The Origin and Evolution of Human Language
Prof. Suzanne Kemmer
The evolution of the opposable or prehensile thumb is usually associated with Homo habilis, the 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.
The most important factors leading to the habile hand (and its thumb) are:
[SK comment: the above is poorly written. I believe the writer means that apes, unlike us, still crucially depend on their hands (specifically knuckles) for walking; and the freeing of the hands was one of the consequences of the adoption by pithecanthropoid and anthropoid primates of erect bipedal walking. This statement says that upright bipedal walking in hominoids allowed the hands to be freed for other things such as tool/weapon use.]
[The author of the above paragraph suggests that an advanced hand structure with an opposable thumb could have preceded bipedal walking in the hominid line. Although this hypothesis seems to be treated as an opposing one to the hypothesis above, I don't see any contradiction. It is entirely possible that first came the fancy hand, then upright bipedal walking; then the freed up hands could be used for holding, carrying, and throwing. Maybe the hands did some further evolving after bipedal gait, too. S.K. ]
The thumb, unlike other fingers, is opposable, in that it is the only digit on the human hand which is able to oppose or turn back against the other four fingers, and thus enables the hand to refine its grip to hold objects which it would be unable to do otherwise. The opposable thumb has helped the human species develop more accurate fine motor skills. It is also thought to have directly led to the development of tools, not just in humans or their evolutionary ancestors, but other primates as well. The thumb, in conjunction with the other fingers make humans and other species with similar hands some of the most dexterous in the world.
[S.K. comment: I said in class that only humans had opposable thumbs, because many works on hominids speak of the opposable thumb as an exclusive innovation of the hominid line. (For example see Footnote 3 below.) However, I see now that some qualify the statement and say humans are the only creatures with a fully opposable thumb. How exactly you can have a semi-opposable thumb is not clear to me. But the animals below do have something functionally and structurally analogous to our thumbs, either on hands or feet or both. I do not know what exact structural and functional differences there are between humans' "fully opposable" thumb and the thumb-type digits of other primates. The pictures of chimp and gorilla hands and feet that I put in the Resources page on Owlspace (folder: Great apes) are useful for seeing the similarities, and perhaps the gross shape differences in hands and feet between the pongidae and to genus Homo, but the pictures alone do not allow us to derive a full understanding of what are the really important structural and functional innovations in our hands.]
Many animals, primates and others, also have some kind of opposable thumb or toe:
* Bornean Orangutan - opposable thumbs on all four hands. The
interdigital grip gives them the ability to pick fruit.
* Gorillas - opposable on all four hands.
* Chimpanzees have opposable thumbs on all four hands.
* Lesser Apes have opposable thumbs on all four hands.
* Old World Monkeys, with some exceptions, such as the genera, Piliocolobus and Colobus.
* Cebids (New World primates of Central and South America) - some have opposable thumbs
* Koala - opposable toe on each foot, plus two opposable digits on each hand
* Opossum - opposable thumb on rear feet
* Giant Panda - Panda paws have five clawed fingers plus an extra bone that works like an opposable thumb. This "thumb" is not really a finger (like the human thumb is), but an extra-long sesamoid bone that works like a thumb.
* Troodon - a birdlike dinosaur with partially opposable thumbs.
* Raccoon - a common mammal with thumbs, which are not opposables.
Footnotes (I am still linking these --S.K.)