Human ancestors were 'grounded,' new analysis shows

Human ancestors were “grounded,” new analysis shows
An evolutionary tree depicting the relationships among living apes, Ardi, and modern humans. Each branch on the tree represents a species and their intersections represent their common ancestors. The dots represent hypothetical evolutionary changes associated with the evolution of ground-living adaptations in the common ancestor of African apes and humans as well as the evolution of bipedalism, which is supported by the analysis. This shows that human bipedalism evolved from an ancestral form similar to the living African apes. Credit: Thomas Prang

African apes adapted to living on the ground, a finding that indicates human evolved from an ancestor not limited to tree or other elevated habitats. The analysis adds a new chapter to evolution, shedding additional light on what preceded human bipedalism.

African apes adapted to living on the ground, a finding that indicates human evolved from an ancestor not limited to tree or other elevated habitats. The analysis adds a new chapter to evolution, shedding additional light on what preceded human bipedalism.

"Our unique form of human locomotion evolved from an ancestor that moved in similar ways to the living African apes—chimpanzees, bonobos, and gorillas," explains Thomas Prang, a in New York University's Department of Anthropology and the author of the study, which appears in the journal eLife. "In other words, the common ancestor we share with chimpanzees and bonobos was an African ape that probably had adaptations to living on the ground in some form and frequency."

The way that humans walk—striding bipedalism—is unique among all living mammals, an attribute resulting from myriad changes over time.

"The has been dramatically modified by over the last several million years in ways that happened to make us better walkers and runners," notes Prang.

Much of this change is evident in the , which has evolved to be a propulsive organ, with a big toe incapable of ape-like grasping and a spring-like, energy-saving arch that runs from front to back.

These traits raise a long-studied, but not definitively answered, question: From what kind of ancestor did the human foot evolve?

In the eLife work, Prang, a researcher in NYU's Center for the Study of Human Origins, focused on the fossil species Ardipithecus ramidus ('Ardi'), a 4.4 million-years-old human ancestor from Ethiopia—more than a million years older than the well-known 'Lucy' fossil. Ardi's bones were first publicly revealed in 2009 and have been the subject of debate since then.

In his research, Prang ascertained the relative length proportions of multiple bones in the primate foot skeleton to evaluate the relationship between species' movement (locomotion) and their skeletal characteristics (morphology). In addition, drawing upon the Ardi fossils, he used statistical methods to reconstruct or estimate what the common ancestor of humans and chimpanzees might have looked like.

Here, he found that the African apes show a clear signal of being adapted to ground-living. The results also reveal that the Ardi foot and the estimated morphology of the human-chimpanzee last is most similar to these African ape species.

"Therefore, humans evolved from an that had adaptations to living on the ground, perhaps not unlike those found in African apes," Prang concludes. "These findings suggest that human bipedalism was derived from a form of locomotion similar to that of living African apes, which contrasts with the original interpretation of these fossils."

The original interpretation of the Ardi foot fossils, published in 2009, suggested that its foot was more monkey-like than chimpanzee- or gorilla-like. The implication of this interpretation is that many of the features shared by living great apes (chimpanzees, bonobos, gorillas, and orangutans) in their and elsewhere must have evolved independently in each lineage—in a different time and place.

"Humans are part of the natural world and our locomotor adaptation—bipedalism—cannot be understood outside of its natural evolutionary context," Prang observes. "Large-scale evolutionary changes do not seem to happen spontaneously. Instead, they are rooted in deeper histories revealed by the study of the fossil record.

"The study of the Ardi fossil shows that the evolution of our own ground-living adaptation—bipedalism—was preceded by a quadrupedal ground-living adaptation in the common ancestors that we share with the African apes."

Explore further

New findings shed light on origin of upright walking in human ancestors

More information: Thomas Cody Prang. The African ape-like foot of Ardipithecus ramidus and its implications for the origin of bipedalism, eLife (2019). DOI: 10.7554/eLife.44433
Journal information: eLife

Citation: Human ancestors were 'grounded,' new analysis shows (2019, April 30) retrieved 19 August 2019 from
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Apr 30, 2019
The thing that made us human was our technology, which first manifested in the ability to throw rocks and spears, overhand, with accuracy and effect. Living on the ground made this necessary in order to drive off predators. But the act of propelling oneself swinging from branch to branch is what gave us the necessary physiology.

Apr 30, 2019
It is often noted that humans are the least derived of the extant apes.

In fact, humans, chimps and gorillas each have their own unique locomotion system. The mechanics of the lighter chimp hand while quadripedal is a knuckle lock while the gorillas have to use the palm more often.

But it is satisfying to see that the feet share more of a common locomotion pattern. The analysis is a straightforward principal component analysis in morphospace (where it is already clear Ardi cluster with the apes), mapped to a phylogenetic tree.

Apr 30, 2019
The thing that made us human was our technology
But there's one non neurological area where humans are physically competitive or superior to many other animals-- long distance running. We can run down prey for hours and we haven't lost that capability as we developed our present brain capacity. That makes me think it must be an important factor what made us human.

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