Evolving by Daniel J. Fairbanks

Author:Daniel J. Fairbanks
Language: eng
Format: epub
Publisher: Prometheus Books

THE HUMAN IMMUNODEFICIENCY VIRUS (HIV), WHICH CAUSES AIDS, IS RAPIDLY EVOLVING.

Let's now turn our attention to one of the most feared modern-day viruses—the human immunodeficiency virus (HIV), which causes AIDS. This virus evolved very quickly to become one of the greatest health threats in modern history. It is so dangerous because it attacks the very cells that normally fight viral infections—cells of the human immune system. People who suffer from AIDS typically do not die from AIDS itself but from other infectious diseases that exploit their weakened immune systems.

The first death related to AIDS in the United States happened in 1959, although it was not recognized as AIDS because the disease had yet to be described. Later analysis of the man's tissue confirmed that he was infected with HIV. By compiling molecular information, the infection history, and the geographic distributions of the virus, evolutionary biologists have reconstructed its evolutionary history and determined how it overcame natural immunity in humans and evolved into the distinctly human form known as HIV.2

HIV is a retrovirus, which means it uses RNA as its genetic material. Upon infection, it replicates very quickly and easily acquires mutations, resulting in exceptionally high genetic diversity. These two features allow it to rapidly evolve. The virus traces its evolutionary history to related viruses called SIVs (simian immunodeficiency viruses), which infect apes and monkeys. In the heart of Africa, an unknown wild chimpanzee became infected with two different SIVs, one called SIVrcm, which infects red-capped mangabeys (rcm = red-capped mangabey), and another called SIVgsn, which infects greater spot-nosed monkeys (gsn = greater spot-nosed). In the infected chimpanzee—who probably killed and ate these monkeys as chimpanzees sometimes do—the RNA of these two viruses fused to form a new hybrid virus that contained a portion of SIVrcm combined with a portion of SIVgsn. This new virus evolved to become an infective virus in chimpanzees called SIVcpz (cpz = chimpanzee).

The most common form of HIV (called HIV-1 group M) arose when SIVcpz jumped from chimpanzees to humans, probably in the 1920s in the southeast corner of Cameroon. The infection most likely resulted from blood-to-blood transmission when someone who butchered chimpanzees to consume the meat was accidentally cut during the process. From there the virus spread through person-to-person transmission. Infected people carried the virus to the city of Kinshasa in the Democratic Republic of Congo, where in an urban area multiple sexual transmissions allowed the virus to infect a large number of people. There the virus diversified by mutating and then spread to people in the rest of Africa and ultimately the entire world. The highest genetic diversity of HIV-1 group M remains in this part of Africa, evidence that it first evolved there.3

To successfully make this jump, the virus had to mutate into a form that could overcome natural immunity to SIV in humans. Mammals have a gene that encodes a protein called tetherin. This protein has evolved to confer resistance to retroviruses by tethering them to the inside of the cell they infect (hence its name), and preventing the viruses from replicating.

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