Fine Toothcomb: New fossils add to primate-origins debate

March 26, 2003 at 3:22 pm

Fossil hunters working in Egypt have unearthed jaw fragments and teeth belonging to the oldest known members of one of the main evolutionary branches of primates.

TANGLED TREE. Primate evolutionary diagram shows the two main living groups, strepsirrhines and haplorhines, with known fossil forms of strepsirrhines. Traditional model of primate evolution (dark lines) contrasts with DNA-based version (shaded lines). Field Museum/Beerli-Bieler/Seagard

The 40-million-year-old specimens represent two ancient groups, one an ancestor of modern lorises–complete with a comb-like set of lower front teeth that confirms its identity–and the other, of bushbabies, say anthropologist Erik R. Seiffert of Duke University in Durham, N.C., and his colleagues. These new finds double the age of the sparse fossil record for lorises and bushbabies, which with lemurs make up a primate group called the strepsirrhines. It originated in Africa at least 50 million years ago, the scientists conclude in the March 27 Nature.

“The new fossils date to near the evolutionary split of lorises and bushbabies from lemurs, which occurred perhaps 45 million years ago,” Seiffert says.

Such estimates fit with the traditional notion that the first primates appeared around 65 million years ago. However, other researchers using DNA analyses have concluded that primates arose approximately 90 million years ago (SN: 4/20/02, p. 243: Available to subscribers at Older Ancestors: Primate origins age in new analysis).

In a comment published with the new report, Robert D. Martin of the Field Museum in Chicago cites the genetic data in support of his view that strepsirrhines originated in southern Asia around 80 million years ago. In his view, lorises and bushbabies took a unique evolutionary route 10 million years later, perhaps when lemurs became isolated on the island of Madagascar after it separated from India.

“The timing and location of primate origins remains a complex problem,” cautions evolutionary biologist Anne D. Yoder of Yale University, who has conducted primate-DNA studies. “Still, the new fossils from Egypt are terribly exciting.”

Seiffert’s group excavated a site in the Fayum Depression on the eastern edge of the Sahara Desert. They assign some of their finds to the genus Saharagalago, an ancestor of living bushbabies that weighed around 1/4 pound. The rest come from the genus Karanisia, an ancient loris that tipped the scales at an estimated 2/3 pound.

Of particular interest, the researchers say, are teeth at the front of a Karanisia lower-jaw fragment that form a toothcomb like that of today’s strepsirrhines. The toothcomb consists of elongated, flattened teeth that angle sharply forward.

Microscopic grooves on the fossil teeth indicate that Karanisia used its toothcomb for grooming, just as its living relatives do, Seiffert says.

Despite an intriguing find in Pakistan several years ago, no definitive lemur fossils have been found, Martin holds (SN: 10/20/01, p. 245: Available to subscribers at Lemurs reveal clues to ancient Asian roots).

“We need to find lemur fossils [at Fayum],” Seiffert says. Without such a find, he notes, fossil reconstructions of primate origin will remain shaky.

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