Written by Michele Wheat
Study of the wrist has uncovered fascinating hints about human evolution. Over time, researchers have guessed that hominids gradually evolved from stances that involved knuckle-walking or swinging from trees to walking on two feet. Studying the entire skeleton provides clues about this evolution, but the wrists give special clues. Wrist bones have changed over time, possibly because of the weight-bearing necessary for knuckle-walking. As hominids evolved to walk upright, this freed their hands for other activities. Scientists have noted skeletal adaptations that seem to correlate with this evolution.
Researchers have been studying the Tiktaalik with special interest. This ancient species could have been as long as 9 feet, and it had gills and primitive lungs. Covered with scales and fins, the Tiktaalik also had large forefins with key skeletal anatomy. The forefins featured shoulders, elbows, and even wrists that worked in a limited fashion to help the creature support itself on land. Parts of the pelvis have also been discovered, which seem to suggest that the Tiktaalik had a ball-and-socket hip joint. This type of joint would likely have made it possible for the creature to move in a variety of ways. Researchers surmise that the Tiktaalik could certainly swim, but it may also have been able to perform some type of weight-bearing gait.
Evolution of Animal Wrist Bones
Animals have what is called a pentadactyl limb. This limb is present in various animals, but it looks and operates differently for different species. For example, the pentadactyl limb in animals related to the horse looked and worked very differently millions of years ago: In ancient times, the pentadactyl limb of horse ancestors had more bones. As time has moved forward, the limb has changed to be one long and thick bone with generous muscle surrounding it. These changes have made it possible for the modern horse to run as it does. Scientists have even discovered startling things about the limbs of the whale and dolphin. Within the front flippers, whales and dolphins have rudimentary bones that resemble the arm, wrist, hand, and fingers. Early ancestors of these animals seem to have had arm buds and leg buds that they may have been able to use for some activities.
Evolution of Dinosaurs to Birds
Researchers have studied dinosaur skeletons extensively. Some dinosaurs had special wrists that suggest their evolution into birds. Wing flapping and folding in birds requires flexing of their wrists, and the way birds can move their wings makes flight possible. However, many dinosaurs did not use these wrist features for flight. Instead, their special wrists gave the dinosaurs flexibility, which could have been helpful for hunting. Other researchers hypothesize that this innate flexibility was present to protect feathers that might have been present on the dinosaurs' skin.
Wrist Bone Evolution in Apes
Scientists have studied the changes in wrist bones through the millennia. Physical characteristics in the wrists would likely have been necessary to enable ancestors to bear weight on their knuckles or even swing through the trees. As hominids switched from these possible methods of locomotion to walking on two feet, wrists would likely have had to adapt to this change. Walking on two feet freed the hands for other uses. Ancestors became able to use their hands for throwing and clubbing. The wrists must move in specific ways to enable these activities. For example, when comparing human capabilities to chimpanzee capabilities, chimpanzees do not have the same capacity for extension of the wrists. This could suggest that changes in the wrist occurred to give humans these capabilities. Major changes also occurred in hominid hand structure, which made it possible for ancestors to begin gripping, grasping, and releasing tools with precision. These changes have had a significant impact on behavior and the success of the species.