Primate evolution claim challenged

An analysis of 37 million year old primate fossils is fueling a debate over the existence of an evolutionary link between lemur-like and monkey-like primates -- a link that could more fully explain human evolution. The linkurl:study,;http://www.nature.com/nature/journal/v461/n7267/ published in this week's issue of Nature, challenges the linkurl:claim;http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0005723 that Darwinius -- a rare, almost-complete skeleton whose unveiling caused a media firestorm last May -- is the possible stem species to today's anthropoid primates, which include monkeys, apes, and humans.

Teeth and jaw remains of
Afradapis longicristatus
Image: Erik Seiffert et al, Nature 2009

"The paper is the first thorough, systematic treatment of the question" of whether there is an ancestral connection between the two primate subgroups, said linkurl:Chris Beard,;http://www.carnegiemnh.org/vp/cv/beard.htm chair of vertebrate paleontology at the Carnegie Museum of Natural History, who was not involved with the research. The new Nature paper, by linkurl:Erik Seiffert,;http://www.anat.stonybrook.edu/eseiffert/people.html a paleontologist at Stony Brook University in New York, and colleagues, presents a phylogenetic analysis of Afradapis longicristatus in which the fossilized specimen is classified as an adapiform -- or "adapoid" -- primate that left no known descendants. Adapiforms are typically described as having "lemur-like" characteristics and are evolutionarily linked to today's lemurs, lorises and galagos. But a small contingent of scientists believes the primates could be the evolutionary ancestors of anthropoids, commonly referred to as "higher primates." The analysis of the 47 million year old Darwinius fossils gave momentum to this alternative view. Seiffert and his colleagues shoot down the notion that adapiforms -- including both Afradapis and Darwinius -- represent the convergence of lemur-like and monkey-like primates and could have been the evolutionary stepping stone for higher primates. Instead, the researchers argue, the two sets of fossils are evidence there was a diverse group of competing species occupying similar ecological niches during the Eocene epoch 55 million years ago. Seiffert and his team first discovered the Afradapis fossils in northern Egypt in 2001 and have slowly been piecing together the entire upper and lower sets of teeth and anatomy of the jaw since. They compared more than 100 isolated teeth and jaw fragments to 360 morphological features in 117 living and extinct primate species. The researchers classified Afradapis as an adapiform because of the fossils' lemur-like dentition and jaw structure. Afradapis is also the largest leaf-eating primate ever documented in Afro-Arabia. The scientists wrote that they did not find enough anthropoid-like features to convince them that Afradapis is a direct ancestor of monkey-like primates; instead, they believed that those anthropoid-like features the fossils did have were the result of convergent evolution. "Our analysis indicates that Darwinius and its adapiform relatives played no role in the origin of Anthropoidea" -- that is, higher primates -- "and in fact are more closely related to the living lemurs and lorises," Seiffert wrote in an email from Egypt, where he is conducting further field work. "The last common ancestor that Darwinius shared with us was the same common ancestor that gave rise to all primates."

Proposed placement of Afradapis and Darwinius
within the primate family tree
Image: E.R.Seiffert, Stony Brook University

In the Darwinius study, published in PLoS last May, linkurl:JL Franzen,;http://www.nmb.bs.ch/jens-lorenz-franzen a paleontologist at the Senckenberg Research Institute in Germany, and his colleagues used cautious language to describe the evolutionary connection. Darwinius "could represent a stem group from which later anthropoid primates evolved, but we are not advocating this here," they wrote in the study. But the researchers made bolder claims to the press. The Times of London linkurl:quoted;http://www.timesonline.co.uk/tol/news/science/article6350095.ece# Franzen as saying the fossil, nicknamed Ida, was "the eighth wonder of the world." The same article quoted linkurl:Jorn Hurum;http://www.nhm.uio.no/om-museet/seksjonene/forskning-samlinger/ansatte/jhhurum/bio-eng.html of the University of Oslo, one of the study's coauthors, as saying that the "fossil rewrites our understanding of the evolution of primates." Headlines shouted that scientists had found the "missing link" between lemur-like and monkey-like primates, a discovery that offered new clues to the evolution of humans. The History Channel ran a documentary and two popular science writers published a book, The Link, soon after the announcement. Several scientists, however, have criticized the Darwinius paper's conclusions. "The Darwinius article in PLoS is a travesty," said linkurl:Callum Ross,;http://pondside.uchicago.edu/oba/faculty/ross_c.html a paleontologist from the University of Chicago. "The description is solid, but the analysis is outdated... and [there is] no comparison with [other] fossils." The Afradapis paper, said linkurl:Richard Kay,;http://evolutionaryanthropology.duke.edu/people?subpage=profile&Gurl=%2Faas%2FBAA&Uil=rich_kay a paleontologist at Duke University, is the correct analysis of Darwinius: specifically, that the skeleton's anthropoid-like features, and also those of Afradapis, such as jaw structure, do not mean it is a stem species to today's higher primates, but evidence that there was much more primate diversity than previously thought. The analysis, he said, is "elegant, thorough, and clear." Hurum disagreed with the Afradapis study's conclusions and stood behind his team's Darwinius interpretation. "We never claimed Darwinius to be an anthropoid as the authors of the paper insinuate," Hurum wrote in an email to The Scientist. "Although most [lemur-like primates] have obviously no phylogenetic descendants and some are contemporaneous with the earliest anthropoids, this does not exclude at least some of them from being members of the stem group from which all higher primates evolved." Hurum also noted that the Darwinius skeleton is much more complete than Afradapis and shows additional higher primate characteristics not available for analysis on Seiffert's fossils, such as bones in the foot. Seiffert and his team are still excavating the region in northern Egypt that produced the Afradapis fossils in hopes of finding additional bones from the species as well as from other primates. They also are trying to find bones from earlier and later time periods so they can "develop a better understanding of how primate communities changed in Africa through the middle and late Eocene," he wrote.
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