Madagascar split off from the African continent at least 100 million years ago, when there were no primates on Earth. Lemurs later appeared in Europe, North America, and Africa. They probably traveled from Africa to Madagascar when the island was not as far as it is today from Africa, on mats of vegetation flushed out of rivers. There they established and speciated (today there are 30 species of lemur on the island), while their ancestors elsewhere died out.
Lentiviruses cause chronic infections in a variety of mammals. The best known lentivirus is HIV-1, which has infected millions of humans and originated from the chimpanzee lentivirus simian immunodeficiency virus. It has been difficult to gauge the age of lentiviruses. Phylogenetic analysis of sequence data from lentiviruses can provide an idea of their recent history, but to trace further back – millions of years – requires the discovery of endogenous viruses integrated into the host cell genome. Up until 2007, no endogenous lentiviral genome had been found.
This situation changed in 2007 with the discovery of an endogenous lentivirus in the genome of the European rabbit. This finding made it possible to determine that lentiviruses are at least 7 million years old.
A year later, an endogenous lentivirus was discovered in the genome of the gray mouse lemur (pictured; image credit). Subsequently lentiviral insertions were discovered in the genomes of seven species of lemur from two different genera. These viruses are thought to have invaded the lemur genome approximately 4.2 million years ago – when the animals were firmly ensconced on Madagascar. How were they infected?
It€™s possible that the lemur arrived on Madagascar already infected with lentiviruses, but they did not invade the genome until much later. Or that a lentivirus-infected primate arrived on the island after the lemurs and infected them; these hypothetical animals subsequently must have died out. Finally, a vector capable of traversing the 400 km distance from Africa to Madagascar might have brought the virus over.
Which of these scenarios is correct is unknown, hence how lemurs became infected with lentiviruses remains a mystery.