Origins continue to fascinate. When it comes to biology, Darwin set the stage and interpreted the play. Following the revolution in molecular biology, we know how to decode the digital information stored in DNA, and can look back in wonder at the phylogenetic trees of protein sequences such as cytochrome c or α-globin. Phylogenetic analysis was a fairly scholarly occupation until the polymerase chain reaction (PCR) discombobulated us all in the late 1980s. PCR allows billion-fold (and more) amplification of a precise segment of DNA — single DNA molecules can be analysed, and almost every living thing has had some part of its DNA amplified by now. Even Neanderthal bones in a museum drawer have harboured enough DNA to yield to PCR, so bringing new life to an old debate1. And laboratory freezers the world over contain old serum samples, from Africa and elsewhere. Which brings us back once again to the human immunodeficiency virus, and to the paper by Zhu et al.2 on page 594 of this issue.
Where did HIV come from? Both of the AIDS viruses, HIV-1 and HIV-2, originated in Africa, where the spread of AIDS preceded development of the disease elsewhere on the globe. How? As is often the case with microbes, a jump from one species to another is probably to blame. This conclusion comes from the observation of viruses with qualitatively identical genetic make-ups among chimpanzees (for HIV-1) and sooty mangabeys (for HIV-2), in geographically overlapping regions. Of the two viruses, HIV-1 is wreaking global havoc — although HIV-2 is capable of generating an AIDS epidemic, HIV-1 has a faster disease course and more efficient transmission.
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