The First Chimpanzee

Marilyn hardly seemed destined to make history. Unlike other chimpanzees kept at Holloman Air Force Base in New Mexico during the 1960s and '70s to study the effects of space travel--like Ham, the first primate in orbit--she never made it off the ground. Instead she did her duty as a breeder, a friendly, easily handled chimp mom who gave birth at least a dozen times before her untimely death in 1985 at age 26.

Now, long after the Air Force abandoned its corps of chimps and monkeys to other scientific custodians, Marilyn is getting belated recognition. In this week's Nature, researchers at the University of Alabama in Birmingham report that Marilyn's frozen tissue, carefully preserved all these years, may have solved a pair of lingering medical mysteries: where the dominant form of the AIDS virus originated in the animal world, and how it made the deadly leap to humans. More than brilliant scientific detective work, the Alabama research, if it turns out to be correct, could lead to new treatments and possibly even a cure for a fatal disease that afflicts more than 35 million people around the globe.

For some time, scientists have known that a small African monkey called the sooty mangabey is the natural reservoir of a virus very similar to HIV-2, which causes a milder form of AIDS found largely in western Africa. But the source of HIV-1, the dominant cause of the AIDS pandemic, has remained elusive to virus hunters like Alabama's Dr. Beatrice Hahn. Long on the trail of links between HIV and kindred simian viruses, she jumped at the chance to examine old tissue samples (stored, as it turns out, in a freezer at the National Cancer Institute) from the only chimp in the Air Force colony to have tested positive for HIV.

Subjecting these cells from Marilyn's spleen to the sort of genetic wizardry unavailable during the animal's lifetime, Hahn and her colleagues, including Feng Gao, husband George Shaw and Paul Sharp, set about amplifying, sequencing and analyzing Marilyn's virus. Except in the rarest cases, chimps like the sooty mangabeys never show AIDS-like symptoms. Even so, when the researchers compared the viral DNA with the three known types of SIV (simian immunodeficiency virus), they found it had a substantially different genetic makeup. And when they compared Marilyn's genetic makeup with that of other chimps, they determined that she belonged to a different subspecies than the chimps that harbor the other SIV strains; those kindred chimps live farther east in the equatorial African rain forest. More important, Marilyn's virus closely matched the three major groupings of HIV-1 strains, whereas the other simian viruses appeared only remotely like the human virus.

To the researchers, this suggested that the chimp virus had mutated and crossed over to humans on at least three separate occasions, each time finding man a more congenial host than ape. The momentous leaps, Hahn speculates, could have occurred when hunters came in contact with infected blood while butchering chimps for food, a common practice in Africa. As it happens, the first documented case of AIDS goes back to 1959, when a man living in Kinshasa, just across the Congo River from Gabon, home of Marilyn's kin, died of the disease.