How did mammals stop laying eggs?

Scientists believe that, as surprising as it may seem, it all started with an encounter with a virus.

Initially, mutations must have changed the way the egg was treated by the organism until it was kept inside the body for longer, thanks to changes in the genes involved in the embryo’s retention and development.

A protein called syncytin then helped fuse the egg’s cell membranes together to create the placenta.

This kind of protein is what remains of an ancient form of glycoprotein that forms the outer shell of some viruses.

It is therefore thought that retroviruses may have attacked early mammals and that part of their genetic code was transferred horizontally into the host. That viral DNA was then reused for a different purpose in new mammalian genes.

It is also thought that mammary glands underwent parallel evolution, and that they initially had the function of keeping eggs hydrated before becoming a source of food for the offspring.