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As soon as HIV was identified in 1983, scientists started trying to understand where it had come from, when it had arisen, and why it had spread. Were they too late? To answer most of their questions, they would have had to witness the virus’s evolution. Scientists can track new pathogens such as SARS and avian flu because they produce obvious symptoms almost immediately, but HIV is a stealth virus that takes as many as 10 years to present symptoms; by the time researchers knew enough to wonder about its origins, those origins were in the distant past. For the last 23 years, scientists have been trying to peer into that past. Jon Cohen, a correspondent for Science who has written extensively about the virus, compares the work to fossil hunting, using a few precious shreds of evidence to construct a possible history. “Everybody’s always looking for certainty. It doesn’t exist [in this field],” he says. “In a sense, it’s all theory.”
For the last 23 years, scientists have been trying to peer into that past. Jon Cohen, a correspondent for Science who has written extensively about the virus, compares the work to fossil hunting, using a few precious shreds of evidence to construct a possible history. “Everybody’s always looking for certainty. It doesn’t exist [in this field],” he says. “In a sense, it’s all theory.” Nonetheless, the theory rests on facts, and at least a few of them are undisputed — including, most significantly, HIV’s family tree. There are two species of the virus, HIV-1 and HIV-2. The first evolved from a simian immunodeficiency virus (SIV) found in chimpanzees, while the second came from an SIV in a type of monkey called the sooty mangabey. HIV-1, which is responsible for the vast majority of AIDS cases worldwide, is divided into three groups — the “major” group M, and the much rarer “outlier” group O and “new” group N — that have diverged over years of mutation and evolution. Within the M group — which makes up 90 percent of all infections worldwide — there are at least nine strains, known as “clades,” of HIV-1 that are constantly mutating and merging with each other, creating yet more new varieties.
As soon as HIV was identified in 1983, scientists started trying to understand where it had come from, when it had arisen, and why it had spread. Were they too late? To answer most of their questions, they would have had to witness the virus’s evolution. Scientists can track new pathogens such as SARS and avian flu because they produce obvious symptoms almost immediately, but HIV is a stealth virus that takes as many as 10 years to present symptoms; by the time researchers knew enough to wonder about its origins, those origins were in the distant past. For the last 23 years, scientists have been trying to peer into that past. Jon Cohen, a correspondent for Science who has written extensively about the virus, compares the work to fossil hunting, using a few precious shreds of evidence to construct a possible history. “Everybody’s always looking for certainty. It doesn’t exist [in this field],” he says. “In a sense, it’s all theory.” Nonetheless, the theory rests on facts, and at least a few of them are undisputed — including, most significantly, HIV’s family tree.
Nonetheless, the theory rests on facts, and at least a few of them are undisputed — including, most significantly, HIV’s family tree. There are two species of the virus, HIV-1 and HIV-2. The first evolved from a simian immunodeficiency virus (SIV) found in chimpanzees, while the second came from an SIV in a type of monkey called the sooty mangabey. HIV-1, which is responsible for the vast majority of AIDS cases worldwide, is divided into three groups — the “major” group M, and the much rarer “outlier” group O and “new” group N — that have diverged over years of mutation and evolution. Within the M group — which makes up 90 percent of all infections worldwide — there are at least nine strains, known as “clades,” of HIV-1 that are constantly mutating and merging with each other, creating yet more new varieties. “The M group epidemiologically has overwhelmed what else is out there,” says Dr. Beatrice Hahn of the University of Alabama at Birmingham, who has conducted much of the research into HIV’s origin. HIV-2, on the other hand, is not as virulent and is largely confined to West Africa, where it originated. In May 2006, an international group of researchers led by Hahn answered two major questions about the origin of HIV-1 M, the deadliest and most widespread form of the virus: Where was its cradle, and what kind of chimp did it come from?
HIV-1, which is responsible for the vast majority of AIDS cases worldwide, is divided into three groups — the “major” group M, and the much rarer “outlier” group O and “new” group N — that have diverged over years of mutation and evolution. Within the M group — which makes up 90 percent of all infections worldwide — there are at least nine strains, known as “clades,” of HIV-1 that are constantly mutating and merging with each other, creating yet more new varieties. “The M group epidemiologically has overwhelmed what else is out there,” says Dr. Beatrice Hahn of the University of Alabama at Birmingham, who has conducted much of the research into HIV’s origin. HIV-2, on the other hand, is not as virulent and is largely confined to West Africa, where it originated. In May 2006, an international group of researchers led by Hahn answered two major questions about the origin of HIV-1 M, the deadliest and most widespread form of the virus: Where was its cradle, and what kind of chimp did it come from? Answering the questions was literally messy work — researchers collected 599 waste samples from wild chimpanzees and analyzed the viral particles they contained — but the results were immaculate. Three populations of Pan troglodytes troglodytes (chimpanzees) living in southern Cameroon provided the crucial data. Two of those populations currently carry SIVs that are molecular dead ringers for HIV-1 M, while many chimps in the third group are infected with an SIV remarkably similar to HIV-1 N. Group O’s simian sibling is probably lurking in other chimp populations in West Central Africa, says Hahn, adding that she has “a pretty good idea where it’s going to be . . . and we’re going to find it.”
Nonetheless, the theory rests on facts, and at least a few of them are undisputed — including, most significantly, HIV’s family tree. There are two species of the virus, HIV-1 and HIV-2. The first evolved from a simian immunodeficiency virus (SIV) found in chimpanzees, while the second came from an SIV in a type of monkey called the sooty mangabey. HIV-1, which is responsible for the vast majority of AIDS cases worldwide, is divided into three groups — the “major” group M, and the much rarer “outlier” group O and “new” group N — that have diverged over years of mutation and evolution. Within the M group — which makes up 90 percent of all infections worldwide — there are at least nine strains, known as “clades,” of HIV-1 that are constantly mutating and merging with each other, creating yet more new varieties. “The M group epidemiologically has overwhelmed what else is out there,” says Dr. Beatrice Hahn of the University of Alabama at Birmingham, who has conducted much of the research into HIV’s origin. HIV-2, on the other hand, is not as virulent and is largely confined to West Africa, where it originated. In May 2006, an international group of researchers led by Hahn answered two major questions about the origin of HIV-1 M, the deadliest and most widespread form of the virus: Where was its cradle, and what kind of chimp did it come from?
See video: HIV Origins in Africa
See video: Study Reveals Origins of AIDS Pandemic
See video: Study Reveals Origins of AIDS Pandemic
Most AIDS researchers believe that the “bushmeat trade” allowed the HIV-1 virus, and separately HIV-2, to enter the human bloodstream several times. Hunters who kill and butcher chimps and monkeys are regularly exposed to animal blood teeming with SIVs. If the hunters have cuts, bites, or scratches — and given the nature of their work they almost always do — they can catch the viruses from their prey. Hunters going after chimps in Cameroon could have caught the first strains of HIV-1. Sooty mangabeys, hunted and kept as pets in West Africa, could have transmitted HIV-2 to humans. Africans have hunted chimps and monkeys and kept them as pets for centuries; they’ve presumably been exposed to SIVs during most of that time. But the conditions needed for HIV to spread widely weren’t in place until after the continent was colonized and urbanized. The first victims would have found it easier to unwittingly spread the virus to sexual partners far and wide as roads and vehicles started connecting previously isolated villages and cities. Hospitals may have played a role, too. Strapped for cash, some of them probably re-used dirty needles, unknowingly infecting patients in the process.
