The earliest distinguishing feature between humans and our simian cousins is our ability to walk on two legs – a feature known as bipedalism. Among mammals, only humans and our ancestors perform this act of atypical equilibrium. New research conducted by an anatomy professor at Case Western Reserve University Medical School provides evidence of a greater reliance on terrestrial bipedalism for a human ancestor than previously suggested in the ancient fossil record.
Scott W. Simpson, PhD, conducted an analysis of a 4.5-million-year fragmentary female skeleton of the human ancestor Ardipithecus ramidus that was discovered in the study area of the Gona Project in Afar Regional State, Ethiopia.
The recently analyzed fossils document a larger, but not perfect, adaptation to bipedalism in the Ar. Ankle ramidus and hallux (toe) than previously recognized. "Our research shows that while Ardipithecus was a bad biped, it was a little better than we thought before," Simpson said.
Fossils of this age are rare and represent a little known period of human evolution. By documenting more fully the function of the hip, ankle and foot in the Ardipithecus locomotion, Simpson's analysis helps illuminate the current understanding of the time, context and anatomical details of the ancient standing walk.
Previous studies of other Ardipithecus fossils have shown that he was capable of terrestrial bipedalism as well as being able to climb trees but lacked the anatomical specializations seen in Gona's fossil examined by Simpson. The new analysis, Journal of Human Evolution, therefore, points to a diversity of adaptations during the transition to how modern humans walk today. "The fact that Ardipithecus can walk, albeit imperfectly, and run on trees, is a crucial figure in our human lineage," Simpson said.
Key to the adaptation of bipedality are changes in the lower limbs. For example, unlike monkeys and apes, the toe of the human foot parallels the other toes, allowing the foot to function as a propulsion lever when walking. While Ardipithecus had a large useful thumb to climb on trees, Simpson's analysis shows that he also used his thumb to help propel it, demonstrating a mixed and transient adaptation to terrestrial bipedalism.
Specifically, Simpson looked at the area of the joints between the arch of the foot and the big toe, allowing him to reconstruct the range of motion of the foot. While the articular cartilage no longer remains for the Ardipithecus fossil, the surface of the bone has a characteristic texture that shows that it has already been covered by cartilage. "This evidence of cartilage shows that the thumb was used in a more humane way to get rid of it," Simpson said. "It is a foot in transition, which shows primitive physical characteristics of afforestation, but also presents a more humane use of the foot to walk erect." In addition, when the chimpanzees are standing, their knees are "outside" the ankle, that is, they are crooked legs. When the humans are standing, the knees are directly above the ankle – what Simpson discovered was also true for the fossil Ardipithecus.
The Gona Project has been conducting continuous field research since 1999. The study area is located in the Afar portion of East Africa and its fossil-rich deposits span the last 6.3 million years. Gona is best known for documenting the earliest evidence of Oldowan stone tool technology. The earliest fossils of Ardipithecus ramidus in Gona were discovered in 1999 and described in the journal Nature in 2005. Gona also documented one of the earliest known human fossil ancestors – dated 6.3 million years ago. The Gona Project is co-directed by Sileshi Semaw, PhD, a research scientist at the CENIEH research center in Burgos, Spain, and Michael Rogers, PhD, Southern Connecticut State University. Geological and contextual research for the current research was led by Naomi Levin, PhD, University of Michigan, and Jay Quade, PhD, University of Arizona.
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