"The earliest mammals, ancestors of the spiny anteater and the duck-billed platypus, laid eggs. Then, at least a hundred million years ago, embryos, instead of growing in a shell, essentially became parasites. While only balls of cells, they began to implant themselves in the lining of the womb. The result was the placenta, which permits the embyros to take nourishment from the mother’s blood, while preventing immune cells or bacteria from entering. The placenta is essentially a modified egg. In the early nineteen-seventies, biologists who were scanning baboon placentas with an electron microscope were surprised to see retroviruses on a layer of tissue known as the syncytium, which forms the principal barrier between mother and fetus. They were even more surprised to see that all the animals were healthy. The same phenomenon was soon observed in mice, cats, guinea pigs, and humans. For many years, however, embryologists were not quite sure what to make of these placental discoveries."
Apparently some tiny percentage of retroviruses manage to infect egg or sperm cells, inserting their RNA into the egg or sperm DNA, and making it into our germ line. 1.5%-2% of our DNA is 'active', and in comparison, this article asserts that about 8% of our genome consists of bits and pieces of old retroviruses.
Also some stuff about how some viruses can cause cancers, like leukemia, not new stuff, but I didn't know it.
"Retroviruses cause cancers in chickens, sheep, mice, and other animals, but their effect on humans became clear only in the late nineteen-seventies, with the identification of two viruses that cause forms of leukemia."
Genetics is really fascinating.