4-million-year-old hand debunks a popular theory of human evolution

Tarzan swinging from tree to tree It may seem like a Hollywood attempt to imagine the life of primitive men, but the new findings suggest that our ancestors really were swingers.

The study apparently resolves a long-standing scientific debate about our ancestor’s ability to brachiation – the ability to swing from tree branches with only the arms. Before this ancestor underwent an evolutionary shift toward using their hands as tools and their legs for walking, they probably knuckle-walked the ground and slid down awnings.

What’s new – Research published Wednesday in the journal Scientific advances suggests the last common ancestor of hominids – a category of great apes that includes chimpanzees, gorillas, orangutans and humans – climbed and swung in trees.

“Our findings support the view that humans and chimpanzees evolved from an ancestor that had similarities to modern apes in its locomotive adaptation,” says lead author Thomas C. Prang, an assistant professor at Texas A&M University. Reverse.

Some background – Most scientists recognize that the highly dexterous human hand appears to differ in shape and form from the hands that primates use to swing from trees.

However, this evidence has led to a controversial hypothesis: Humans evolved from a quadruped ancestor that used all four limbs to move on the ground, rather than a bipedal ancestor that is suspended from trees.

A chimpanzee in a tree. The researchers suggest that the ancestor of humans swayed from trees like chimpanzees. Getty

Proponents of this hypothesis believe that the last common ancestor was more “ape-like” and less similar to, for example, chimpanzees or bonobos.

The researchers in this study were skeptical of this idea and wanted to prove its merits.

How they did it? The researchers used a sample of more than 400 specimens, spanning both living primates and ancient hominoid fossils.

First, the researchers analyzed the ancient bones of the hand of Ardipithecus ramidus, which believers of the controversial hypothesis use to support their idea of ​​a last quadruped common ancestor. Ardipithecus ramidus it is a human ancestor that lived almost 4.4 million years ago. Our understanding of it is predominantly related to a partial skeleton found in 2009, nicknamed ‘Ardi’.

The initial interpretation of this hand suggested that the last common ancestors of humans and chimpanzees used a form of locomotion called “climbing over branches,” Prang explains.

The remains of Ardipithecus ramidus.Raphael GAILLARDE / Gamma-Rapho via Getty Image

He doubts this interpretation for a reason: monkeys and lemurs are the only primates that use climbing above branches, and their much smaller bodies use external tails to help them climb trees, unlike the subject of your study.

“The inference of adaptations ‘above the branch’ in Ardipithecus it is somewhat problematic as it is the size of a chimpanzee and lacks an external tail [like all apes and humans]”Says Prang.

To prove it, Prang and his colleagues reconstructed the evolution of the hominid hand and how it may have adapted to ancient environments.

A figure from the study showing the evolution of the hands in various hominoids, including humans and Neanderthals.

What they found – The results showed that Arkansas. ramidus it was more similar to chimpanzees, bonobos, and orangutans compared to “non-suspensive” monkeys. Overall, they compared the specimen with a sample of 53 species of anthropoid primates.

Arkansas. ramidus had these suspensory traits, which allowed them to hang from tree branches, before significant evolutionary change occurred with the lineages of Homo (humans) and Australopithecus, an ancient ancestor of hominids, which includes humans and chimpanzees.

“The hand of Ardipithecus suggests that the last common ancestor of humans and chimpanzees adapted to climb tree trunks and suspend the body under branches, “says Prang.

The study, in turn, is framed as a refutation of the previous hypothesis suggesting that hominins evolved from an ancestor “with a generalized hand that lacked suspensory adaptations.”

According to Prang, the study also indicates an important evolutionary step related to the development of tool use.

“We show a great evolutionary leap between the hand of Ardipithecus and all later hominins that match the loss of tree-climbing adaptations in the foot and the oldest known stone tools and fossils of animals marked with stone tools, “says Prang.

This finding supports the idea that Arkansas. ramidus showed an early form of bipedalism, or the ability to walk upright on two legs, which helps us understand how human hands and feet evolved.

“Our study provides some support for the hypothesis that human hands and feet ‘coevolved’, which previous studies have suggested based on comparisons of patterns of hand-foot trait relationships, and evolutionary simulations, between humans and chimpanzees. “said Prang. He says.

The researchers refer to Charles Darwin, the father of evolutionary theory, when discussing the implications of their findings.Getty

Delving into the details – The researchers’ new findings date back to the works of more historical evolutionary scholars.

“Our analysis is much more consistent with what people like Thomas Henry Huxley and Sir Arthur Keith proposed in the late 19th and early 20th centuries based on anatomical comparisons between humans and apes,” says Prang.

The most notable of these historical scholars is Charles Darwin, the father of evolution. Prang connects Darwin’s work with his findings on bipedalism in the ancient specimen, that can help explain human evolution.

“The classic idea attributed to Darwin is that bipedalism ‘freed the hands’ from their leading role in quadruped locomotion, allowing natural selection to push the anatomy of the hand in a new direction. [directly or indirectly] related to manual dexterity, possibly useful for making and using stone tools, “says Pran.

Because it is important According to the study, these findings “resolve a long-standing debate about the role of suspension in human ancestry.”

Alexandros Karakostis, a manual biomechanics expert not affiliated with the study, describes the findings for Reverse as “very intriguing”. It provides a solid answer to “a heated debate,” says Karakostis, although it is a debate that is likely to continue.

“In this context, this new study identifies suspensive adaptations in 4.4 million-year-old hand remains. Ardipithecus ramidus, suggesting that the morphology of the human hand may have arisen from an evolutionary change between Ardipithecus Y Australopithecus,” he says.

Representation of a sculptor of the hominid Australopithecus afarensis. The researchers in this study discuss the evolution of Australopithecus. Getty

Whats Next – In the future, the study team wishes to examine the Ardipithecus hand in more detail.

Ameline Bardo, a postdoctoral research associate at the University of Kent who is not affiliated with the study, agrees that a more detailed analysis of the bones in the hand would be necessary to “better understand the links between the shape and function of your hand.” . This analysis, says Bardo Reverse, can contribute to the understanding of the ancient creature’s movements.

Overall, Bardo sees the study as “very well done” and contributes to the idea that “early hominins evolved from an ancestor with a varied positional repertoire including suspension and vertical climbing.”

The study team is very excited to explore the implications of the article for the evolution of great apes and humans.

“If it’s true that humans and chimpanzees evolved from an African ape-like ancestor, it implies that each African ape lineage evolved at a different rate,” says Prang.

“It will be important to think about the evolutionary histories of African ape populations and how the evolutionary process might have shaped their anatomy and behavior over the past millions of years.”

Summary: The morphology and positional behavior of the last common ancestor of humans and chimpanzees are critical to understanding the evolution of bipedalism. Anatomical research from the early 20th century supported the view that humans evolved from a suspensory ancestor that resembled apes. However, the 4.4 million-year-old hominin hand Ardipithecus ramidus supposedly provides evidence that the hominin hand was derived in a more generalized way. Here, we use comparative morphometric and phylogenetic methods to show that Ardipithecus retains hand morphologies adapted to suspensions shared with chimpanzees and bonobos. We identify an evolutionary change in the morphology of the hand between Ardipithecus and Australopithecus that renews the questions about the coevolution of the manipulative capacities of hominids and the forced bipedalism initially proposed by Darwin. Overall, our results suggest that early hominins evolved from an ancestor with a varied positional repertoire that included suspension and vertical climbing, which directly affects the viable range of hypotheses about the origin of our lineage.

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