Human Thumbs underwent a major upgrade 2 million years ago, sparking cultural revolution, study finds

The human thumb is a marvel of evolution, allowing our ancestors to make stone tools and radically expand their food choices. New research suggests that our nimble, dexterous thumbs appeared 2 million years ago, in a development that irrevocably changed the course of human history.

Many primates have opposable thumbs, but none look like ours. The human thumb, set in opposition to other fingers, allows for precision grasping, which anthropologists consider a physical attribute necessary for making tools.

Scientists are naturally interested in knowing when this extra dexterity evolved. and whether it coincided with the emergence of the production of stone tools and other cultural innovations.

Katerina Harvati, lead author of the new study and paleoanthropologist at the University of Tübingen, says most studies of the history of hominid dexterity rely on a direct comparison between the modern human hand and the hands of early hominids. . New research challenges this methodological approach and instead rates each hand on its own merits. It is possible, hypothetically speaking, that an earlier version of the hominin hand is superior in terms of thumb dexterity.

As a reminder, our species, homo sapiens, appeared about 300,000 years ago, which means we were behind in the human spectacle. Other humans (now extinct), such as Homo, Man standing, man Naledi, and Homo neanderthalensis (otherwise known as the Neanderthal) existed much earlier, with the very first humans appearing around 2.8 million years ago and maybe even earlier.

Key to the new study, published today in Current Biology, is an anatomical concept called “opposition of the thumb.” It is “the act of putting the thumb in contact with the fingers,” Harvati explained in an email. This efficiency, she said, is “greatly improved in humans” compared to other primates like chimpanzees (who also have opposable thumbs) and is a “central component of human-like manual dexterity”.

Going into the new study, Harvati and his colleagues wanted to know if this increased effectiveness of thumb opposition could be detected in early hominid fossils, and if so, which ones. Given that some of the oldest stone tools in archaeological records date back over 3 million years, it seemed possible that another hominin genus, namely Austraopithecus, also had human-like thumb dexterity. That the appearance of the dexterous thumb could somehow relate to the chronology of the cultural evolution of mankind was another line of inquiry pursued by the team.

For analysis, the researchers studied hand-held fossils of modern humans, chimpanzees and a large number of hominids from the Pleistocene era, including Homo neanderthalensis, man Naledi, three kinds of Australopithecus, and two specimens found at the Swartkrans site in South Africa, which are presumed to belong to a but not identified Homo species or Paranthropus robustus (who could in fact be a member of Australopithecus). The researchers took two factors into account for the analysis: bone anatomy and inferred soft tissues.

“As the muscles themselves are not preserved in the fossils, we inferred their presence and location in the skeleton of the hand based on their distinctive areas of attachment on the bone surface,” Harvati wrote. “Interestingly, our study focused on one muscle, adversens pollicis, whose general location, function and muscle attachment sites are equivalent in great apes, providing an appropriate comparative basis for our sample.

Taken together, this allowed scientists to create virtual models of hominid hands and calculate the manual dexterity available for each species.

“Our methodology integrates state-of-the-art virtual muscle modeling with a three-dimensional analysis of bone shape and size,” said Alexandros Karakostis, paleoanthropologist at the University of Tübingen and first author of the study, in a Cell Press declaration.

The results showed that all Pleistocene humans assessed in the study showed increased effectiveness of thumb opposition, highlighting “the importance of this functional feature in the bio-cultural evolution of our gender,” the team wrote in their paper.

This dexterity has been seen in man Naledi, a human with a small brain unrelated to stone tools, and in the 2-millions of years old bones found at the Swartkrans Cave site in South Africa, setting a timeline for the emergence of this morphological trait. Indeed, and as the authors argue, this period coincides with the increase in levels of tool use in Africa and the emergence of cultural complexity.

“Our study indicates that this human ability, the increased efficiency of thumb opposition, or dexterity of the thumb, has already evolved at the dawn of Homo line and was perhaps a crucial founding element of the very important bio-cultural developments that took place after 2 million years ago, ”Harvati explained. “These include a more systematic use of stone tools, the development of more complex stone tool industries, a gradual increased exploitation of animal resources, and of course the emergence of Man standing, a large-brained, larger-bodied hominid whose geographic distribution included both Africa and Eurasia.

At the same time, however, the dexterity of the thumb Australopithecus was found to be similar to that of living chimpanzees. It is somewhat surprising, but members of this genre would still have been able to use tools, as chimpanzees are today, according to Harvati. In addition, they may have produced the first stone tools, the the oldest of which have been found in Kenya and date back approximately 3.3 million years. Despite this, Australopithecus “The increased dexterity seen in humans had not yet evolved,” Harvati said, including Australopithecus sediba, “Whose hand, and in particular the thumb, has been described as particularly human, suggesting that it was associated with behaviors related to tools.”

Erin Marie Williams-Hatala, associate professor of biology at Chatham University who was not involved in the new research, had problems with the paper, citing the focus on a single muscle attachment site, known as enthesis, as a major limitation.

The authors used “aspects of the shape and size of a muscle attachment complex to approximate the shape and functional capabilities of the associated small muscle in the hand,” she wrote in an email. . This particular muscle is very important for moving the thumb, but “the idea that muscle morphology – and by extension, muscle and organ function – can be gleaned from the associated attachment site is old and very tempting which continues to hold true. be the subject of much debate, ”Williams-Hatala said.

Essentially, “we just don’t understand the relationship between the morphology of muscle attachment sites and the morphology, and certainly not the functional capacity of the associated muscle, to confidently say anything about the latter based on the first, ”she said.

Harvati admitted that a significant limitation of his team’s study is that it was only able to focus on a single thumb muscle, “albeit crucial.” This was “necessary because of the fragmentary nature of the fossil record,” and because her team “wanted to include as many specimens of as many fossil hominid species as possible,” she said.

For the future, Harvati would like to study other important fingers and muscles involved in the use of human-like tools. and study the remains of early hominins, including Australopithecus, to learn more about their behavior and the possible use of the tools. She also plans to study the hands of Neanderthals, who were slightly different of ours.