A proviral puzzle with a prosimian twist.

V iruses are obligatory intracellular parasites, and as such they cannot exist in the absence of a host. It follows that the natural history of the viruses is inseparable from that of their animal hosts, and to reconstruct the evolutionary past of a virus is to understand much about the history of those species it colonizes. Unfortunately, viruses do not leave behind physical remnants of their presence: once a viral species becomes extinct, it vanishes without a trace. At best, the evolutionary history of most viruses can only be inferred indirectly through the phylogenetic comparison of modern, living viruses. There is, however, one prominent exception: the Retroviridae. The genomes of all animal species have accumulated (over hundreds of millions of years) the proviral remnants of ancient, largely extinct, retroviral species. This vast archive of viral ‘‘fossils’’ comprises millions upon millions of elements, a tiny fraction of which are coming to light as a consequence of genome sequencing efforts. From the genome of the gray mouse lemur, a diminutive primate ( 100 g) found only on the island of Madagascar, Gifford and colleagues (1) have now unearthed a retroviral fossil unambiguously related to the modern AIDS viruses, as reported in this issue of PNAS. How it came to be there is an intriguing, and as yet unsolved, evolutionary mystery.