One of the problems with decoding the human genome was discovering how little of the human genome was actually... well, human. Are we really made up by mostly virus DNA?
As biologist Frank Kelly explains, scientists sequencing the human genome discovered that not only were there around 5 times less genes than originally anticipated, they made up less of the genome than expected - 1.5% - with more than 34% of the remaining being made up of retrotransponsons, which Kelly describes as "mysterious virus-like entities... pieces of selfish DNA that appear to serve no function other than to make copies of themselves":
All in all, the virus-like components of the human genome amount to almost half of our DNA. This would once have been dismissed as mere "junk DNA", but we now know that some of it plays a critical role in our biology.
The key, argues Kelly, is that all the "junk DNA" is the result of evolutionary processes to keep us safe from harm - a process that he calls Virolution:
[T]he human genome has evolved as a holobiontic union of vertebrate and virus... It is also probable that this "virolution" is continuing today. HIV belongs to a group of retroviruses called the lentiviruses. Until recently virologists thought that lentiviruses did not endogenise, but now we know that they have entered the germ lines of rabbits and the grey mouse lemur. That suggests that HIV-1 might have the potential to enter the human germ line, perhaps taking our evolution in new and unexpected directions. It's a plague to us - but it could be vital to the biology our descendants.
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On the one hand, it's good to know that HIV may end up serving some purpose. On the other, evolution really is a fucker if that's the way it wants to work.
I, virus: Why you're only half human [New Scientist]