The human thumb is a marvel of evolution, allowing our ancestors to craft stone tools and radically expand their food choices. New research suggests that our agile, dexterous thumbs appeared two million years ago, in a development that irrevocably changed the course of human history.
Many primates have opposable thumbs, but none are quite similar to ours. The human thumb, located in opposition to the other fingers, allows to take with precision, that the anthropologists consider a necessary physical attribute to elaborate tools.
Scientists are naturally interested in knowing when this added skill 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 that study the history of hominin dexterity are based on a direct comparison between the modern human hand and the hands of the former. hominins. The new research calls into question this methodological approach and instead evaluates each hand on its own merits. It is possible, hypothetically speaking, that an earlier version of the hominid hand was superior in terms of thumb dexterity.
As a reminder, our species, homo sapiens, emerged about 300,000 years ago, which means we arrived late to the human spectacle. Other humans (now extinct), such as Man, Right man, home Naledi, i Homo neanderthalensis (also known as Neanderthals) were much earlier, with the first humans appearing about 2.8 million years ago. and possibly even before.
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Key to the new to study, published today in Current Biology, is an anatomical concept known as “thumb opposition.” It’s about “the action of putting your thumb in contact with your fingers,” Harvati explained in an email. This efficiency, he said, “is greatly improved in humans” compared to other primates such as chimpanzees (who also have opposable thumbs) and is a “central component of human manual dexterity.”
Entering the new study, Harvati and colleagues wanted to know if this improved efficiency of thumb opposition could be detected in the first hominid fossils and, if so, which ones. Given that some of the oldest stone tools in the archaeological record date back more than 3 million years, it seemed possible that another hominid genus, namely Austraopithecus, also had human-like thumb dexterity. The fact that the appearance of the right thumb could be related in some way to the timeline of the cultural evolution of humanity was another line of research that the team followed.
For analysis, researchers studied fossils from modern humans, chimpanzees, and a large number of Pleistocene-era hominins, including Homo neanderthalensis, home Naledi, three species of Australopithecus, and two specimens found at the Swartkrans site in South Africa, which are supposed to belong to an early but unidentified Man species or Paranthropus robustus (who could be a member of Australopithecus). The researchers considered two factors for the analysis: bone anatomy and inferred soft tissue.
“Because the muscles themselves are not preserved in fossils, we inferred their presence and location in the skeleton of the hand based on their different areas of attachment to the bone surface,” Harvati wrote. “It is worth noting that our study focused on a muscle, opposens thumbs, overall location, function, and muscle fixation sites are equivalent among great apes, 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 for each species.
“Our methodology integrates cutting-edge virtual muscle models with three-dimensional analyzes of bone shape and size,” Alexandros Karakostis, a paleoanthropologist at the University of Tübingen and lead author of the study, told Cell Press statement.
The results showed that all Pleistocene humans evaluated in the study showed increasing the efficiency of opposition to the thumb, emphasizing the “importance of this functional feature in the biocultural evolution of our gender,” the team wrote in its work.
This skill was seen in home Naledi, a small – brained human being not tied to stone tools, and al 2-million-year-old bones found at the Swartkrans Cave site in South Africa, setting a time period for the appearance of this morphological trait. In fact, and as the authors state, this period of time coincides with rising levels of tool use in Africa and the emergence of cultural complexity.
“Our study indicates that this human capacity, the increased efficiency of thumb opposition or thumb dexterity, already evolved at the dawn of the Man lineage and it was perhaps a crucial founding block of subsequent very important biocultural developments that took place after two million years ago, “Harvati explained.” These include a more systematic use of stone tools, the development of industries. of more complex stone tools, the gradual increase in the exploitation of animal resources and, of course, the emergence of Right man, a hominid with a large brain and a larger body, whose geographical distribution included both Africa and Eurasia. “
At the same time, however, the dexterity of the thumb Australopithecus it was found to be similar to that of living chimpanzees. This is a bit surprising, but members of this genre he would still have been able to use tools, as they are today, according to Harvati. Moreover, it is possible that they produced the first stone tools, the older of which were found in Kenya and date back to about 3.3 million years ago. Despite this, Australopithecus “The increase in skill observed in humans had not yet evolved,” Harvati said, including Australopithecus sediba, “Whose hand, and in particular his thumb, has been described as especially human, suggesting that it was associated with tool-related behaviors.”
Erin Marie Williams-Hatala, an associate professor of biology at Chatham University who was not related to the new research, he had some problems with the paper, citing the focus on a single muscle binding site, known as enthesis, as an important limitation.
The authors used “aspects of the shape and size of a muscle attachment complex to approximate the shape and functional capabilities of the small muscle associated with the hand,” he 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 body function, can be picked up from the associated junction site is old and very tempting that it’s still very much debated, ”Williams-Hatala said.
Basically, “we just don’t understand the relationship between the morphology of the muscle binding sites and the morphology, and certainly not the functional capacity of the associated muscle, to confidently say anything about the second based on the first,” he said.
Harvati admitted that a major limitation of his team’s study is that they were only able to focus on a single “though crucial” thumb muscle. This was “necessary because of the fragmentary nature of the fossil record” and because his team “wanted to include as many specimens of as many hominid fossil species as possible,” he said.
Looking to the future, Harvati would like to investigate other important fingers and muscles involved in using people-like tools. i study the remains of the first hominins, included Australopithecus, for more information on their behaviors and the possible use of the tools. He also plans to study the hands of the Neanderthals, who were a little different of ours.