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Scientists have known for decades that the genomes of animals can sometimes harbor DNA from the viruses that have infected them. When I first learned of this fact some years ago, it blew my mind. The notion that any animal could be a little bit viral blurred nature’s boundaries.
The viruses that scientists discovered in host genomes were of a particular sort, known as endogenous retroviruses. Retroviruses, which include HIV and a number of viruses that can trigger cancer, have to insert their genetic material into their host’s genome in order to reproduce. The cell reads their genetic instructions along with its own, and then builds new viruses. It made a certain intuitive sense that retroviruses might sometimes get trapped in their host genomes, to be passed down from one generation to the next.
The first endogenous retroviruses scientists identified were still relatively functional. Under certain circumstances, their genes could still give rise to new viruses that could break out of their host cell. But gradually, scientists identified more and more fossil viruses, which had mutated so much that they...
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