The evolution of the opposable thumb is often accompanied by the rise of stone tools.
Without the ability to grip or manipulate objects with power and precision, some scientists believe our early human ancestors could never have brought our species to such great success.
It’s an interesting assumption, but opposable thumbs and tools may not be as closely related as we think. In fact, early humans may have made and used tools long before their thumbs matched ours.
A new study has found that opposable thumbs evolved around 2 million years ago, and there is evidence of human ancestors making stone tools before that.
In recent years, Australopithecines, a genus of primitive hominids, have emerged as possibly the first humans with indirect evidence of tool use, between 2 and 3 million years ago.
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. The new research suggests that their thumbs at that time were more similar to those of chimpanzees than ours.
Compared to their primate ancestors, Australopithecines had better manual dexterity, researchers say. But although their hands are similar in proportions to modern humans, their thumbs might not achieve the same degree of effective opposition that we have today.
“Increased manual dexterity in the form of effective thumb opposition was one of the early defining characteristics of our lineage, providing a tremendous adaptive advantage to our ancestors.” explains paleoanthropologist Katerina Harvati from Eberhard Karls University in Tübingen, Germany.
These consistently high skill levels in species of Homo they testify to the great adaptive value of the opposition of the thumb to human biocultural evolution, ”he adds.
The idea that our opposable thumbs played a role in the evolution of human tools has been hinted at for decades, but until now we still haven’t figured out exactly when the effective use of the thumb first appeared in the world. human history and how it relates to our tool. development.
In 2015, researchers discovered Australopithecus africanus it had a bony pattern on the thumb and palms that would have allowed strong thumb opposition over 2 million years ago.
In 2011, another type of Australopithecus, known as A. sediba, which lived over 2 million years ago, has also been interpreted as a human precision shot.
The new research refutes these earlier studies. It is not based on comparative anatomy, like them, but on the biomechanics of the thumb itself.
To achieve this, the team focused on a single muscle and single joint in the hominid’s hand. The muscle, known as the opposite thumb, is believed to be crucial for the opposition of the thumb, allowing flexion at the trapeziometacarpal joint (TMC).
The location of this muscle, its path, and its general areas of attachment have been compared among a variety of fossils from human species.
“Our methodology integrates advanced virtual muscle modeling with three-dimensional analysis of bone shape and size.” explains biomechanics expert Alexandros Karakostis from the University of Tübingen.
“Above all, we were able to validate the predictions of our models by confirming that the differences observed between living taxa (chimpanzees and modern humans) mirror those reported in previous experimental studies.”
The difference between the thumb muscles of humans and modern chimpanzees. (Harvati, Karakostis and Haeufle)
While our own kind, including Neanderthals (Homo neanderthalensis) showed equally high degrees of manual dexterity, unlike other hominids.
Thumb efficiency and dexterity in all australopithecines was consistently lower.
Even the last kind of Australopithecus, A. sediba, showed less flexion in the TMC joint. Interestingly, however, the earliest hominids from this period, found at the Swartkrans site in South Africa, had hands much more similar to ours.
In fact, scientists say that Swartkrans thumb mechanics are halfway between chimpanzees and modern humans.
“ One of the biggest surprises was to discover that the hominid hand fossils from the Swartkrans site in South Africa, dating from around 2 million years ago and attributed to Homo or the side branch of the extinct hominid Paranthropus robustus, it could achieve a thumb dexterity similar to that of modern humans, ”says Karakostis.
Such dexterity could be one of the reasons the hominids found at the Swartkans site are considered the first users of fire and the first to start killing large animals in South Africa.
This change, the authors note, has potentially led to “a significant evolutionary advantage, which could have been part of the crucial biocultural developments that take place after 2 million years”.
Virtually from then on, the first humans had the tools they needed to harness the resources, disperse outside Africa, and develop a complex culture.
But it is possible that even before that, species of Australopithecus they used stone tools with less precision.
The authors now hope to take a closer look at the hands of specific groups of hominids, including Neanderthals, to better understand the timeline of our own thumbs and what they have helped us achieve over millions of years.
The study was published in Current biology.
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