Through the use of sophisticated genetic analysis scientists have determined that a monkey virus called SIV, which is believed to have mutated into HIV after it crossed over from monkeys to humans, may have existed in certain primate populations for at least 32,000 years. Amazingly, HIV’s genetic ancestor may be even older than this estimate-which has lead some to wonder why the global HIV pandemic didn’t happen centuries earlier.
SIV doesn’t cause AIDS in a species of monkeys called sooty mangabeys, as it has evolved to replicate itself to very high levels in the blood of these primates without significantly damaging their immune system. However, when SIV is transmitted between primates to a Asian rhesus Macaque, then the animal can develop a version of primate AIDS.
The researchers who completed the study of SIV believe that the virus may have only been able to “take off” in the human population once large cities were established in Africa. The higher density of humans living in cities may have allowed HIV to spread much more quickly. At the beginning of the 20th Century in Africa there were not cities larger than 10,000 people, whereas now many contain millions of people. Meaning that a person living in a rural village may have developed HIV and AIDS centuries ago, but they would have had much less of a chance to pass the virus on, and the infection could have “burned out” and not have been passed beyond their village.
However, it is also possible that a number of other factors have contributed to the spread of HIV in Africa, such as the construction of a freeway system and the breakdown of traditional African culture due to apartheid and colonialism.
In addition, it is also possible that certain genetic factors may predispose people living in Africa to higher rates of HIV infection. One of these genetic factors has been elucidated and is a mutation which blocks the production of a protein present on the surface of red blood cells. This mutation which protects against certain types of malaria, appears to have had the effect of increasing the susceptibility to infection with HIV. It is possible that other factors, perhaps genetic or a co-infection with another infectious organism, could have helped to precipitate the HIV/AIDS crisis in hard-hit regions in Africa.
Island Biogeography Reveals the Deep History of SIV
Worobey et al.
Science 17 September 2010: 1487