The role of Ardipithecus ramidus in the evolution of bipedalism in humans

An article published in the journal “eLife” reports the claim that the foot morphology of Ardipithecus ramidus, a hominin that lived about 4.4 million years ago, is the most similar to that of chimpanzees and gorillas among apes. Thomas Prang of the New York University’s Department of Anthropology examined the characteristics of Ardipithecus ramidus’ feet to assess the relationship between the type of locomotion of a species and its skeletal characteristics. The aim is to reconstruct the evolution of bipedalism and estimate the appearance of the last common ancestor of humans and chimpanzees.

Ardipithecus ramidus (Photo of the skeleton of the individual nicknamed Ardi ©Chartep) is a hominin whose fossil remains have been discovered since 1992 in the Afar, Ethiopia, not far from the place where they found the fossil bones of Lucy, the Australopithecus afarensis which at the time of her discovery was the oldest known hominin with her 3.2 million years. The dating of the first individual discovered, a female nicknamed Ardi, indicated that she was much older going back to about 4.4 million years ago.

Also thanks to new discoveries of other fossils, although partial, over the years it was possible to establish that Ardipithecus ramidus had characteristics closer to apes than to humans. This new research focuses on this species as a key to understanding the evolution of bipedalism.

According to Thomas Prang, humans evolved from ancestors who adapted to living on the ground, perhaps in a similar way to African apes. However, after the discovery of Ardipithecus ramidus fossils the first researchers’ interpretation offered in a study published in 2009 was that their feet looked more like those of monkeys. Instead, according to Prang, the morphology of the these hominins’ feet suggests that human bipedalism is derived from a form of locomotion similar to those of African apes.

The top image (Courtesy Thomas Prang. All rights reserved) shows an evolutionary tree with the relationships between living apes, Ardipithecus ramidus and Homo sapiens. The circles represent evolutionary changes hypothesized to explain the adaptations to life on the ground in the common ancestor of African apes and human but also the evolution of bipedalism.

Thomas Prang emphasized the importance of the natural evolutionary context in which ancient hominins developed bipedal locomotion, whose history is revealed by the study of fossils. He added that bipedalism was preceded by an adaptation to quadrupedal locomotion on the ground in the common ancestors of Homo sapiens and apes.

This is just the latest of various studies on Ardipithecus ramidus, a really interesting hominin to understand the evolution of human beings and the diversification with respect to apes. The discoveries of hominin fossils and the understanding of the environments in which they lived is leading to the reconstruction of the evolutionary drives that led different populations of ancient hominins to develop different adaptations that had as the result the species existing today: Homo sapiens, chimpanzees, bonobos, gorillas, orangutan, and gibbon.

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