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Carl Zimmer is a science writer, lecturer, and frequent guest on such radio programs as Fresh Air and This American Life. His books include "Soul Made Flesh," "Evolution: The Triumph of an Idea," and "Parasite[…]

Everyone knows we have hereditary viruses in our genome. What scientists are just learning is how many there are—and how many we’ve come to depend on.

Question: Can beneficial viruses be transmitted through the human genome?

Carl Zimmer: Well it’s just been recently that scientists have realized that actually viruses don’t just make us sick. They can actually sometimes end up in our genomes. In other words a virus sort of pastes itself into our own DNA and if that DNA happens to be in an egg or a sperm cell it can get carried down to the next generation, so that virus DNA goes along with our own DNA and it’s in our kids and this happens actually incredibly often, maybe not to everybody in every generation, but if you go over millions and millions of years it actually happens so often that we now have maybe 100,000 pieces of virus DNA in our genome and if you add it all up it’s huge, so about 1.2% of the human genome is made up of genes, things that encode for proteins, the stuff that we consider us. There is about 8.3% that’s a virus. In other words we’re probably about seven times more virus than we are human genes, which is kind of a weird way to thinking about yourself, so if you’re looking for your own idea of your own identity you know the human genome may not be the best place to look for it. You’re just looking at a bunch of viruses.

Now a lot of those viruses, once they get into the genome and they go down through generation after generation they mutate, the get crippled, they can’t make new copies of themselves after awhile and then after awhile a lot of them are just dead. They’re just sort of filler, but there are some cases where our own genomes over millions of years have actually domesticated some of this virus DNA, so what happens is that there is a mutation to one of these virus genes and it starts to switch on and actually make proteins that we can use. So for example ironically there are some proteins from viruses that will help cells defend against invading viruses, so we basically are taking these viruses in and we’re using them as kind of guard dogs to keep the wolves out because they will make proteins that will block receptors that these viruses might use to invade our cells. Another way that viruses have become a part of us is when we’re all embryos in the womb. Before we were born we were sitting inside our mothers and there was this nice placenta that was giving us food. The attachment where the placenta connects to the mother’s body in the uterus has some very special proteins that create the right kind of barrier to support the placenta there and they’re essential. They’re called centisan, and these proteins come from viruses. If you knock those virus genes out, say in a mouse, in an experiment that mouse cannot have babies. Its placenta won’t develop properly. So we really depend on viruses for our complete survival.

Question: What new discoveries are emerging from the study of viruses in the genome?

Carl Zimmer: Okay. Let’s see. So one of the big challenges now is to figure out just how many viruses there really are in the human genome. So far the estimate is 8.3% of our genome is virus, but it actually could be a lot higher. The problem is that over millions of years the viruses in our genome mutate more and more so the look less and less and less recognizable as viruses and so if there was a virus that infected our pre mammal ancestors like 250 million years ago, which it probably did, we can’t see it because it just looks totally random now. So it’s hard to work back that far in time. Our view of things gets fuzzy. I mean that’s true of evolution in general and so it’s also true of this fossil virus record, but the surprising thing is that a lot of different viruses can get into our genome and that’s actually a big surprise. It used to be thought that only a certain kind of virus could get into our genome and it’s called a retrovirus and that’s a virus that might be HIV for example. That’s an example of a retrovirus and all these retroviruses replicate in the same way and what they do is they make copies of their genes and insert them into our DNA, so they actually splice them in there and then our cells churn out all these new viruses and then the cell dies and the viruses go and infect other cells, so that is retroviruses and so you could see how a virus could get into your genome. I mean that is what they do in general, so if they just happen to get trapped in there and then get passed onto the generation you get viruses in the genome.

But now it turns out that you can get other viruses in the genome, so for example, the first non retrovirus that scientists have found in our genome is something called Borna virus and this is actually very weird. So Borna viruses, you probably haven’t heard of them. The reason is that they may or may not infect people. They mostly are known in mammals, so for example they can make horses sick. They get into these horses brains and they do really weird things. The horses go running around in circles or they might bang their head against the wall of their stall until actually they crack their skulls or they starve themselves. It’s really horrendous and some people have actually suggested that schizophrenia and some other psychiatric diseases might be connected to Borna virus, but it’s still very mysterious and controversial. So Borna virus is not a retrovirus. It doesn’t actually insert its own genes into our cells. What it does is just hangs out near our DNA and uses some of the molecular machinery to copy itself. That’s how it works. Well it turns out that there is Borna virus in our DNA. We have Borna virus genes. We’re part Borna virus, which is weird, but apparently our cells and our genomes in a weird way might actually be grabbing these viruses, grabbing genetic material from the viruses that are infecting it and pulling them into their own genome. So we may be sucking in all sorts of viruses and we really don’t know the full range of them. Maybe we’ve got flu virus inside of us. That’s a possibility. Maybe we’re part flu.

Recorded on January 6, 2010
Interviewed by Austin Allen