The human thumb is a miracle of evolution, allowing our ancestors to make stone tools and radically expand their food choices. New research suggests our nimble, handy thumbs appeared 2 million years ago, in a development that irrevocably changed the course of human history.
Many primates have opposable thumbs, but none are very like ours. The human thumb, placed in contrast to the other fingers, allows for precise gripping, which anthropologists consider a necessary physical feature for tool making.
Scientists are naturally interested in knowing when this extra dexterity developed and whether it coincided with the rise of stone tool production and other cultural innovations.
Katerina Harvati, the lead author of the new study and paleoanthropologist at the University of Tübingen, says most of the studies on the history of human agility are based on a direct comparison between the modern human hand and the hands of early hominins. The new research challenges this methodological approach and instead judges each hand on its own merits. It is hypothetically possible that an earlier version of the hominin hand was superior in terms of dexterity.
As a reminder, our kind, homo sapiens, came into existence about 300,000 years ago, meaning we were latecomers to the human show. Other people (now extinct) like Gay, Standing man, husband Naledi, and Homo neanderthalensis (also known as Neanderthals) were there much earlier, with the very first humans appearing about 2.8 million years ago and possibly even earlier.
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Key to the new study, published today in Current Biology, is an anatomical concept known as ‘thumb opposition’. This is the “act of bringing the thumb into contact with the fingers,” Harvati explains in an email. This efficiency, she said, is “vastly improved in humans” compared to other primates such as chimpanzees (which also have opposable thumbs) and is a “central part of humanoid crafts.”
When Harvati and her colleagues went to the new study, they wanted to know if this improved thumb opposition efficiency could be detected in early hominin fossils, and if so, which ones. Given that some of the oldest stone tools in the archaeological record date back to over 3 million years ago, it seemed possible that another genus of the hominids, namely Austraopithecus, too humanoid dexterity of the thumb. That the appearance of the handy thumb could somehow be related to the timeline of humanity’s cultural evolution was yet another line of research pursued by the team.
For the analysis, the researchers studied hand fossils from modern humans, chimpanzees and a large number of Pleistocene hominins, including Homo neanderthalensis, husband Naledi, three kinds Australopithecus, and two specimens found at the Swartkrans site in South Africa, believed to belong to an early one but not identified Gay kind or Paranthropus robustus (which may well be a member of Australopithecus). The researchers took two factors into account in the analysis: botanical and derived soft tissue.
“Because muscles themselves are not preserved in fossils, we deduced their presence and location in the hand skeleton based on their characteristic attachment areas on the bone surface,” Harvati wrote. “It is worth noting that our study focused on a muscle, opponent of pollicis, whose general location, function and muscle attachment sites are similar in great apes, providing a good basis for comparison for our sample.”
All told, this enabled the scientists to create virtual models of the human-like hands and calculate the manual dexterity available for each species.
“Our methodology integrates advanced virtual muscle modeling with three-dimensional analysis of bone shape and size,” explains Alexandros Karakostis, a paleoanthropologist at the University of Tübingen and the study’s lead author, 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 biocultural evolution of our gender,” the team wrote in their paper.
This dexterity was seen throughout husband Naledi, a human with a small brain not attached to stone tools, and in the 2-million-year-old bones found in the Swartkrans Cave in South Africa, setting a time frame for the emergence of this morphological feature. Indeed, and as the authors claim, this period coincides with an increased use of tools in Africa and the onset of cultural complexity.
“Our study indicates that this human ability, the increased efficiency of the thumb opposition, or the dexterity of the thumb, has already evolved at the beginning of the Gay pedigree and was perhaps a crucial foundation block of the subsequent very important biocultural developments that took place 2 million years ago, ”explained Harvati. These include a more systematic use of stone tools, the development of more complex stone tool industries, the gradual increase in the exploitation of animal resources and of course the emergence of Standing man, a large-brain, larger-body hominin whose geographic reach included both Africa and Eurasia. “
At the same time, however, the thumb turns into dexterity Australopithecus was found to be comparable to that of living chimpanzees. That’s somewhat surprising, but members of this genus would still have been able to use tools, as chimpanzees are todayHarvati said. In addition, they may have produced the earliest stone tools, the eldest of which were found in Kenya and date back to about 3.3 million years ago. Despite this, Australopithecus “Had not yet developed the increased agility we see in humans,” Harvati said, inclusive Australopithecus sediba, “Whose hand, and especially the thumb, has been described as particularly human-like, leading to the suggestion that it is associated with tool-related behavior.”
Erin Marie Williams-Hatala, an associate professor of biology at Chatham University who was not involved in the new investigation, had some problems with the paper, citing the focus on a single muscle attachment site, known as an 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 thumb movement, but the ‘idea that muscle morphology – and by extension muscle and organism function – can be deduced from its associated attachment site is old and very tempting, which is still much debated. Williams-Hatala said.
In essence, “we simply do not understand the relationship between muscle attachment site morphology and morphology, and certainly not the functional ability of the associated muscle, to confidently say anything about the latter based on the former,” she said.
Harvati admitted that a major limitation of her team’s study is that they could only focus on a single, “ albeit crucial ” muscle of the thumb. This was “necessary because of the fragmentary nature of the fossil record” and because her team “wanted to include as many specimens as possible of as many fossil hominins as possible,” she said.
Looking ahead, Harvati would like to explore other important fingers and muscles involved in the use of human tools and study the remains of early hominins, inclusive Australopithecus, for more information about their behavior and possible use of aids. She also plans to study the hands of Neanderthals, which were a little different of ours.