Analysis of tissues preserved by doctors in the colonial-era Belgian Congo shows that the most pervasive strain of the human immunodeficiency virus (HIV) began spreading among humans at some point between 1884 and 1924.

"The diversification of HIV-1 in west-central Africa occurred long before the recognised AIDS pandemic," they announced in the British-based science journal Nature.

AIDS first came to public notice in 1981, when alert US doctors noted an unusual cluster of deaths among young homosexuals in California and New York.

It has since killed at least 25 million people, and 33 million others are living with the disease or HIV, the virus that causes AIDS by destroying immune cells.

Epidemiologists trying to date the history of HIV have until now been limited to only one laboratory source that long precedes the detected start of the outbreak.

This is a now-legendary blood sample called ZR59, which was taken in 1959 from a patient in Leopoldville, now Kinshasa, then capital of the Belgian Congo, now the Democratic Republic of Congo (DRC).

HIV is highly mutating virus, with as much as one percent of its genome diverging per year.

This rate of mutation gives rise to a measurement called a "molecular clock," a timescale at which the HIV deviates from previous strains and from its animal ancestor, the simian immunodeficiency virus (SIV).