Another Scientific Paper Challenges Ardi's Place as a Bipedal Human Ancestor - Evolution News & Views

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Another Scientific Paper Challenges Ardi's Place as a Bipedal Human Ancestor

Ardipithecus.jpgWhen Ardipithecus ramidus ("Ardi") was first unveiled to the world in 2009, the media went bananas. The Discovery Channel ran the headline "'Ardi,' Oldest Human Ancestor, Unveiled," and quoted a scientist stating that the fossil is "as close as we have ever come to finding the last common ancestor of chimpanzees and humans." The Associated Press headline read "World's oldest human-linked skeleton found," and stated "the new find provides evidence that chimps and humans evolved from some long-ago common ancestor."

The journal Science officially introduced Ardi with an article titled "A New Kind of Ancestor: Ardipithecus Unveiled." UC Berkeley paleoanthropologist Tim White promised the fossil would be the "Rosetta stone for understanding bipedalism" -- basically an ape-like bipedal ancestor of humans. Since that time, cooler heads have prevailed.

We previously covered papers in Nature and Science that challenged Ardi's status as a bipedal human ancestor. Now, another paper has been published in the journal HOMO: Journal of Comparative Human Biology titled "Behavioral and phylogenetic implications of a narrow allometric study of Ardipithecus ramidus." The authors, E.E. Sarmiento and D.J. Meldrum, argue that a closer analysis of Ardi shows she was not a habitual biped:

Comparatively short hands and upper limbs suggest Ardipithecus was less adept at forelimb suspension and vertical climbing than are great apes. Its tibial and tarsal lengths suggest bonobo-like leaping ability. Its short lower limbs, but long toes relative to humans, are not conducive to habitual bipedality. When terrestrial, Ardipithecus would have engaged in palmigrade quadrupedality.

(E.E. Sarmiento and D.J. Meldrum, "Behavioral and phylogenetic implications of a narrow allometric study of Ardipithecus ramidus," HOMO: Journal of Comparative Human Biology, Vol. 62:75-108 (2011).)

The authors continue, explaining why Ardipithecus was not bipedal:
Ardipithecus exhibits more generalized proportions than what may be expected for a fully arboreal or a fully terrestrial bipedal or quadrupedal ape. It lacks the derived lengths associated with the quadrumanous suspensory behaviors expressed in the extreme in orangutans. Conversely, it does not show the committed segment length proportions of the lower limb linked to habitual bipedalism or the compact hand and foot segment lengths associated with committed terrestriality. In some respects it more closely approximates gorillas, an animal retaining some degree of arboreality, whose proportions reflect both increased mass and some modifications for terrestriality (Sarmiento, 1994). This approximation need not reflect a shared ancestry. In lacking committed arboreal or terrestrial proportions Ardipithecus is likely to find resemblance with a variety of large-bodied Miocene apes.

[...]

Ardipithecus does not show the attenuated toes of humans that are associated with committed terrestrial bipedality or the attenuated fingers of baboons that reflect commitment to terrestrial quadrupedality. ... Nothing in its segment lengths or in the described anatomy suggests that it would have engaged in bipedal behaviors at higher frequencies than do any of the living great apes.

In the article's view: "Ardipithecus dimensions reflect a generalized ape, able to move in trees and on the ground, and exploit food sources in woodlands, grasslands and/or flooded terrain." As a result, the authors argue: "Parsimonious reconstruction of the common human/African ape ancestor suggests the short upper limbs and metacarpals of Ardipithecus are too derived to belong to an exclusive human ancestor."


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