Pathogenesis: How germs made history: A Sunday Times Science Book of the Year

by Jonathan Kennedy

Another mind-blowing example of a function that human ancestors acquired from retroviruses is the ability to give birth. When animals first evolved, they reproduced by laying eggs, and most creatures in the animal kingdom continue to give birth this way. Then, between 100 million and 200 million years ago, a shrew-like creature developed the capacity to gestate her young inside her own body – an extraordinary evolutionary advance because a foetus is much safer growing inside its mother’s body. It is only possible because of the placenta, a temporary organ that attaches to the uterus and allows nutrients and oxygen to pass from mother to baby, and carbon dioxide and waste to travel in the other direction, without provoking a devastating response from the mother’s immune system. There is nothing like this interface between the placenta and womb anywhere else in our bodies. When geneticists looked at the gene responsible for creating it, they realized that it was almost identical to those used by retroviruses to produce the proteins that attach to cells they are infecting without triggering an immune response.27 The scientists concluded that a crucial function of the placenta didn’t emerge gradually as a result of evolution by natural selection but was suddenly acquired when a retrovirus inserted its DNA into our ancestor’s genome. So if one of our distant ancestors hadn’t been infected by this tiny virus hundreds of millions of years ago, humans would reproduce by laying eggs.