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


hominins fossils Ardipithecus ramidis Afar Regional State Ethiopia

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

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

4.5 million-year old fossil shows evidence of greater reliance on bipedalism than previously suggested

 

The oldest distinguishing feature between humans and our ape cousins is our ability to walk on two legs - a trait known as bipedalism. Among mammals, only humans and our ancestors perform this atypical balancing act. New research led by a Case Western Reserve University School of Medicine professor of anatomy provides evidence for greater reliance on terrestrial bipedalism by a human ancestor than previously suggested in the ancient fossil record.

Scott W. Simpson, PhD, led an analysis of a 4.5 million-year-old fragmentary female skeleton of the human ancestor Ardipithecus ramidus that was discovered in the Gona Project study area in the Afar Regional State of Ethiopia.

The newly analyzed fossils document a greater, but far from perfect, adaptation to bipedalism in the Ar. ramidus ankle and hallux (big toe) than previously recognized. "Our research shows that while Ardipithecus was a lousy biped, she was somewhat better than we thought before," said Simpson.

Fossils of this age are rare and represent a poorly known period of human evolution. By documenting more fully the function of the hip, ankle, and foot in Ardipithecus locomotion, Simpson's analysis helps illuminate current understanding of the timing, context, and anatomical details of ancient upright walking.

Previous studies of other Ardipithecus fossils showed that it was capable of terrestrial bipedalism as well as being able to clamber in trees, but lacked the anatomical specializations seen in the Gona fossil examined by Simpson. The new analysis, published in the Journal of Human Evolution, thus points to a diversity of adaptations during the transition to how modern humans walk today. "The fact that Ardipithecus could both walk upright, albeit imperfectly, and scurry in trees marks it out as a pivotal transitional figure in our human lineage," said Simpson.

Key to the adaptation of bipedality are changes in the lower limbs. For example, unlike monkeys and apes, the human big toe is parallel with the other toes, allowing the foot to function as a propulsive lever when walking. While Ardipithecus had an offset grasping big toe useful for climbing in trees, Simpson's analysis shows that it also used its big toe to help propel it forward, demonstrating a mixed, transitional adaptation to terrestrial bipedalism.

Specifically, Simpson looked at the area of the joints between the arch of the foot and the big toe, enabling him to reconstruct the range of motion of the foot. While joint cartilage no longer remains for the Ardipithecus fossil, the surface of the bone has a characteristic texture which shows that it had once been covered by cartilage. "This evidence for cartilage shows that the big toe was used in a more human-like manner to push off," said Simpson. "It is a foot in transition, one that shows primitive, tree-climbing physical characteristics but one that also features a more human-like use of the foot for upright walking." Additionally, when chimpanzees stand, their knees are "outside" the ankle, i.e., they are bow-legged. When humans stand, the knees are directly above the ankle - which Simpson found was also true for the Ardipithecus fossil.

The Gona Project has conducted continuous field research since 1999. The study area is located in the Afar Depression portion of the eastern Africa rift and its fossil-rich deposits span the last 6.3 million years. Gona is best known as documenting the earliest evidence of the Oldowan stone tool technology. The first Ardipithecus ramidus fossils at Gona were discovered in 1999 and described in the journal Nature in 2005. Gona has also documented one of the earliest known human fossil ancestors - dated to 6.3 million years ago. The Gona Project is co-directed by Sileshi Semaw, PhD, a research scientist with the CENIEH research center in Burgos, Spain, and Michael Rogers, PhD, of Southern Connecticut State University. The geological and contextual research for the current research was led by Naomi Levin, PhD, of the University of Michigan, and Jay Quade, PhD, of the University of Arizona.

hominins fossils bipedalism Ardipithecus ramidis Afar Regional State Ethiopia
This is a fossil hominin talus from site GWM67 (2005) at the time of its discovery. Credits: Case Western Reserve University School of Medicine

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This research was made possible by use of the human and ape skeletal collections housed at the Laboratory of Physical Anthropology, Cleveland Museum of Natural History. Major financial support was provided by the L.S.B. Leakey Foundation, Spain's Ministerio de Economia, Industria y Competitividad, Marie Curie EU Integration Grant, U.S. National Science Foundation, Case Western Reserve University, the National Geographic Society, and the Wenner-Gren Foundation.

Simpson, S., et al. "Ardipithecus ramidus postcrania from the Gona Project area, Afar Regional State, Ethiopia." Journal of Human Evolution. DOI: 10.1016/j.jhevol.2018.12.005

For more information about Case Western Reserve University School of Medicine, please visit case.edu/medicine.

Press release from Case Western Reserve University