Ardipithecus ramidus Ardi bipedalism quadrupedalism

Human ancestors were 'grounded,' new analysis shows

Human ancestors were 'grounded,' new analysis shows

Primates adapted to living on the ground, adding new chapter to human evolution

 

Ardipithecus ramidus bipedalism
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, NYU

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 doctoral candidate 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 human body has been dramatically modified by evolutionary processes 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 human foot, 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 common ancestor is most similar to these African ape species.

"Therefore, humans evolved from an ancestor 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 foot 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."

 

Press release from the New York University


human face hominins evolution

Need for social skills helped shape modern human face

Need for social skills helped shape modern human face

The modern human face is distinctively different to that of our near relatives and now researchers believe its evolution may have been partly driven by our need for good social skills

This is professor Paul O'Higgins from the University of York. Credit: University of York

The modern human face is distinctively different to that of our near relatives and now researchers believe its evolution may have been partly driven by our need for good social skills.

As large-brained, short-faced hominins, our faces are different from other, now extinct hominins (such as the Neanderthals) and our closest living relatives (bonobos and chimpanzees), but how and why did the modern human face evolve this way?

A new review published in Nature Ecology and Evolution and authored by a team of international experts, including researchers from the University of York, traces changes in the evolution of the face from the early African hominins to the appearance of modern human anatomy.

They conclude that social communication has been somewhat overlooked as a factor underlying the modern human facial form. Our faces should be seen as the result of a combination of biomechanical, physiological and social influences, the authors of the study say.

The researchers suggest that our faces evolved not only due to factors such as diet and climate, but possibly also to provide more opportunities for gesture and nonverbal communication - vital skills for establishing the large social networks which are believed to have helped Homo sapiens to survive.

"We can now use our faces to signal more than 20 different categories of emotion via the contraction or relaxation of muscles", says Paul O'Higgins, Professor of Anatomy at the Hull York Medical School and the Department of Archaeology at the University of York. "It's unlikely that our early human ancestors had the same facial dexterity as the overall shape of the face and the positions of the muscles were different."

human face hominins evolution
These are skulls of hominins over the last 4.4 million years. Credit: Rodrigo Lacruz

Instead of the pronounced brow ridge of other hominins, humans developed a smooth forehead with more visible, hairy eyebrows capable of a greater range of movement. This, alongside our faces becoming more slender, allows us to express a wide range of subtle emotions - including recognition and sympathy.

"We know that other factors such as diet, respiratory physiology and climate have contributed to the shape of the modern human face, but to interpret its evolution solely in terms of these factors would be an oversimplification," Professor O'Higgins adds.

The human face has been partly shaped by the mechanical demands of feeding and over the past 100,000 years our faces have been getting smaller as our developing ability to cook and process food led to a reduced need for chewing.

This facial shrinking process has become particularly marked since the agricultural revolution, as we switched from being hunter gatherers to agriculturalists and then to living in cities - lifestyles that led to increasingly pre-processed foods and less physical effort.

"Softer modern diets and industrialised societies may mean that the human face continues to decrease in size", says Professor O'Higgins. "There are limits on how much the human face can change however, for example breathing requires a sufficiently large nasal cavity."

"However, within these limits, the evolution of the human face is likely to continue as long as our species survives, migrates and encounters new environmental, social and cultural conditions."

 

 

The Evolutionary History of the Human Face is published in Nature Ecology and Evolution. The review was carried out in collaboration with colleagues from international institutions including the New York University College of Dentistry, the Natural History Museum, Arizona State University and Universidad Complutense de Madrid.

Press release from the University of York