Skilful thumbs on the trail

Researchers have developed a virtual model to calculate thumb mobility. (Photo: Katerina Harvati, Alexandros Karakostis, Daniel Herdle)

They enable precise gripping and refined dexterity: the thumbs are the key elements of our hands. A study now sheds light on when the particular performance of the thumb developed in human evolution: The development began around two million years ago, emerges from the modeling of the dexterity of different human forms, which is based on the analysis of fossil thumbbones.

Not only the superior mind – the hand also became the secret of success for humans: The highly developed fine motor skills enabled complex skills such as the manufacture of tools and thus opened up many new possibilities for our ancestors. The dexterity of the human hands became the basis of biocultural evolution. The thumb is of particular importance: the fact that this finger can be positioned very efficiently in relation to the others made it possible to use the tweezers precisely and to use tools. So far, however, it is unclear when and with which representative of the human family tree the corresponding adaptations developed.

Fossil thumbs in the sights

Previous attempts to study the evolution of thumb dexterity have relied on comparisons between the skeletal anatomy of modern humans and earlier species of hominins. In the new study, the researchers led by Alexandros Karakostis from the University of Tübingen have now chosen a more comprehensive approach. “Our methodology integrates modern virtual muscle modeling with the three-dimensional analysis of bone shape and size,” explains Karakostis. “This process involves the precise 3D examination of the areas of the bones where the muscles attach in life. For the first time, we were able to take into account the influence of the shape of the thumb bone and the muscle tissue that is no longer preserved in fossils. This made it possible to compare the skill of different fossil human forms, ”says Karakostis.

As part of their study, the researchers first scanned the thumbbones of various representatives of the human family tree. These included species from the genus Homo – early anatomically modern humans, Neanderthals and Homo naledi – as well as representatives of the historically older Australopithecines. Using a biomechanical model, the scientists then reconstructed the respective forces of the thumb muscles in order to deduce the dexterity. In addition, the data was compared with the thumb usage of today’s great apes. “Our approach focuses on the efficiency of the so-called thumb opposition,” says Karakostis.

About two million years ago

The evaluations of the results showed that the particular efficiency and dexterity of the thumb became clear for the first time in the hominins who lived in South Africa about two million years ago. In contrast, it turned out that the australopithecines, considered to be the earliest tool manufacturers, had not yet fully developed this pronounced dexterity – it was comparable to that of today’s great apes. This also included Australopithecus sediba, which existed about two million years ago. This finding is particularly noteworthy because it was previously assumed that A. sediba’s human-like thumb proportions reflected a comparable ability to manufacture tools.

“In contrast, the analyzes of the roughly two-million-year-old hominin hand fossils from the Swartkrans site in South Africa, which are ascribed to early members of the genus Homo, showed a similar dexterity in the use of the thumb as in modern humans,” says Karakostis. The results also show that all examined later species from the genus Homo exhibited similarly high levels of manual dexterity. These findings apply to Neanderthals, the early Homo sapiens and also to the Homo naledi, although this species has not yet been found in connection with stone tools.

“These consistently high levels of skill in different homo species are an indication of the great adaptive value of the thumb for the biocultural evolution of humans,” summarizes senior author Katerina Harvati from the University of Tübingen. The researchers now want to keep their thumbs in their sights: For example, they are planning to take a closer look at the Neanderthals, in order to possibly uncover details of their manual dexterity. Because maybe there were subtle differences to modern humans that could have been linked to the different success of the two species.

Source: Cell Press, University of Tübingen, specialist article: Current Biology, doi: 10.1016 / j.cub.2020.12.041

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