The Darwin Awards Countdown to Extinction Read Online Free PDF Page B

scientists can determine when they evolved from a common parent. This detective method, called phylogenetics, tells us, for example, that the DNA sequences of chimpanzees and humans differ by only 2 percent and—based on the mutation rate—that we shared Thanksgiving dinners with them five to seven million years ago.

    But viral evolution takes place on a much faster time scale. Reproducing in days, not decades, with the higher mutation rate of an RNA genome, HIV evolves a million times faster than humans. A fascinating evolutionary history has unfolded in just a single century, and using phylogenetics, scientists have been able to reconstruct this history.
     
    The History of HIV
    The AIDS virus (HIV-1 M) first gained a foothold in what is now the city of Kinshasa in the Democratic Republic of the Congo. The city experienced a population explosion in the 1940s that helped the virus build the critical mass it needed to seed a worldwide epidemic. By analyzing samples stored in the early eighties, scientists have shown that the virus traveled from Africa to Haiti around 1966 (likely carried by a single person) and then from Haiti to the U.S. around 1969, twelve years before the first cases were recognized by physicians. Like Haiti, the U.S. happened to be an early stop on the virus’s globe trot. HIV spread from the U.S. to Canada and parts of Europe. HIV also spread directly from Africa to Europe and Asia, seeding slightly different strains of the epidemic.
    Genetic similarities prove that HIV evolved directly from SIV (simian immunodeficiency virus), which infects African monkeys. SIV was transmitted from monkeys to humans on multiple occasions resulting in several different strains of HIV. One of these strains—HIV-1 M—gave rise to the current global pandemic, while the others remain confined to small groups in Africa. It’s clear that HIV-1 M came from chimpanzees in eastern Cameroon, but it is hotly debated as to when the original transmission occurred. Older studies put the date at about 1930, but a 2008 study shows it was most likely 1908.
    In that study, scientists compared the oldest and second-oldest HIV samples: blood (1959) and tissue (1960) from the city of Kinshasa. The HIV gene sequences differed by a whopping 12 percent. Clearly the two strains had diverged from a common ancestor long before 1959. Next, the scientists arranged decades of HIV samples into a genetic tree based on their similarities and converted the genetic distances on the tree into units of time, using the known mutation rate of HIV and some fancy computer modeling. The roots of the tree converged about one hundred years ago.
    Between 1884 and 1924, AIDS was born.
    Deadly virulence is the hallmark of a recently evolved disease. Deadly is not a beneficial trait, as killing the host kills the virus. More evolved diseases are less virulent, thus more successful at spreading their genes.
    HIV and the Evolutionary Speed Limit
    HIV has been evolving for a century, the equivalent of 100 million years of mammalian evolution, so one might expect that HIV would now be an ultra-evolved superbug, even more infectious than the original version. But, surprisingly, most of the genetic changes that have accumulated in the past century are random—driven by genetic drift rather than natural selection.

    Genetic drift: genetic changes that accumulate in a species’s genome randomly, rather than through natural selection.
    HIV is a retrovirus, meaning it stores its genetic information in RNA. Copying RNA back to DNA is a messy, error-prone process. The mutation rate is so high that retroviruses are said to live at the evolutionary speed limit—if they mutated any faster, they would fall apart. So HIV racks up scads of mutations that don’t confer an immediate advantage. It’s genetic drift is fast enough to be a riptide!
    But don’t think that HIV doesn’t also undergo classic Darwin-style natural selection. HIV is a marvel at escaping the immune