 Finally, holobionts can also be manifestors of adaptations, as in the case of the evolution of placental mammals in the acquisition by horizontal gene transfer from a retrovirus of a crucial gene coding for the protein syncytin (Lloyd forthcoming; Dupressoir, Lavialle, &amp; Heidemann 2012). Syncytin allows fetuses to fuse to their mother’s placenta, a role crucial to the evolution of placental mammals. Moreover, it seems that several retrovirally derived enhancers played critical roles in the formation of a key cell in the uterine wall, also crucial for maintaining pregnancy, enhancing the holobiont’s role as a manifestor of adaptation (Wagner et al. 2014).