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See Inside August 2009

Weak Link: Fossil Darwinius Has Its 15 Minutes

Skepticism about a fossil cast as a missing link in human ancestry

On May 19 the world met a most unlikely celebrity: the fossilized carcass of a housecat-size primate that lived 47 million years ago in a rain forest in what is now Germany. The specimen, a juvenile female, represents a genus and species new to science, Darwinius masillae, although the media-savvy researchers who unveiled her were quick to give her a user-friendly nickname, Ida. And in an elaborate public-relations campaign, in which the release of a Web site, a book and a documentary on the History Channel were timed to coincide with the publication of the scientific paper describing her in PLoS ONE, Ida’s significance was described in no uncertain terms as the missing link between us humans and our primate kin. In news reports, team members called her “the eighth wonder of the world,” “the Holy Grail,” and “a Rosetta Stone.”

The orchestration paid off, as Ida graced the front page of countless newspapers and made appearances on the morning (and evening) news programs. Gossip outlets, such as People and Gawker, took note of her, too. And Google incorporated her image into its logo on the main search page for a day.

But a number of outside experts have criticized these claims. Not only is Ida too old to reveal anything about the evolution of humans in particular (the earliest putative human ancestors are a mere seven million years old), but she may not even be particularly closely related to the so-called anthropoid branch of the primate family tree that includes monkeys, apes and us.
Scientists have long debated the origin of the anthropoids, also known as the higher primates. The predominant view holds that a group of tarsierlike creatures known as the omomyiforms spawned the anthropoids. Some authorities, however, believe that anthropoids instead arose from a group of extinct primates called the adapiforms.

Enter Ida. University of Oslo paleontologist Jørn H. Hurum and his team classify Ida as an adapiform and contend that she also exhibits a number of anthropoidlike characteristics, such as the spatulate shape of her incisor teeth, the absence of a so-called grooming claw on her second toe, and a partially fused lower jaw. They believe that Ida could be on the line leading to anthropoids, thus linking that group and the adapiforms.

Critics concur that Ida is an adapiform, but they dispute the alleged ties to anthropoids. Robert Martin of the Field Museum in Chicago charges that some of the traits used to align Ida with the anthropoids do not in fact support such a relationship. Fusion of the lower jaw, for instance, is not present in the earliest unequivocal anthropoids, suggesting that it was not an ancestral feature of this group. Moreover, the trait has arisen independently in several lineages of mammals—including some lemurs—through convergent evolution. Martin further notes that Ida also lacks a defining feature of the anthropoids: a bony wall at the back of the eye socket. “I am utterly convinced that Darwinius has nothing whatsoever to do with the origin of higher primates,” he declares.

Adapiforms “are related to the strepsirrhine group of living primates that include lemurs from Madagascar and galagos [bush babies] and lorises from Africa and Asia,” insists paleontologist Richard F. Kay of Duke University. Claims by the authors to the contrary notwithstanding, he adds, “they are decidedly not in the direct line leading to living monkeys, apes and humans.” Kay and others believe that a primitive primate from China called Eosimias is a better candidate ancestor of anthropoids than is Darwinius.

If the detractors are right, Ida is irrelevant to the question of anthropoid—and thus, human—origins. That does not mean she is without value, though. Unlike Eosimias, which is known only from its fossilized teeth and jaws, Ida is spectacularly complete. Her entire skeleton is preserved, as well as traces of her last meal and impressions of her body contour and fur. Already Hurum’s team has deduced that Ida was good at running and leaping in the trees of her rain forest home, that she grew up relatively quickly, that she dined on leaves and fruits, and that she may have been nocturnal.

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