HIV In Humans For 100 Years

Fascinating story from Science Magazine:

The human immunodeficiency virus (HIV-1) responsible for most of the AIDS cases in the world infected people approximately 100 years ago, more than 20 years earlier than previously believed, according to findings presented here this week at the Evolution 2008 meeting. Its lesser known cousin, HIV-2, jumped into humans decades later, from a monkey species that carried the virus for just a couple of hundred years, not the millions of years researchers had assumed, according to other research presented at the meeting.

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Marlea Gemmel, analyzed HIV-1 genetic material obtained from lymph tissue collected in 1960 from the University of Kinshasa pathology department in the Democratic Republic of the Congo--only the second HIV sequence predating 1976 deciphered to date. Thus far, she has sequenced about 1000 DNA bases, which she has compared with the previously reported sequence of HIV-1 extracted from a frozen blood sample from 1959. Since it entered into humans, HIV-1 has been evolving into different substrains--but the 1960 and 1959 sequences were much more divergent than expected, Gemmel reported at the meeting. "It reflects a long past of diversification before 1960," she said.

By comparing the two sequences with more recent ones, Gemmel was able to show that HIV-1 first entered humans about 1908, not 1931, as earlier analyses with just the 1959 sample found. Her analysis also indicates that the virus existed in low levels in humans until the middle of the 20th century. "That matches the rise of population centers," Gemmel explained, suggesting that urbanization around that time paved the way for the AIDS epidemic.

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AIDS experts have assumed that the disease is so severe in humans--yet less so in most monkeys--because humans have not had the time to evolve the proper defenses against the virus that many other primates have. "But the origins are around the same order of magnitude," and still the monkeys don't get sick, says Hillis. "It points out that there are other directions we need to go to understand [virulence]."

The key question seems to be if monkeys have only been infected with SIV for a hundred years longer than humans have had HIV, a mere blip in evolutionary terms, by what mechanism have they been able to withstand the virus?