New study reinforces the idea that humans evolved from ancestors who walked on their knuckles. The analysis traces the evolution of articulation that made our species so dexterous.
Humans are the only primates that walk always in an upright positionan adaptation that freed our hands to build tools with more dexterity, transport food and perform other tasks that require manual skill.
Hidden in the eight small bones of the wrist is a anatomical clue about the origin of this gift of grasping.
In a recently published Proceedings of the Royal Society Bthe most comprehensive analysis of primate wrist bones to date concludes that our wrists more similar to gorillas and chimpanzees than any other group of primates.
This is a similarity that the authors associate with a possible past of locomotion supported on the knucklesthe main hypothesis about how our ancestors moved before they walked upright on two legs.
The study also concludes that the bone structures linked to the sophisticated use of tools emerged surprisingly late in human evolutionin the last hundreds of thousands of years.
Philip Renoa developmental biologist at the Philadelphia College of Osteopathic Medicine who was not involved in the study, calls the scope of the new work “impressive,” noting that it “raises many kinds of interesting questions related to the mosaic of hominin evolution.”
The new research addresses one of the most persistent debates in human evolution: whether our ancestors walked on their knuckles before developing the ability to stand upright, explains .
Scientists have searched the wrist anatomy clues about our evolutionary past, comparing our wrists with those of other current primates that display different styles of locomotion, including knuckle walking, what do chimpanzees and gorillas do?and with the palm settled on the ground, as capuchins and rhesus monkeys do.
The distinction is importantbecause walking supported on the knuckles and walking in an upright position share a fundamental characteristic: in both, the pulse is not bent back.
Animals that walk with their palm resting on the ground, on the contrary, carry their wrist in extension with each stepa stance that would have required a very different skeletal architecture.
But studying the pulses of fossil hominins for signs of these types of adaptations has proven complicated. The wrist is a complex joint systemwith eight or nine interconnected bones. Until now, most studies looked at just one or two bones at a time.
To address this problem, a paleoanthropologist Laura Hunterfrom the University of Chicago, and his colleagues used CT and 3D laser surface scans to digitally reconstruct the exterior of 2,037 wrist bones from several living and extinct species, including monkeys and apes.
To quantify the ssubtle grooves and ridges of each bonewhich record the mechanical pressures exerted on the wrist during movement, the team then represented each bone as a three-dimensional digital figure high resolution. The method allowed Hunter to confirm what he had observed with the naked eye.
In almost all of the bones analyzed, the human wrist bones resembled the equivalents of African apes who walk on their knuckles much more than those of any other group of primates.
Human wrists also have features that help stabilize the wrists of other primates during knuckle walking, including knuckle fusion. scaphoid and the centraltwo bones on the side of the thumb.
But humans don’t walk on their knuckles. “If these characteristics have remained in our lineage, it is certainly not because we walk on our knuckles”, observes Hunter.
Instead, evolution repurposed them. Features that once stabilized the pulse of our distant ancestors during locomotion became the structural basis for adaptations that allowed our wrists to manipulate objects with dexterity.
This transformation occurred gradually over millions of years. Stone tools first appear in the fossil record more than 3 million years ago, made by a group of ancient human ancestors belonging to the genus of the famous Lucy, o Australopithecus — which, according to a study last year, .
2 million years agothe first members of our genus, Homo, were already chipping simple stone tools.
However, the specific characteristics of the pulse associated with more sophisticated tool making, a set of thumb side changes, identified as typically human, they only became constant in later members of the genus Homo.
These characteristics are present in both humans and Neanderthals, which suggests that they date back at least to the common ancestors of the two hominin species, more than 550,000 years ago.
Hunter and his colleagues say their study provides essential data for understand the anatomy of the wrist throughout the extensive chronology of primate and human evolution. “We have become the human lineage,” says Hunter, “but understanding the starting point This is what tells us how we got here“.
