The ancestry of our opposable thumbs

Since the appearance of smart phones and the explosion of social media our thumbs have found a new niche; typing while holding a mobile. At a desktop keyboard, most of us don’t use thumbs very much, unless we have mastered fast touch typing, but for a huge variety of manual tasks thumbs are essential. The first makers of sophisticated stone tools must have been able to grip between fingers and thumb to manipulate the materials from which they were made and to perform the various stages in creating a razor sharp edge. To do that, as most of us are aware, the tip of the thumb must be capable of touching the tips of all four fingers; an opposable thumb is essential for the ‘precision grip’. Being able to tell when opposable thumbs evolve depends, of course, on finding hand-bone fossils. Being made of many bones disarticulated hands are a lot more fragile than long bones or those of the skull. Complete fossil hands are rare, as are feet, but a number have been found more or less complete. Whichever hominin had evolved opposable thumbs, their potential would have given them a considerable advantage over those that hadn’t.

The main muscles that control the movements of modern human fingers and thumb (Credit: Wikipedia)

Simply comparing the shapes of fossilised bones of fingers and thumbs with those of modern humans and other living primates has, so far, not proved capable of resolving with certainty which hominin groups either did or did not have opposable thumbs. The key lies in the muscles that operate them. It has become commonplace to reconstruct faces and even whole bodies from fairly complete skeletal remains by modelling musculature from the positioning and shape of the points of attachment of muscles to bone. But that become increasingly difficult for the small-scale and intricate attachments in hands. The critical muscle for opposable thumbs is known as the Opponens pollicis (the Latin for thumb is Digitus pollex); a small triangular muscle that operates in conjunction with three others (with pollicis in their Latin names).

Fotios Karakostis and six colleagues from German, Swiss and Greek universities have devised software that can model muscles in 3-D (F.A. Karakostis et al. 2021. Biomechanics of the human thumb and the evolution of dexterityCurrent Biology, v.31,  online; DOI: 10.1016/j.cub.2020.12.041). Based on the anatomy of human and chimpanzee hand muscles and the positions of their attachment to individual bones, they have been able to establish a series of parameters that clearly distinguish the morphological and probably functional characteristics of the thumbs of these living primates. Complete sets of thumb bones from four Neanderthal skeletons show that they were significantly, but only slightly, different from anatomically modern humans. Those from three species of Australopithecus (africanus, sediba and afarensis) lie between ours and chimps’, with significantly closer affinity to chimpanzees. It seems that australopithecines of whatever age were not equipped with opposable thumbs and were possible tool producers and users with the very limited capabilities of modern chimps; holding, pounding and poking. A single set of hominin thumb bones from about two million years ago that were found in the famous Swartkrans Cave in South Africa show just as close affinity in thumb opposability to humans as do Neanderthals. So at 2 Ma there was a hominin species sufficiently dextrous to make and use sophisticated tools. The problem is, the bones are not directly associated with others and have been ascribed by different authors either to H. habilis or Paranthropus robustus. Interestingly, this paranthropoid has also been suggested (controversially) to have been the first known hominin to use fire, and it also used digging sticks. No one has ever suggested that the genus Homo descended from a paranthropoid ancestor or vice versa; these massively jawed beings did coexist with early humans in East Africa for over a million years. The other hominin who left hands in the geological record was Homo naledi; a controversial species because it was found in a barely accessible cave chamber, and took a while to date. This context gave rise to the notions that it was the direct ancestor of humans and that it buried its dead in a special place. However, it turned out to be relative recent, at about 280 ka (see: Homo naledi: an anti-climax; May 2017). Homo naledi does seem to have had opposable thumbs, but there is no associated evidence to suggest either tool making or use.

Fascinating as the methodology outlined by Karakostis et al. is, their findings do not take early human capabilities very much further than what is already known. Tools were made and used as far back as 3.3 Ma ago, and we know that H. habilis was doing this by about 2.6 Ma; i.e. long before the first evidence for opposable thumbs, and who had them first is uncertain. What is clear is that sophisticated tools, such as the bifacial Acheulian artifacts whose manufacture demands great dexterity, only appeared after the potential for nimble dexterity (about 1.8 Ma). The same goes for the first migration out of Africa, at about the same time, which demanded resourcefulness that may have sprung from the ability to manipulate natural materials effectively and carefully

See also: Handwerk B. 2012. How dexterous thumbs may have helped shape evolution two million years ago. (Smithsonian Magazine, 28 January 2021); Bower, B. 2021. Humanlike thumb dexterity may date back as far as 2 million years ago. (Science News, 28 January 2021)