Thumb - Evolution

Evolution

A primitive autonomization of the first carpometacarpal joint (CMC) may have occurred in dinosaurs. A real differentiation appeared perhaps 70 mya in early primates, while the shape of the human thumb CMC finally appears about 5 mya. The result of this evolutionary process is a human CMC joint positioned at 80° of pronation, 40 of abduction, and 50° of flexion in relation to an axis passing through the second and third CMC joints.

Opposable thumbs are shared by many primates, including most simians, and some prosimians. The climbing and suspensory behaviour in orthograde apes, such as chimpanzees, has resulted in elongated hands while the thumb has remained short. As a result, these primates are unable to perform the pad-to-pad grip associated with opposability. However, in pronograde monkeys such as baboons, an adaptation to a terrestrial lifestyle has led to reduced digit length and thus hand proportions similar to those of humans. Consequently, these primates have dexterous hands and are able to grasp objects using a pad-to-pad grip. It can thus be difficult to identify hand adaptations to manipulation-related tasks based solely on thumb proportions.

The evolution of the fully opposable thumb is usually associated with Homo habilis, a forerunner of Homo sapiens. This, however, is the suggested result of evolution from Homo erectus (around 1 mya) via a series of intermediate anthropoid stages, and is therefore a much more complicated link.

It is possible, though, that a more likely scenario may be that the specialized precision gripping hand (equipped with opposable thumb) of Homo habilis preceded walking, with the specialized adaptation of the spine, pelvis, and lower extremities preceding a more advanced hand. And, it is logical that a conservative, highly functional adaptation be followed by a series of more complex ones that complement it. With Homo habilis, an advanced grasping-capable hand was accompanied by facultative bipedalism, possibly implying, assuming a co-opted evolutionary relationship exists, that the latter resulted from the former as obligate bipedalism was yet to follow. Walking may have been a by-product of busy hands and not vice versa.

HACNS1 (also known as Human Accelerated Region 2) is a gene enhancer "that may have contributed to the evolution of the uniquely opposable human thumb, and possibly also modifications in the ankle or foot that allow humans to walk on two legs". Evidence to date shows that of the 110,000 gene enhancer sequences identified in the human genome, HACNS1 has undergone the most change during the human evolution since the chimpanzee-human last common ancestor.