40-million-year-old Primate Fossils Debated
Field Museum scientist challenges theory that primates originated in Africa
CHICAGO — Fossils recently discovered in Egypt fill one of the gaps in the evolutionary tree of primates and double the known age of one of its main branches.
Because there are still so many gaps in the tree, statistical calculations push the origin of primates, including the earliest ancestors of humans, back to about 90 million years ago — at least 20 million years earlier than previously thought.
Analyses of DNA sequences confirm this early date and indicate that, contrary to a widespread assumption, primates did not originate in Africa.
The new fossil evidence supports a novel proposal that lemurs, lorises and bushbabies, which comprise a sister group to higher primates, might have originated in Indo-Madagascar rather than in Africa.
So says Robert D. Martin, Provost and Vice President of Academic Affairs at The Field Museum in Chicago, and author of a commentary about a paper by Seiffert and colleagues that describes the new primate fossils. Both the paper and the commentary will be published in the March 27 issue of Nature.
Martin calls the fossils “spectacular,” but disagrees with the researchers’ own interpretations. “It’s great how in one fell swoop, the researchers have doubled the known age of lorises and bushbabies,” he says. “Unfortunately, however, they are still trying to squeeze their findings into a traditional model of primate evolution that is based on an insufficient fossil record, which leads to a serious underestimation of divergence times for the entire group.”
Consisting of teeth and jaw fragments, the fossils were found in the Fayum Depression on the eastern edge of the Sahara Desert in Egypt, the best site for early African fossil mammals.
They represent Saharagalago (a bushbaby) and Karanisia (a loris), adding two new genera to only four that were previously known for bushbabies and lorises.
The well-preserved teeth date from about 40 million years ago, twice as old as the already known fossils for these two groups of primates.
In Karanisia, the teeth at the front of the lower jaw formed a toothcomb like that typically seen in living strepsirrhine primates, which includes bushbabies, lorises and lemurs, as one of their defining characteristics.
Amazingly, wear patterns on the teeth indicate that Karanisia used its toothcomb for grooming, just as its modern relatives do today. The authors report that the sides of the teeth show irregularly spaced microscopic grooves created by the passage of hair during grooming.
Seiffert and colleagues conclude that the new finds are compatible with the widely accepted view that the lorises and bushbabies (as a group) diverged from lemurs 50 million to 53 million years ago, and that their last common ancestor lived in Afro-Arabia.
Martin’s alternative interpretation stems from statistical models indicating that gaps in the fossil record have led scientists to underestimate divergence times throughout the primate evolutionary tree. Furthermore, new molecular trees also indicate that primates originated much earlier than generally accepted.
“One possibility is that the strepsirrhine primates originally inhabited Indo-Madagascar, rather than Africa, and that lemurs became isolated when Madagascar separated from India,” Martin says. “Subsequently, lorises could have migrated to Africa after India collided with Asia.”
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