From Carrie Arnold at Nova:
One of the few survivors of the asteroid impact 65 million years ago was a small, furry, shrew-like creature that lived in underground burrows and only ventured out at night, when predators weren’t active. The critter—already the product of some 100 million years of evolution—looked like a modern mammal, with body hair and mammary glands, except for one tiny detail: according to a recent genetic study, it didn’t have a placenta. And its kind might never have evolved one if not for a chance encounter with a retrovirus.
Unlike most viruses, which infect, replicate, and then leave their host, retroviruses elbow their way into their host’s genome where they are copied and passed on to daughter cells for the life of the host. This retrovirus, however, managed to sneak its way into one of our ancestor’s sperm or egg cells, able to be passed on to every cell in every subsequent generation. Virus and host had become one.
In that would-be mammal living 160 million years ago, a symbiotic retrovirus enabled it to evolve a placenta over many generations. In order to let a fetus mature inside a mother’s uterus, an animal needed a way to provide oxygen and nutrients while removing waste and keeping both blood supplies separate. More.
That took a lot of accidental engineering, right?
Scientists are discovering that the so-called “junk DNA”—a significant portion of which is from symbiotic viruses—is actually a potent force in the evolution of new species. Although the evolution of pregnancy via the placenta might be some of the most persuasive evidence that viruses stashed deep within the genome can help give rise to new species, it’s not the only proof. New studies revealing the role of endogenous retroviruses in the more recent evolution of humans show that these snippets of DNA are helping to blur the boundary between human and virus. Humans are, in a very real sense, part virus.
Yet viruses are not becoming human. And we actually don’t know very much about viruses.
Note the role of supposed junk DNA.
See also: Human origins: The war of trivial explanations
Another accidental use for junk DNA
Why is devolution counterintuitive?
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