The oldest documented specimen of the virus that causes acquired immune deficiency syndrome (AIDS), contained in a blood sample collected in 1959, may be a common ancestor of major strains of the virus that have infected tens of millions of people worldwide since the 1980s, according to a study published in the journal Nature on February 5, 1998.

Study authors said that genetic analysis of the virus, a form of the human immunodeficiency virus (HIV) known as HIV-1, suggests that two of the ten major subtypes of HIV-1, and possibly all of them, evolved from a single strain introduced into sub-Saharan Africa. They also noted that HIV-1, the most common form of the AIDS virus, probably first infected humans in the 1940s or early 1950s, about 10 to 20 years earlier than many researchers had estimated.

In 1986 scientists discovered evidence of HIV, a virus known to mutate rapidly, in 1 of 1213 blood samples collected in Africa between 1959 and 1982. The sample was taken in 1959 from a Bantu man who lived in Leopoldville, Belgian Congo, now Kinshasa, Democratic Republic of the Congo. The virus in the sample was identified as HIV-1, but the sample's relevant genetic material, its deoxyribonucleic acid (DNA), had degraded, making comparisons to other known strains of HIV difficult.

The new study, conducted at the Aaron Diamond AIDS Research Center in New York City, used sophisticated tools of genetic analysis to identify key DNA fragments in the sample. The researchers then compared the virus to other strains of HIV-1 and determined that it was probably a common ancestor of two major HIV-1 subtypes, B and D.

Because HIV is believed to mutate at a constant rate, the researchers concluded that the sample collected in 1959 was a primitive form of the virus, and that it was probably transmitted from monkeys to humans one or two decades earlier. The major subtypes of HIV-1 then likely evolved in humans from this common ancestor virus, the researchers said. The conclusion contradicts a widespread theory that the proliferation of different strains of HIV in humans derived from numerous human contacts with monkeys carrying variations of the virus.