Koalas
are having a very hard time. At least
100 years ago they picked up a retrovirus, much like the Feline Leukemia
retrovirus that has been attacking domestic cats, and it has spread throughout
the population. When the diseases get
this contagious and prevalent they can insert themselves into the genetic code
of their host.
PastPandemics Are in Our Genes
Carl Zimmer, 5 December 2012 (hat tip: The Browser)
To understand what it means to be human, you have to
understand koalas. Or, to be more precise, you have to understand how they are
dying from a bizarre viral outbreak that has been raging for the past 150 years
or so. The koalas are now going through something our ancestors experienced 31
times over the past 60 million years…
Eventually, these outbreaks ended, and the viruses became
trapped in their hosts. But they didn’t lose all their viral powers. They could
still parasitize their host’s genome. Sometimes a cell would make an extra copy
of the viral genes and then insert them back elsewhere in its genome. As a
result, our 31 viral invasions gave rise to 100,000 separate chunks of virus
DNA. Altogether, they make up at least 8 percent of the human genome.
One
specific example of note is that human European population sometime in the
distant past was infected by an Aids-like entity. There are some remnants of this in some
European’s genetic code, and it has been theorized that this remnant gives
Europeans a slightly better possibility of immunity to the disease.
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