Eight percent of the genome is old virus

About eight percent of human DNA is the remains of retroviruses that inserted themselves into the germline and were inherited. Nearly all of it is broken.

When the human genome was drafted, the accounting of its repetitive sequence put endogenous retroviruses at about eight percent. These are not a disease. They are copies, made by viruses that reverse-transcribed themselves into a cell that became a person, and then copied again with every generation. Mutations have since punched holes in almost all of them. They cannot build a virus. They can still be transcribed, and a few have been put to work.

Syncytin is the plainest case. A viral envelope gene, domesticated independently in several lineages of mammals, is part of how cells in the placenta fuse into a layer. The virus’s tool for entering a cell was turned around into a tool for making one tissue. Eight percent is a graveyard. A handful of the graves were robbed on purpose, by selection, and the robbery is why a placenta holds.