The human thumb is a miracle of evolution, allowing our ancestors to make stone tools and radically expand their food choices. New research suggests that our agile and dexterous fingers appeared 2 million years ago, in a development that has irrevocably changed the course of human history.
Many primates have opposable thumbs, but none resemble ours. The human thumb, placed in opposition to the other fingers, allows precise grasping, which anthropologists consider a physical attribute necessary for the manufacture of instruments.
Scientists are naturally interested in knowing when this added 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 hominin dexterity are based on a direct comparison between the modern human hand and the hands of early hominins. New research challenges this methodological approach and instead evaluates each hand on its own merits. It is possible, hypothetically speaking, that an earlier version of the hominin hand is superior in the dexterity of the thumb.
As a reminder, our species, homo sapiens, appeared about 300,000 years ago, which means we arrived late to the human spectacle. Other people (now extinct), such as Man, Standing man, om Naledi, and Homo neanderthalensis (also known as Neanderthals) were around much earlier, the first humans appearing about 2.8 million years ago. and possibly even earlier.
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The key to the new study, published today in Current Biology, is an anatomical concept known as “opposition thumb”. This is “the action of bringing the thumb in contact with the fingers,” Harvati explained in an email. This efficiency, she said, is “much improved in humans” compared to other primates like it chimpanzees (who also have opposable thumbs) and is a “central component of human-like manual dexterity.”
Entering the new study, Harvati and her colleagues wanted to know if this increased effectiveness of thumb opposition could be detected in early hominin fossils and, if so, which of them. Given that some of the oldest stone tools in the archaeological record date back more than 3 million years ago, it seemed possible that another hominin genus, namely Austraopithecus, also had the dexterity of the human-like thumb. That the appearance of the thumb could somehow relate to the chronology of the cultural evolution of humanity was yet another line of investigation pursued by the team.
For analysis, the researchers studied hand fossils of modern humans, chimpanzees and a large number of Pleistocene hominins, including Homo neanderthalensis, om Naledi, three species of Australopithecus, and two specimens found at the Swartkrans site in South Africa, which are believed to belong to an early but unidentified Man species or Paranthropus robustus (who may in fact be a member of Australopithecus). The researchers considered two factors for the analysis: bone anatomy and deduced soft tissues.
“Because the muscles themselves are not preserved in fossils, we deduced their presence and location in the skeleton of the hand based on their distinct areas of attachment to the bone surface,” Harvati wrote. “It is noteworthy that our study focused on a muscle, oposens pollicis, whose general location, function, and sites of muscle attachment are equivalent among large monkeys, providing an adequate comparative basis for our sample.”
Taken together, this allowed scientists to create virtual models of hominin hands and calculate the manual dexterity available to each species.
“Our methodology integrates state-of-the-art virtual muscle modeling with three-dimensional analysis of bone shape and size,” Alexandros Karakostis, a paleoanthropologist at the University of Tübingen and the first author of the study, explained in a Cell Press. statement.
The results showed that all Pleistocene people evaluated in the study showed the increased efficiency of the thumb opposition, highlighting “the significance of this functional feature in the bio-cultural evolution of our genre”, the team wrote in their paper.
This dexterity was seen in om Naledi, a man with a small brain who is not tied to stone tools and in 2-million-year-old bones found at the site of Swartkrans Cave in South Africa, setting a time frame for the appearance of this morphological feature. Indeed, and as the authors argue, this period of time coincides with increased levels of instrument use in Africa and the emergence of cultural complexity.
“Our study indicates that this human capacity, the increased efficiency of thumb opposition or the dexterity of the thumb, have already evolved at dawn. Man offspring and it was probably a crucial founding block of subsequent 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, the gradual increased exploitation of animal resources and, of course, the emergence of Standing man, a large-brained, larger-bodied hominin whose geographical area included both Africa and Eurasia. ”
At the same time, however, the dexterity of the thumb Australopithecus it turned out to be similar to that of live chimpanzees. It is somewhat surprising, but members of this genre he would still have been able to use the instruments, as chimpanzees are today, according to Harvati. Moreover, you may have produced the oldest stone tools, the oldest of which were found in Kenya and date back about 3.3 million years. Despite this, Australopithecus “The increased dexterity observed in humans had not yet evolved,” Harvati said Australopithecus sediba, “Whose hand, and especially the thumb, has been described as rather human-like, suggesting that it was associated with instrument-related behaviors.”
Erin Marie Williams-Hatala, associate professor of biology at Chatham University, who was not involved in new research, had some issues with paper, citing the emphasis on a single site of muscle attachment, 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 abilities of the associated small hand muscles,” she wrote in an email. This particular muscle is very important for the movement of the thumb, but “the idea that muscle morphology – and by extension, muscle and body function – can be gathered from the associated attachment site is an old and very tempting one, which continues to be hotly debated. “Williams-Hatala said.
In essence, “we simply do not understand the relationship between muscle attachment morphology and morphology and certainly not the functional ability of the associated muscle to confidently say something about the latter based on the former,” she said.
Harvati acknowledged that an important limitation of his team’s study is that they were able to focus on only one “crucial” thumb muscle. 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 hominin fossil species as possible,” she said.
Looking ahead, Harvati would like to investigate other important fingers and muscles involved in using a human-like tool. and studies the remains of early hominins, inclusive Australopithecus, to learn more about their behaviors and the possible use of tools. She also intends to study the hands of the Neanderthals, who were a little bit different from ours.