E. R. Seiffert, Stony Brook University; E. R. Seiffert et al., Nature 2009 (Inset)

Dead end. The discovery of Afradapis suggests that Ida (Darwinius) has no connection to modern humans.

New Primate Fossil Poses Further Challenge to Ida

Ann is a contributing correspondent for Science

Pour more rain on Ida's parade. Last May, a team of researchers announced that the 47-million-year-old lemurlike fossil represented a missing link between primitive primates and humans. Many paleoanthropologists were skeptical, however, and shortly thereafter a new find challenged Ida's status. Now, another skeleton has come out of Ida's closet: a younger relative from Egypt that shows Ida's resemblance to apes and humans is only superficial.

Researchers have named the new primate Afradapis longicristatus, a name that pegs it as an African member of an extinct group of primitive primates known as Adapiformes, to which Ida also belongs. Some researchers have long argued that Adapiformes might be primitive relatives of anthropoids--the higher primates including monkeys, apes, and humans--rather than ancestors of lorises and lemurs. But new fossils found over the past 20 years in Asia and Africa proved to be better candidates for the earliest anthropoids. The discovery of anthropoid-like features in the remarkably complete skeleton of Ida (Darwinius masillae), however, led the researchers who analyzed her remains to resurrect the view that Ida and, hence, adapids, were direct ancestors of anthropoids.

If Ida were truly a missing link between early primates and us, then Afradapis, which appeared 10 million years after Ida, should also share the same traits with the earliest undisputed anthropoids that were alive at about the same time and in the same place. But that's not what Erik Seiffert of Stony Brook University in New York state and Elwyn Simons of Duke University in Durham, North Carolina, and their colleagues found when they unearthed Afradapis in the Fayum desert of Egypt in 2001. Working with 100 teeth and jaws from multiple individuals--all about 37 million years old--they compared 360 morphological traits in 117 living and extinct primates, in the most complete analysis assembled so far of extinct primates. When they scored Ida and Afradapis against those other primates, Seiffert and colleagues found that adapids do share some traits with anthropoids, such as the loss of a third upper and lower premolar. But these traits evolved more than once among primates, the team reports tomorrow in Nature. They are the result of convergent evolution, which is the acquisition of the same biological trait in unrelated lineages--and, thus, do not indicate inheritance of the trait from a shared ancestor.

Indeed, other extinct primates, including two species from the Fayum in Egypt that are about the same age as Afradapis, had not evolved those so-called anthropoid-like traits (such as the loss of the premolar). And another new discovery of a 37-million-year-old primate from Asia called Ganlea megacanina also had features that suggest it is an early anthropoid--but not the same ones that supposedly tie Ida or Afradapis to our lineage. All these lines of evidence suggest that those "anthropoid-like" features emerged later and independently in anthropoids, says Seiffert. "It's coincidental."

One of the researchers who studied Ida, however, responds that Ida and Afradapis look more like the group that gave rise to anthropoids than the group that gave rise to lemurs and lorises--and that there are too many traits to dismiss as convergent evolution. "The complete convergence postulated for Afradapis seems implausible to me," says paleontologist Philip Gingerich of the University of Michigan, Ann Arbor.

Other researchers, however, say that the new analysis convincingly shows that adapids, including Ida, were not our direct ancestor. "Adapids never were a good bet for [higher primate] relatives, and the more data we get, the worse the case gets," says paleontologist Callum Ross of the University of Chicago in Illinois, who was not involved in either the Ida or Afradapis papers. Adds paleontologist K. Chris Beard of the Carnegie Museum of Natural History in Pittsburgh: "This debate over adapids would have been dead long ago if it weren't for Ida."

Posted in Paleontology