Thumb study reveals why one ancient human had the upper hand over another

Humans are a relatively young species. Before modern Homo sapiens took over, Neanderthals (Homo neanderthalensis) roamed the planet.

Scientists have discovered many shared attributes between our species and our ancient peers — a material culture, and similar biological development, for example. We were so similar, in fact, that our species also intermixed, and some populations of modern humans carry genes which appear to have come down from Neanderthals.

But there are differences between modern humans and Neanderthals, and these may be the key to understanding why our species thrived, while theirs did not.

A new study published Thursday in Scientific Reports sheds further light on these ancient humans, revealing a critical distinction to do with how humans and Neanderthals used tools.

Humans Vs. Neanderthals — In the study, scientists used 3D scanning technology to compare the bones of five Neanderthal fossils to those of five early humans, as well as a group of modern-day humans.

Specifically, they wanted to learn how the bones responsible for thumb movement differed between our species and Neanderthals. To do this, they compared different bones from species' joints.

"We specifically looked at the shape covariation between the trapezium — a wrist bone at the base of your thumb — and [the] proximal end of the first metacarpal — the first bone in your thumb that joins the wrist — in Neanderthals in comparison to early and recent humans," Ameline Bardo, a postdoctoral research associate at the University of Kent, tells Inverse.

The researchers placed landmarks on the bones' surfaces, measuring whether a change in one side of the joint — at the base of the thumb — matched the other side. Doing so could reveal "which thumb positions would be favored by different joint shapes," Bardo says.

The comparison revealed a surprising finding — Neanderthals and Homo sapiens held tools very differently.

"We found that the differences in the covariation of shape and orientation of these bones in both species suggest that Neanderthals had different repetitive thumb movements compared with modern humans," Bardo says.

Neanderthals appear to have favored a different thumb position compared to humans, adapting over time to the tools that they used.

"The joint at the base of the thumb of the Neanderthal fossils is flatter with a smaller contact surface between the bones, which is better suited to an extended thumb positioned alongside the side of the hand," Bardo says. "This thumb posture suggests the regular use of power ‘squeeze’ grips, which is the grip we use when we hold tools with handles, like a hammer."

On the other hand, human "joint surfaces are generally larger and more curved," a shape which lends itself well to "gripping objects between the pads of the finger and thumb, known as a precision grip," Bardo explains.

It's not that the Neanderthals couldn't use precision grips, but rather they would have found them difficult. Instead, they adapted better to power grips for handling tools.

The study does not prove humans were superior to Neanderthals in the way they preferred to grip tools. The two species just had different styles.

"Their hand anatomy and the archaeological record makes abundantly clear that Neanderthals were very intelligent, sophisticated tool users, and used many of the same tools that contemporary modern humans did," Bardo says.

In some ways, Bardo says, the findings show another overlooked similarity between humans and our ancient relatives.

"We were surprised that both Neanderthals and recent humans demonstrate high intraspecific variation in shape covariation," Bardo says. "In other words, Neanderthals as a group were quite variable and recent humans as a group were quite variable."

Tool Box — Building upon this study, Bardo and her colleagues want to compare the Neanderthal fossil hands to different tools the ancient species would likely have used. In doing so, scientists can better understand not only Neanderthals, but, also, ourselves.

This comparison "will help us to infer past behavior and better understand the evolution of modern human manipulative abilities," Bardo says.

Unfortunately, our investigations into this ancient human species are limited by the fact there are so few well-preserved specimens to study.

"Without a time machine, we will always struggle to know for certain exactly how our fossil relatives used their hands, used their tools, and how they behaved," Bardo laments.

But advances in 3D technology of the kind used in this work could help fill in some of the blanks about our ancient kin.

"There are a lot of ‘unknowns’ in paleoanthropology," Bardo says. "By using more sophisticated methods — like the 3D shape analysis we used — we hope to get as much information as possible from the fossils we do have."

Abstract: Much research has debated the technological abilities of Neanderthals relative to those of early modern humans, with a particular focus on subtle differences in thumb morphology and how this may reflect differences in manipulative behaviors in these two species. Here, we provide a novel perspective on this debate through a 3D geometric morphometric analysis of shape covariation between the trapezial and proximal first metacarpal articular surfaces of Neanderthals (Homo neanderthalensis) in comparison to early and recent humans (Homo sapiens). Results show a distinct pattern of shape covariation in Neanderthals, consistent with more extended and adducted thumb postures that may reflect habitual use of grips commonly used for hafted tools. Both Neanderthals and recent humans demonstrate high intraspecific variation in shape covariation. This intraspecific variation is likely the result of genetic and/or developmental differences, but may also reflect, in part, differing functional requirements imposed by the use of varied tool‑kits. These results underscore the importance of holistic joint shape analysis for understanding the functional capabilities and evolution of the modern human thumb.