The human thumb is a marvel of evolution, allowing our ancestors to craft stone tools and radically expand their food choices. New research suggests our agile, dextrous 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 quite like ours. The human thumb, set in opposition to the other fingers, allows for precision grasping, which anthropologists consider a necessary physical attribute for crafting tools.
Scientists are naturally interested in knowing when this added dexterity evolved and whether it coincided with the emergence of stone tool production and other cultural innovations.
Katerina Harvati, the lead author of the new study and a paleoanthropologist at the University of Tübingen, says most studies looking into the history of hominin dexterity rely on direct comparison between the modern human hand and the hands of early hominins. The new research challenges this methodological approach and instead evaluates each hand on its own merits. It’s possible, hypothetically speaking, that an earlier version of the hominin hand was superior in terms of thumb dexterity.
As a reminder, our species, Homo sapiens, emerged around 300,000 years ago, which means we were latecomers to the human show. Other humans (now extinct), such as Homo habilis, Homo erectus, Homo naledi, and Homo neanderthalensis (otherwise known as Neanderthals) were around much earlier, with the very first humans appearing approximately 2.8 million years ago and possibly even earlier.
Key to the new study, published today in Current Biology, is an anatomical concept known as “thumb opposition.” This is the “action of bringing the thumb in contact with the fingers,” explained Harvati in an email. This efficiency, she said, is “greatly enhanced 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 her colleagues wanted to know if this enhanced 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 more than 3 million years ago, it seemed possible that another hominin genus, namely Austraopithecus, also had human-like thumb dexterity. That the appearance of the dextrous thumb might somehow relate to the timeline of humanity’s cultural evolution was yet another line of inquiry pursued by the team.
For the analysis, the researchers studied hand fossils of modern humans, chimpanzees, and a large number of Pleistocene-era hominins, including Homo neanderthalensis, Homo naledi, three species of Australopithecus, and two specimens found at the Swartkrans site in South Africa, which are presumed to belong to an early but unidentified Homo species or Paranthropus robustus (which might actually be a member of Australopithecus). The researchers took two factors into account for the analysis: bone anatomy and inferred soft tissue.
“As muscles themselves are not preserved in fossils, we inferred their presence and location in the hand skeleton based on their distinctive areas of attachment on the bone surface,” wrote Harvati. “ It is worth noting that our study focused on a muscle, opponens pollicis, whose general location, function, and muscle attachment sites are equivalent among great apes, providing a proper comparative basis for our sample.”
Taken together, this allowed the scientists to create virtual models of the hominin hands and calculate the manual dexterity available to each species.
“Our methodology integrates cutting-edge 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.
Results showed that all Pleistocene humans evaluated in the study showed the increased thumb opposition efficiency, highlighting the “significance of this functional feature in the bio-cultural evolution of our genus,” the team wrote in their paper.
This dexterity was seen in Homo naledi, a small-brained human not linked to stone tools, and in the 2-million-year-old bones found at the Swartkrans cave site in South Africa, establishing a timeframe for the emergence of this morphological trait. Indeed, and as the authors contend, this time period coincides with increased levels of tool use in Africa and the onset of cultural complexity.
“Our study indicates that this human ability, the increased thumb opposition efficiency, or thumb dexterity, already evolved at the dawn of the Homo lineage and was perhaps a crucial founding block of the subsequent very important bio-cultural developments that took place after 2 million years ago,” explained Harvati. “These include 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 appearance of Homo erectus, a large-brained and larger-bodied hominin whose geographic range included both Africa and Eurasia.”
At the same time, however, the thumb dexterity in Australopithecus was found to be similar to that of living chimpanzees. That’s somewhat surprising, but members of this genus would still have been capable of tool use, as chimps are today, according to Harvati. What’s more, they may have produced the earliest stone tools, the oldest of which were found in Kenya and date back to approximately 3.3 million years ago. Despite this, Australopithecus “had not yet evolved the increased dexterity seen in humans,” said Harvati, including Australopithecus sediba, “whose hand, and particularly the thumb, has been described as especially human-like, prompting suggestions that it was associated with tool-related behaviors.”
Erin Marie Williams-Hatala, an associate professor of biology at Chatham University who wasn’t involved with the new research, had some issues 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 abilities 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 organismal function—can be gleaned from the associated attachment site is an old and very tempting one that continues to be heavily debated,” said Williams-Hatala.
Essentially, “we simply do not understand the relationship between the morphology of muscle attachment sites and the 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 an important limitation of her team’s study is that they were only able to focus on a single, “albeit crucial,” muscle of the thumb. This was “necessary due to the fragmentary nature of the fossil record,” and because her team “wanted to include as many specimens from as many fossil hominin species as possible,” she said.
Looking ahead, Harvati would like to investigate other important fingers and muscles involved in human-like tool use and study the remains of early hominins, including Australopithecus, to learn more about their behaviors and possible tool use. She also plans to study the hands of Neanderthals, which were slightly different from ours.