Carl Zimmer in his blog, The Loom:
Last November, scientists announced they had revived a virus that had been dead for millions of years. The virus belongs to a special class that multiply by inserting their genetic code into the genome of their host cell. When the cell divides, it makes a new copy of the virus’s genes along with its own DNA. Once it has installed itself in a genome, the virus can liberate itself from time to time, creating new copies. These copies can infect the same cell again, or wander out of the cell to infect another one. Some of these viruses, known as human endogenous retroviruses, may be harmless, while others have been associated with diseases such as cancer. If one of these viruses happens to infect an egg or sperm, it has the chance to get in on the ground floor during the development of an embryo. The virus will be replicated in every one of the trillions of cells in the new growing body. It can then be passed down from one generation to the next along with the rest of the genome. Mutations may strike the DNA of these viruses, making them unable to jump out of the genome. But these dead copies will still be replicated for millions of years. Scientists are scanning the human genome to count up all of the endogenous retroviruses (or their dead remains and fragments). Roughly 100,000 pieces of DNA in our genomes started out as viruses, making up eight percent of the genome all told. A small fraction of them can still produce proteins; the rest are generally just coming along for the ride. Related versions of some viruses are residing in the genomes of our primate relatives, and a number of scientists are busy delving into the sixty-million year history of this viral evolution.
More here.
