It’s All in the Wrist
By Claire Asher, on 20 December 2013
The evolution of the primate wrist has been dramatic, enabling primates to adapt to a wide variety of lifestyles and walking styles, including tree-swinging, climbing and terrestrial walking both on four legs and two. In hominids, the evolution of the bipedal gait freed up the forelimbs for tool use, and the wrist evolved independently from the feet enabling increasing dexterity that was crucial to human evolution. Recent research in GEE has provided a more thorough analysis of primate wrist evolution, and shed light on a long-standing debate in human evolution: did humans evolve from tree-swingers or knuckle-walkers?
Primates use their limbs to move in a wide variety of different ways, many of which are not seen in other animals alive today, such as vertical clinging, swinging and leaping, and upright walking. Furthermore, within primates, some species have moved towards a more upright stance, freeing the forelimbs for other tasks. This is thought to have been a key aspect of human evolution, increasing our ability to develop and use complex tools, and possibly even playing a role in the evolution of gesture and language. The morphological evolution of primate wrist bones has therefore been of great interest to evolutionary biologists.
Comparative studies looking at humans and other living and extinct apes and monkeys have previously attempted to deduce the early evolution of the human skeleton, in particular how our bipedal stance evolved. However, many of these studies have attempted to determine the rate of evolution (the speed of ticking of the evolutionary clock) using morphological characteristics, which may not provide an accurate view. Recent research by GEE academics, in collaboration with the University of Kent and the Max Planck Institute, has attempted a more rigorous analysis of primate wrist-bone evolution by mapping morphological features onto an independently-generated phylogenetic tree, using molecular methods to estimate the speed of evolution. This method allowed the authors to detect multiple independent appearances of the same feature, as well as more accurately measuring the speed of wrist evolution.
Dr Kivell (University of Kent) and UCL’s Anna Barros and Dr Smaers, compared wrist bone features across 24 living primate species and 16 extinct species. Primate wrists are composed of between 8 and 9 separate bones, and they discovered differing evolutionary patterns for different bones, indicating that each bone evolves at least partly independently from the others. Some of the evolutionary changes that occurred during primate evolution are shared between species which move in similar ways, whilst others are shared between closely related species, regardless of locomotion. Hominids tended to show more morphological variation than monkeys, suggesting stronger selection on the hominid wrist, possible relating to rapid and major changes in body size and locomotion in these species.
This study also sheds light on a long-standing debate over the early evolution of bipedalism in hominids; competing hypotheses have suggested that humans evolved to an upright position from a knuckle-walking stance (e.g. modern Gorillas), or that they evolved from an aboreal, tree-swinging ancestor. The results of this study show adaptations in the hominid wrist bone, which appeared in parallel with Gorillas and Chimpanzees, that are consistent with increased weight being placed on the wrist during knuckle-walking. Thus, it seems more likely that humans evolved from a knuckle-walking, terrestrial ancestor.
Our bipedal stance came with a huge number of skeletal adaptations, and enabled us to adapt to new environments. It may also have been crucial in freeing up our hands for other tasks, which in turn played a role in our intellectual development. Understanding how our skeleton, particularly our hands and feet, evolved through the primate lineage therefore sheds light on some of the deepest aspects of humanity. The bones in the primate wrist have evolved at least partly independently from each other, and this has generated a large variety of wrist morphologies, adapting different primate species to different modes of locomotion. Early hominids likely evolved their bipedal stance from a knuckle-walking ancestor, rather than an arboreal tree-swinger.
This research was made possible by funding from the Natural Environment Research Council (NERC), the Natural Sciences and Engineering Research Council of Canada (NSERC), the Fundação para a Ciência e a Tecnologia , the Max Planck Society , and a General Motors Women in Science and Mathematics Award