly high efficiency, much like our own, in the act of bringing the thumb and fingers together. Scientists don’t know which species these hands belong to because Paranthropus robustus and early Homo remains were both found in the cave during this time period.

Very efficient thumb opposition appeared in all members of our own genus Homo that were tested in the study. Those included modern humans but also early Homo sapiens, Neanderthals and even the small-brained species Homo naledi. This last species is intriguing, since Homo naledi hasn’t yet been associated with tools and has a small brain that might or might not have been capable of human like cognitive abilities. “This does not mean that it necessarily made and used tools, but that it would have been able to, from the point of view of its manual capacities,” Harvati says.

On the other hand, the study found that hominids of the genus Australopithecus, which might have been the first hominins known to make very simple tools 3.3 million years ago, scored quite low in thumb dexterity.

Previous studies of hand morphology have suggested that Australopithecus africanus had a human-like ability to grip with an opposable thumb two to three million years ago. The hand of Australopithecus sediba features a long thumb, in relation to the rest of the hand, that other scholars have deemed well-suited for grasping and manipulation of objects use much like our own. While Australopithecus thumbs tend to be long they are also thin, suggesting they would’ve been less powerful than human thumbs.

Harvati’s group believes that a more complete look at the hand’s various bones and joints, along with their muscle reconstruction, also suggest a low range of motion and more limited thumb dexterity. That doesn’t mean that Australopithecus couldn’t make tools. But it suggests that if they did, they may have demonstrated a different kind of hand dexterity that we and our closer relatives in the genus Homo enjoyed. "Australopiths could still be dextrous, capable of tool-making and use, even if their thumb is not as powerful as that of humans,” says Kivell.

Of course, if the Swartkrans hands actually belonged to Australopithecus (Paranthropus) robustus, rather than to an early Homo species, they would suggest that parallel evolution occurred among the two distinct lineages.

Neil Thomas Roach, an evolutionary biologist at Harvard University not involved with the research, says the study’s compelling case that shifts in both hand bones and their associated muscles occurred after the earliest tools appeared isn’t surprising. But it is illustrative. “To me, this reflects one of the most fascinating dilemmas about tool use, namely, who is responsible and how can we know?” he says. “It just might be possible that such questions aren’t [the] right ones and that data like these show that regardless of when the innovation occurred, it was such a fundamental advantage that it became widely adopted across multiple species.”

When the study’s results are considered alongside the archaeological evidence, they support a coherent theory. Some two million years ago hominins began to increasingly rely on tools and feast on animal fats and proteins. This suggests “a shift in hand use at this time that roughly corresponds to [the] disappearance of Australopiths and greater diversity in Homo species,” says Kivell.

The process of making and using increasingly complex tools requires more than dexterous thumbs and hand and eye coordination. Prehistoric toolmakers also must have displayed some levels of forethought, planning and learning along the way. That required better brains, which might have been fed by a meaty diet made possible by better tools, which were crafted by increasingly more capable brains.

“Tool use and cognitive abilities, even possibly including language, are often thought to be part of a complex feedback loop, one reinforcing the other through human evolution,” Harvati says. Exploring which other key evolutionary factors helped to drive the physical and cognitive evolution that led to modern humans is the next step for Harvati and colleagues. “While our study only looked at a small component, the thumb, we plan to expand our work to directly address these questions.”

Brian Handwerk | READ MORE

Brian Handwerk is a science correspondent based in Amherst, New Hampshire.