Humans stand alone among primates for their constant upright posture, an adaptation that freed our hands for more dexterous toolmaking, food carrying, and other tasks requiring fine motor control. Hidden within the eight small bones of our wrist lies an anatomical clue to the origin of this grasping gift.
A study published in the journal Proceedings of the Royal Society B presents the most comprehensive analysis to date of primate wrist bones, revealing that our wrists bear a closer resemblance to those of gorillas and chimpanzees than to any other primate group. The authors link this similarity to the possibility of knuckle-walking in our past, a leading hypothesis for how our ancestors moved before adopting bipedalism. The research also found that the bony structures associated with complex tool use emerged surprisingly late in human evolution, within the past few hundred thousand years.
Philip Reno, a developmental biologist at Penn State University who was not involved in the study, calls the scope of the new work “impressive,” noting that “they raise a lot of interesting questions about the mosaic evolution of hominids.”
The new study tackles one of the most challenging questions in human evolution: did our ancestors knuckle-walk before they evolved the ability to stand upright? Scientists turned to wrist anatomy for clues about our evolutionary past, comparing our wrists to those of other living primates with different modes of locomotion, including knuckle-walking (like chimpanzees and gorillas) and palmigrade quadrupedalism (like capuchin monkeys and macaques).
The distinction is important because knuckle-walking and upright walking share a key feature: in both cases, the wrist is not bent backward. Palmigrade walkers, by contrast, extend their wrists with every step—a posture that would necessitate a different skeletal structure. But examining the wrists of fossil hominins for signs of such adaptations has proven challenging. The wrist is a complex joint system involving eight or nine interconnected bones. Until now, most studies have examined only one or two bones at a time.
To overcome this hurdle, paleoanthropologist Lael Gate, and her colleagues at the University of Chicago, employed CT scanning and surface 3D laser scanning to digitally reconstruct the external appearance of 2,037 wrist bones from a variety of living and extinct species, including monkeys and apes. To quantitatively assess the subtle ridges and grooves on each bone that record the mechanical pressures on the wrist during movement, the team then rendered each bone as a high-resolution digital 3D image. This method allowed Gate to cross-reference what she observed visually.
Across nearly all types of bones examined, human wrists bore a much greater resemblance to the corresponding bones of African apes known for knuckle-walking than to the bones of any other primate group. Human wrists also possess features that help stabilize the wrists of other apes when knuckle-walking, including the fusion of the scaphoid and trapezoid bones—two bones on the thumb side.
But humans do not knuckle-walk. “If these traits persisted in our lineage, it certainly wasn’t because we knuckle-walk,” Gate remarks. Instead, evolution repurposed them. Features that once stabilized the wrist for our distant ancestors during locomotion became the structural foundation for adaptations that enabled our wrists to dexterously manipulate objects.
This transformation occurred gradually over millions of years. Stone tools first appear in the fossil record more than 3 million years ago, crafted by a group of ancient human ancestors belonging to the famous hominin genus of Lucy, Australopithecus. By 2 million years ago, early members of our genus, Homo, were using simple stone tools. However, the specific wrist features associated with more complex toolmaking—a suite of changes on the thumb side of the wrist considered characteristic of humans—only became stable in later members of the Homo genus. These features are present in both humans and Neanderthals, suggesting they date back at least to the common ancestor of the two hominin species, more than 550,000 years ago.
The evolutionary conclusions of the study come with a caveat, says Scott Simpson, a paleoanthropologist at Case Western Reserve University. He points to a notable omission from the analysis: Ardipithecus ramidus, an East African hominin that lived 4.4 million years ago, whose hand bones are preserved well enough to reconstruct its locomotion. This hand showed no clear signs of knuckle-walking, and some researchers suggest that bipedalism may have evolved from a more generalized ancestor, bypassing the knuckle-walking phase entirely.
Simpson adds that the similarity between human and ape wrists may simply reflect our close kinship rather than indicating our ancestors once moved in the same way as modern-day African apes. “Showing morphological similarity between humans and our closest relatives is sort of the null hypothesis, what you’d expect,” Reno said. “To really get at whether natural selection was involved, you need differences.”
Nevertheless, Gate and her colleagues maintain that their study provides important data for understanding wrist anatomy over a long span of primate and human evolution. Future studies that link specific wrist movements to tool use, they add, could help clarify when and why each of these adaptations arose. “We became the human lineage,” Gate said, “but understanding where we started from is what speaks to how we got here.”
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