The Fossil Virus Within Us

Look closely at a human embryo safe within a womb, and you will find something downright unsettling. A cloud of retroviruses, much like the kind that causes AIDS, is swarming the placenta, congregating at the junction between baby and mom.

But there is no reason to be alarmed. The embryo itself is producing them by reading viral genes scattered within the human genome. The viruses pumped out by embryos are a permanent part of us. They are the remnants of an ancient infection, when a so-called retrovirus inserted its genes into our DNA millions of years ago. It may have been a devastating pandemic at first, but over time, this germ lost its bite.

The human genome is littered with fossil viruses. Between 6 percent and 8 percent of our DNA is made up of genes they left behind. Each time we reproduce, we copy those viral genes into our eggs and sperm. They pass from one generation to the next, just like blue eyes or, say, a tendency to fret.

Far from being mere curiosities, some of the domesticated viruses may have become essential. Researchers reported last month that the viruses seen in the placenta apparently play a critical part in the pregnancies of mammals.

That discovery lends weight to an emerging hypothesis that retroviruses may be important drivers of evolutionary change. They may provide new types of genetic innovation -- ones that spark more significant changes than would be expected from a slow accumulation of random mutations. Some virologists even speculate that retroviruses may have helped trigger one of the crucial early steps in our ancestors' evolution.

"It can be proposed that retroviruses were the founding event for introducing the development of the placenta," said Thierry Heidmann, who studies retroviruses at the Institute Gustave-Roussy, in Villejuif, France.

Without viruses, some researchers say, we might still be laying eggs.

Closer to Mom

The earliest mammals were small, furry, egg-laying monotremes, like the modern platypus. About 120 million years ago, though, some mammalian embryos began to grow in a new way. Instead of developing autonomously within a shell, these embryos took a parasitic approach.

Early in pregnancy, when they are still just balls of cells, these invasive embryos burrow into the linings of wombs...