Knuckle walking for knuckleheads? Just kidding!
"Humans are the only primates that walk upright all the time, an adaptation that has freed up our hands to more nimbly build tools, lug around food, and carry out other dexterous tasks. Hidden in the eight small bones of the wrist is an anatomical hint to where that gift of grab originated.
Now, the most comprehensive analysis of primate wrist bones to date ... concludes that our wrists more closely resemble those of gorillas and chimpanzees than any other primate group, a similarity the authors link to a possible knuckle-walking past.
Scientists have long looked to wrist anatomy for clues to our evolutionary past, comparing our wrists to those of other living primates such as chimps and gorillas (which knuckle-walk) or capuchins and macaques (which flat-palm walk). Studying fossil hominins’ wrists for signs of these adaptations has proven tricky, as the wrist is a complex puzzle of eight or nine interlocking bones. So, researchers digitally reconstructed and quantified the exteriors of 2037 wrist bones across multiple living and extinct species, including monkeys and apes.
For nearly every bone examined, human wrist bones resembled the equivalents in knuckle-walking African apes far more than those of any other primate group. Human wrists also feature traits that help stabilize other primates’ wrists during knuckle walking—a sign of evolution’s opportunism. Features that once steadied the wrist in our distant ancestors laid the foundation for adaptations that yielded our dexterous wrists. ..."
"... The study also finds that bone structures tied to sophisticated tool use emerged surprisingly late in human evolution, within the past few hundred thousand years. ..."
From the abstract:
"Hominin forelimbs have evolved from primarily locomotive to manipulative appendages over approximately 6 million years. As such, hand functions in fossil hominins and the Pan–Homo last common ancestor (LCA) are intensely debated, with carpal morphology central to this debate. However, owing to their irregular and challenging shapes, few studies have comprehensively quantified carpal morphology.
We analyse the overall carpal morphology of anthropoids, including fossil hominins, using spherical harmonics and use classification methods to characterize fossil hominins within the context of extant taxa.
Results show that hominins share with African apes derived carpal morphology possibly related to knuckle walking. Furthermore, unique modern human carpal morphology appears to have evolved from these possible knuckle-walking features and in a piecemeal manner, causing some hominin capitates to resemble those of palmigrade monkeys. Striking variation in biomechanically relevant carpal morphology and retention of potentially ancestral features persists as late as Homo naledi, suggesting that most hominins probably neither knuckle walked nor extensively used stone tools.
These results indicate that the hominin carpus evolved from an African ape-like wrist, with radial-side reorganization related to manipulation occurring only recently. Although it remains unclear whether the LCA knuckle walked, our results suggest that this is the most likely existing hypothesis."
Figure 2. Scatterplots of PC1 and PC2 for all examined carpals. Lunate and triquetrum are similar in humans and African apes, with fossil hominins largely within the range of these two groups. Capitate, scaphoid, trapezium and trapezoid of humans appear distinct from those of other taxa, with early fossil hominins largely intermediate between humans and African apes. For extant taxa, symbols represent species averages.
No comments:
Post a Comment