The AIDS virus, previously thought to have been transmitted from chimps to humans in the 1930s, may have leapt the species barrier more than a century ago in west-central Africa, scientists said on Wednesday.

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).

By this calculation, HIV began to spread among humans before 1940, according to ZR59's genes.

Now, though, another precious piece of the jigsaw has emerged.

It is a piece of lymph node tissue that was taken for a biopsy from a woman in Kinshasa in 1960 and preserved in a bed of paraffin wax. It was found in the archives of the Anatomy Department at the University of Kinshasa.

An international team of sleuths pieced together the genetic sequence of the virus -- the sub-group M of HIV-1 -- and then compared telltale regions between ZR59 and the second sample, DRC60.

They found a significant divergence between the two genetic regions, and calculate that this gap must have taken around 40 years to evolve from a common viral ancestor.

In other words, the ancestral virus began to be transmitted among humans at the start of the century -- the estimated range is between 1884 and 1924.

The virus spread only very slowly at first but got a vital foothold thanks to urbanisation during the colonial era, the authors speculate. It was transmitted through sex and then was taken further afield through commerce.

"The founding of and growth of colonial administrative and trading centres such as Kinshasa may have enabled the region to become the epicentre of the HIV/AIDS pandemic," they suggest.

Kinshasa was founded in 1881, Brazzaville (capital of today's Republic of Congo) in 1883 and Yaounde (Cameroun) in 1890, while Bangui (Central African Republic) was established in 1899.

All of these towns were founded before or at around the time that HIV-1 is believed to have entered the human population, the investigators note.

The growth of these towns was at first slow. Until 1910, not one of them had a population of more than 10,000 people.

There are several theories that seek to explain how SIV entered humans, the animal primates' closest relative.

An infected chimpanzee bit a human, or a SIV-infected ape was butchered and sold for bushmeat, and the virus entered the bloodstream through tiny cuts in the hand, according to these hypotheses.

The new research was led by Michael Worobey of the Department of Ecology and Evolutionary Biology at the University of Arizona in Tucson.