Why didn’t we hatch from eggs? Thank viruses, says biologist
Whether you know it or not, you’ve already gone viral — hundreds of thousands of times, in fact.
Viruses are part of who we are in a very concrete way. Fully 8 percent of our genetic code comes not from human ancestors, but from viruses, according to Harmit Malik.
“Things we consider bonafide human genes and human biological processes actually owe their roots to something that came from a virus,” says Malik, a faculty member at the Fred Hutchinson Cancer Research Center.
Malik is especially interested in a type of virus called a retrovirus — the virus that causes HIV is a famous one. A retrovirus enters a host cell, copies its own genetic material, and inserts that copy into the host’s DNA.
If that host cell happens to form a sperm or egg, the viral-origin DNA gets passed on to the next generations. Multiply that a few hundred thousand times (at least — Malik says that’s an underestimate), and you wind up with a human genome full of viral relics.
BIOLOGICAL JIU JITSU
Malik says that DNA derived from ancient encounters with viruses forms a kind of viral “fossil record.”
“So the best analogy I can make is that we are basically archaeologists. We are studying viruses that have not left a fossil record in the traditional sense, but they’ve left a fossil record in our own DNA,” Malik says.
Researchers poring through that fossil record have made some astonishing discoveries.
For instance: In some cases, even as a virus was using its genetic material to hijack a host cell, the cell seems to have done some jiu jitsu in order to claim and repurpose the viral gene.
A dramatic example of that is the syncytin gene, which Malik says originated as the blueprint for a retrovirus’ “envelope,” or outer layer.
That gene was quickly repurposed by our ancestors to perform a very different function: forming the placenta, which allows a pregnant mother to nourish a fetus in the womb instead of laying an egg.
“It turns out that our main mechanism of biological reproduction, which is live birth, may owe its roots to insertion from a virus that landed on our genome many millions of years ago,” he says.
This has probably happened many times. Viruses are one of the most efficient ways nature has devised to mix and match genes among different species.
Often those transactions are harmful (such as in HIV), but they’ve been useful enough times that now we likely have viruses to thank for all kinds of human characteristics: the way our neurons communicate, parts of our immune system and, yes, the fact that you were born live and didn’t hatch from an egg.