The placentas of ruminants and muroid rodents express prolactin (PRL)-related genes whereas the placentas of anthropoid primates express growth hormone (GH)-related genes. The evolution of placental expression is associated with accelerated evolution of the corresponding pituitary hormone and destabilization of conserved endocrine systems. In particular, placental hormones often evolve novel interactions with new receptors. The adaptive functions of some placental hormones may be revealed only under conditions of physiological stress.
Haig, DengReviewEngland2007/11/06 09:00Placenta. 2008 Mar;29 Suppl A:S36-41. Epub 2007 Nov 5.