Just when we're getting over our discomfort with the notion that there are ten bacterial cells in your body for every one of your own, we are reminded that approximately 8 percent of human DNA is composed of sequences that are recognizably viral.
To put a fine point on it, these sequences are remnants of infections by a class of viruses known as retroviruses, which leave nucleic-acid imprints of themselves in our DNA. If a retrovirus had a sense of purpose and a voice, it would explain that by embedding copies of its own genetic material in our DNA, it was laying the groundwork for a future activation of that stretch of DNA. This, in turn, would result in the production of proteins and genes that could then self-assemble to form brand new chips off the old block. To the virus, that's a new lease on life. To us, it's chronic infection.
But three things happen: First, if the viral template gets embedded in a creature's germ cell, it obtains a kind of immortality, in that all the creature's progeny will also host the viral sequences in their own DNA, and so on down the line. Second, with successive retroviral infections over the generations, the quantity of these viral sequences increases over time - there's no magic reset button that rids the DNA of them. And third, over evolutionary time many of these sequences undergo mutations that inactivate them - they can no longer be expressed as components of new viral particles - so they stick around as, well, genomic warts.
Thus, "the human genome represents a fossil record of ancient retroviruses that once infected our ancestors," as two Rockefeller University researchers recently showed by effectively expunging mutations from ancient viral sequences in the human genome and reconstituting an ancient virus in its active form.
Of course, genomic warts can mutate in all kinds of ways other than back to their same ol' used-to-be, and it's likely that in some cases leftover viral genes and their surrounding human sequences have served as raw material in the formation of what have eventually becoming brand-new types of functioning genes. More directly, retroviruses can introduce functioning genes from one species of creature into the genomic material of another. They are thus not only valuable laboratory tools but, by allowing for lateral transmission of heritable traits, probably one of the major engines of evolution itself.
Via Boing Boing