Until now, the evolutionary direction of mammals has been inferred from the evolution of the placenta.It was thought that the cells evolved from pigs and horses to dogs, cats, and humans based on the degree of infiltration of nutrient-carrying cells into the placenta.However, in recent years, it has become clear that genes involved in placental formation have different origins that cannot be explained by natural evolution.

Endogenous viruses are the key to unraveling this mystery.When a virus infects an animal, the gene may be incorporated into the host's DNA.Such viruses are called endogenous retroviruses.The research group extracted all the genes expressed during placental formation in bovine and analyzed them with a computer to identify the genes derived from the endogenous retrovirus and to identify the function of each gene.When the results are integrated, it has become clear that mammals incorporate genes involved in placental formation from viral genes.Furthermore, when infected with a virus that has a gene with a better function, it has become possible to actively incorporate it.This discovery has the potential to overturn the previously thought evolutionary direction of mammals.In the future, it may be indispensable to regulate the expression of virus-derived genes in order to promote and control pregnancy.

Viruses have a strong image of being the cause of illness, but from the perspective of carrying genes, they may also carry useful genes.In addition to the placenta, many of the traits that are beneficial to survival may be caused by the virus.