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

Tarzan swinging from tree to tree might seem like a Hollywood attempt at imagining the life of primitive men, but new findings suggest our ancient ancestors really were swingers.

The study seemingly resolves a long-standing scientific debate over our ancestor's ability for brachiation — the ability to swing from tree limbs only using one's arms. Before this ancestor experienced an evolutionary shift toward using hands for tools and legs for walking, they likely knuckle-walked on the ground and glided across canopies.

What's new — Research published Wednesday in the journal Science 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 their locomotor adaptation," lead author Thomas C. Prang, an assistant professor at Texas A&M University, tells Inverse.

Some background — Most scientists recognize that the highly dextrous human hand seems to differ in shape and form from the hands primates use to swing from trees.

However, this evidence has given rise to a disputed hypothesis: Humans evolved from a quadrupedal ancestor that used all four limbs for movement on the ground, rather than a bipedal ancestor that suspends from trees.

Proponents of this hypothesis believe the last common ancestor was more "monkey-like" and less similar to, say, chimpanzees or bonobos.

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

How they did it — Researchers used a sample of 400-plus specimens, encompassing both living primates and ancient hominoid fossils.

First, researchers analyzed the ancient hand bones of Ardipithecus ramidus, which believers of the disputed hypothesis use to support their idea regarding a quadrupedal last common ancestor. Ardipithecus ramidus is a human ancestor that lived nearly 4.4 million years ago. Our understanding of it is predominantly linked to a partial skeleton found in 2009, nicknamed 'Ardi.'

The initial interpretation of this hand suggested the last common ancestors of humans and chimpanzees used a form of locomotion called "above-branch clambering," Prang explains.

He doubts this interpretation for one reason: monkeys and lemurs are the only primates that use above-branch climbing, and their much smaller bodies use external tails to help them with tree climbing — unlike the subject of their study.

"The inference of 'above-branch' adaptations in Ardipithecus is somewhat problematic since it's chimpanzee-sized and lacks an external tail [like all apes and humans]," Prang says.

To test it, Prang and his colleagues reconstructed the evolution of the hominin hand and how it may have adapted in ancient environments.

What they found — The results showed that Ar. ramidus was most similar to chimpanzees, bonobos, and orangutans compared to "non-suspensory" monkeys. Overall, they compared the specimen across a sample of 53 anthropoid primate species.

Ar. ramidus had these suspensory traits — which enabled them to swing from tree branches — before a significant evolutionary shift occurred with the lineages of Homo (humans) and Australopithecus, an ancient ancestor of hominins, which includes humans and chimpanzees.

"The hand of Ardipithecus suggests that the last common ancestor of humans and chimpanzees was adapted to climbing tree trunks and suspending the body beneath branches," Prang says.

The study, in turn, is framed as a debunking of the earlier hypothesis suggesting 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 major evolutionary jump between the hand of Ardipithecus and all later hominins that happens to coincide with the loss of tree climbing adaptations in the foot and the earliest known stone tools and stone tool-cut-marked animal fossils," Prang says.

This finding provides support for the idea that Ar. ramidus displayed 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 'co-evolved,' which previous studies have suggested on the basis of comparisons of patterns of hand/foot trait relationships, and evolutionary simulations, among humans and chimpanzees," Prang says.

Digging into the details — The researchers' new findings harken 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 century based on anatomical comparisons between humans and apes," Prang says.

The most notable of these historical scholars is Charles Darwin, the father of evolution. Prang connects Darwin's work to their findings on bipedalism in the ancient specimen, which can help explain human evolution.

"The classic idea attributed to Darwin is that bipedalism 'freed the hands' from their primary role in quadrupedal locomotion, which enabled natural selection to push hand anatomy in a new direction [directly or indirectly] related to manual dexterity, possibly useful for the manufacture and use of stone tools," Pran says.

Why it matters — According to the study, these findings "resolve a long-standing debate about the role of suspension in the ancestry of humans."

Alexandros Karakostis, a hand biomechanics expert not affiliated with the study, describes the findings to Inverse as "very intriguing." It provides a robust answer to "a heated debate," Karakostis says — although it's a debate that's likely to continue.

"In this context, this new study identifies suspensory adaptations in the 4.4 million-year-old hand remains of Ardipithecus ramidus, suggesting that human hand morphology may have emerged from an evolutionary shift between Ardipithecus and Australopithecus," he says.

What's next — In the future, the study team wants to examine the Ardipithecus hand in more detail.

Ameline Bardo, a postdoctoral research associate at the University of Kent not affiliated with the study, agrees a more detailed analysis of the hand bones would be necessary to "better understand the links between form and function of his hand." This analysis, Bardo tells Inverse, may contribute to an understanding of the ancient creature's movements.

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

The study team is most excited to explore the paper's implications for the evolution of great apes and humans

"If it is true that humans and chimpanzees evolved from an African ape-like ancestor, it implies that each African ape lineage evolved at different rates," Prang says.

"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 last several million years."

Abstract: The morphology and positional behavior of the last common ancestor of humans and chimpanzees are critical for understanding the evolution of bipedalism. Early 20th century anatomical research supported the view that humans evolved from a suspensory ancestor bearing some resemblance to apes. However, the hand of the 4.4-million-year-old hominin Ardipithecus ramidus purportedly provides evidence that the hominin hand was derived from a more generalized form. Here, we use morphometric and phylogenetic comparative methods to show that Ardipithecus retains suspensory adapted hand morphologies shared with chimpanzees and bonobos. We identify an evolutionary shift in hand morphology between Ardipithecus and Australopithecus that renews questions about the coevolution of hominin manipulative capabilities and obligate bipedalism initially proposed by Darwin. Overall, our results suggest that early hominins evolved from an ancestor with a varied positional repertoire including suspension and vertical climbing, directly affecting the viable range of hypotheses for the origin of our lineage.