Mammals Came from Viruses, and So Did You
Artistic Rendition of the First Placental Ancestor. Illustration Credit: Carl Buell If not for retroviruses, mammals would not exist. Mammals are distinguished from other classes by their live births, a trait made possible by the placenta. The placenta is a temporary organ that connects the developing fetus to its mother via the umbilical cord to the uterine wall. This placenta combines with the mother’s blood to facilitate nutrient uptake, thermo-regulation, waste elimination, gas exchange, and more. The protein syncytin is essential for formation of the placenta and is present in all placental mammals. However, genetic analyses show that the code for syncytin originally came to the genome of ancestral mammals through a retrovirus infection.