Source the-scientist.com Image: Teeth and jaw of Afradapis (Image from the-scientist.com)

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 study, published in this week's issue of Nature, challenges the claim 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.

"The paper is the first thorough, systematic treatment of the question" of whether there is an ancestral connection between the two primate subgroups, said Chris Beard, chair of vertebrate paleontology at the Carnegie Museum of Natural History, who was not involved with the research.

The new Nature paper, by Erik Seiffert, 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 evolutionary 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."

Posted by Katherine Bagley