Viral archeologists have been trying to track down and understand the spread of HIV-1, the strain responsible for most HIV infections worldwide. In particular, researchers are interested in the M (major) group, which is responsible for over 90% of the infections. Although scientists first identified HIV as the cause of AIDS in 1984, the virus has been spreading among humans long before that. Since HIV evolves up to a million times faster than we do, it is advantageous to examine and compare HIV-1 samples from as early as possible. Finding these samples is the limiting factor. Only one early-period HIV-1, called ZR59, was known about until now, but today's Nature unveils another "ancient" sample, giving us a more complete understanding of HIV-1's prehistory.

In a collaboration, eight research groups isolated and studied partial HIV-1 genome fragments from a lymph node that was obtained in 1960 at what is now known as Kinshasa, Democratic Republic of Congo. This "ancient" virus, dubbed DRC60, shares about 88 percent sequence similarity to ZR59. Out of the 11 classified subtypes of group M HIV-1 strains (A-K), DRC60 is most closely related to the ancestor of subtype A, while ZR59 is associated with the one for subtype D. This difference shows that diversification of HIV-1 was already underway half a century ago.

As both samples come from Kinshasa, a comparison of their sequences can reveal the existence of a common ancestor. By performing statistical analyses with models that account for the rates of evolution and various methods of HIV-1 pandemic spread, the researchers found that the viral ancestor existed between 1902 and 1921.

Even though only two "ancient" sequences from Kinshasa are known, scientists had already inferred that the ancestors of all group M viruses originated there, as it is the location with the most diverse number of group M subtypes in modern times. If that is the case, the simian immunodeficiency virus (SIV) that plagued chimpanzees jumped to humans at Kinshasa in the early 1900s and was likely to be related to M group viruses.

This possibility raised a couple of additional questions. First, chimpanzees infected with SIV that are most similar to group M strains live about 435 miles away from Kinshasa, in the southeast corner of Cameroon. So, why did the virus originate in Kinshasa? Second, what made the early 1900s particularly favorable for the start of an infection spread?

The researchers point to the rise of cities as the reason. Kinshasa was known as Léopoldville at the time, and its location near the mouth of the Congo River made it a key transportation spot. When populations grew in the early 1900s, Léopoldville became the largest city in the area. Infected chimpanzees and/or their handlers could have easily arrived there, and the sufficient population density allowed the virus to take hold.

The isolation of DRC60 allowed researchers to make significant progress in their efforts to trace the origin of HIV-1. Samples preserved in other hospitals in the region hold the promise of further discoveries of other early HIV strains, adding more details to the history.

Nature, 2008. DOIs: 10.1038/nature07390 and 10.1038/455605a