The human hand has evolved over millions of years to make the most of tools, while our ape cousins primarily use their hands to move around. Chimpanzees and gorillas, for example, have hands that are perfect for walking on their knuckles. But some of the wrist bones that make apes knuckle-walking experts are shockingly similar to bones in modern humans’ wrists.
A new study published in the Proceedings of the Royal Society B: Biological Sciences has made headway on an age-old question in hominin evolution: did the last common ancestor (LCA) between the genus Pan (chimpanzees and bonobos) and Homo (hominins, including modern humans) walk on its knuckles, or not? Based on bones in the wrists of African apes and humans, we share wrist features with apes that are possibly related to knuckle-walking, and this trait may stem from the common ancestor of both groups.
Trying to Crack a Knuckle-Walking Mystery
One of the biggest questions in the study of ancient hominins is how they moved. Scientists have looked as far back as the last common ancestor (LCA) of Pan and Homo for answers. The two groups split around 7 million years ago, according to Yale University, and in the years that followed, the function of hominins’ hands changed slowly but surely.
In time, hominins’ hands would go from being used for locomotion — climbing trees and swinging from branch to branch — to being used for tool manipulation. African apes, meanwhile, continued to mostly use their hands for knuckle-walking.
At the root of human hand evolution, scientists have wondered how the hand functioned in the enigmatic Pan–Homo LCA, of which we have no direct fossil evidence. Some hold true to the idea that it walked on its knuckles like chimpanzees and gorillas, while others raise an alternative hypothesis that it used its hands for climbing and clambering in trees instead.
The latter hypothesis is based on the skeleton of Ardipithecus ramidus, an early hominin that lived around 4.4 million years ago in the Afar region of present-day Ethiopia; proponents of this hypothesis point to limb and digit similarities between this species and palmigrade monkeys (“palmigrade” meaning they put the entire palm on the ground as they walk, as opposed to walking on knuckles).
Read More: 773,000-Year-Old Fossils Capture a Moment Before Human Lineages Split
It's All in the Wrist
The new study aimed to gain insight into how the Pan-Homo LCA may have evolved by examining the bones of the carpus (wrist) in living primates and 55 fossil hominins. To analyze these bones, researchers used a method called spherical harmonics (SPHARM) to quantify the shape of each carpal bone. They then classified similarities between the bones through machine learning methods.
The researchers ultimately found many similarities between the carpal bones of modern humans and African apes. Of the eight bones in the wrist, the two most similar to each other are the lunate (close to the forearm) and the triquetrum (near the pinky side of the hand).
While some of the other carpal bones (including the trapezoid, capitate, and scaphoid) are more distinct in humans, the researchers write that extant human-African ape differences in carpal morphology are outweighed by their similarities.
Because of those similarities, the researchers propose that the hominin wrist evolved from an African ape-like wrist, suggesting that the LCA may have knuckle-walked.
How Human Hands Evolved
Despite sharing a common ancestor, modern humans clearly don’t walk on their knuckles like African apes. So what happened over those millions of years after Pan and Homo split that made hominin hands ideal for using tools rather than for moving around?
The study suggests that hominins entered a lengthy experimental period during which they didn’t commit the forelimb to either locomotion or manipulation. It likely wasn’t until much later, the researchers said, that the wrist bones in hominins began to slowly reorganize into a structure that supported tool production and use.
The researchers conclude that new fossils from hominins like Homo habilis and Homo erectus — both of which lack complete carpals in their fossil records — will be needed to confirm the true timing of the emergence of humans’ distinct wrists.
Read More: Hominins Moved Into the Neighborhood of the Hobbit Humans Around 1.04 Million Years Ago
Article Sources
Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:
- This article references information from a study published in the Proceedings of the Royal Society B: Biological Sciences: Did modern human carpal morphology evolve from knuckle walking traits?
- This article references information from Yale University: Arched Foot
