Revealing new details about the origins of AIDS, scientists said on Monday half the lineages of the main type of human immunodeficiency virus, HIV-1, originated in gorillas in Cameroon before infecting people, probably via bushmeat hunting.

HIV-1, which causes AIDS, is composed of four groups, each coming from a separate cross-species transmission of a simian version of the virus from apes to humans.

Previous research identified chimpanzees in southern Cameroon as the source of HIV-1 group M, which has infected more than 40 million people worldwide and triggered the AIDS pandemic, as well as the geographically limited group N, identified in only about 20 people.

Until now, the source of the two other groups, known as O and P, had not been confirmed. The new research, published in the journal Proceedings of the National Academy of Sciences, showed those groups originated in western lowland gorillas in southern Cameroon.

"Thus, both chimpanzees and gorillas harbor viruses that are capable of crossing the species barrier to humans and causing major disease outbreaks," said virologist Martine Peeters of the Institute for Research and Development and University of Montpellier in France.

The researchers examined fecal samples from different gorillas across central Africa, including western lowland gorillas, eastern lowland gorillas and mountain gorillas in Cameroon, Gabon, the Democratic Republic of Congo and Uganda, for evidence of the gorilla version of HIV.

Genetic analyses implicated the western lowland gorillas as the source of HIV-1 groups O and P.

"The mode of transmission is most likely exposure to infected blood and/or tissues during hunting and butchering for bushmeat," Peeters said.

Group O viruses, the second most common HIV-1 lineage, have spread across Cameroon, Gabon, Nigeria and neighboring countries and have infected about 100,000 people. Group P viruses have been documented in just two Cameroonian patients.

The researchers said group O emerged at the beginning of the 20th century. Group P arose sometime later that century.

University of Pennsylvania microbiologist Beatrice Hahn, the study's other co-leader, said there does not appear to be any inherent viral property that prevented the group O lineage from becoming a pandemic-causing pathogen like group M. "Hence, on this occasion, humans got lucky," Hahn said.

Another virus type, called HIV-2, is mainly restricted to West Africa, less easily transmitted than HIV-1 and has a slower progression to AIDS. It was transmitted from monkeys called sooty mangabeys to humans in West Africa.