Each newly sequenced genome allows scientists to determine how humans stack up against other members of the animal kingdom. As it turns out, our genome is rather similar to that of the puffer fish, and we have only about twice as many genes as a worm or a fly. Now a report published online this week by the Proceedings of the National Academy of Sciences identifies a gene specific to hominoids--the group made up of apes and humans--that arose as recently as 21 million years ago. Because the gene is expressed primarily in the testes, the researchers say it could have played a role in speciation, eventually helping humans become humans.

The gene described in the new work, Tre2, was first identified by the Human Genome Project. After noticing that mice and other mammals lacked the gene, Charles A. Paulding of Harvard Medical School and his colleagues set out to trace its evolution in primates. By comparing its sequence to genetic data from animals as varied as the C. elegans worm, chickens, chimpanzees and gorillas, the team determined that Tre2 was a hybrid of two different genes, USP32 and TBC1D3. The former is evolutionarily ancient and exists in many mammals. The latter, in contrast, is present only in primates that are closely related to humans. The scientists posit that the two genes fused to form Tre2 between 21 and 33 million years ago, around the same time the hominoid lineage appeared.

Unlike its predecessors, which are expressed in tissues throughout the body, the authors report that Tre2 is testis-specific. "The abrupt creation of a mosaic gene with novel functions, especially for genes involved in reproduction, could potentially lead to reproductive barriers," they write. These, in turn, could result in speciation. But much more specific comparisons to other primate genomes will be required to determine just what role, if any, Tre2 played in generating human-specific features.

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