Obviously, evolutionary biologists believe in some kind of magic – it would be a miracle if such a placental structure would evolve once, by random unguided process without foresight, because placental structures require an interaction / cooperation of both, the mother and fetus. However, here we read, that the miracle happened many times. First, these placental structures evolved, then devolved, and then re-evolved many times …
The still apt definition of a placenta is that coined by Mossman, namely apposition or fusion of the fetal membranes to the uterine mucosa for physiological exchange. As such it is a specialized organ whose purpose is to provide continuing support to the developing young. By this definition, placentas have evolved within every vertebrate class other than birds …
This epithelial layer is lined with microvilli, while the surrounding tissue is highly vascularized, presumably to facilitate maternal-fetal exchange (Kwan, et al. 2015). This type of placental structure has arisen several times during Poeciliopsis evolution (Kwan, et al. 2015) and has been subsequently lost and regained many times, making it a model for the evolution of placentation (Pollux, et al. 2014).
similar miracle should have happened here:
Have Wings Come, Gone and Come Again?
Can complex traits be re-evolved by lineages that have lost them? Phylogenetic study now suggests that wings may indeed have reappeared several times within the ancestrally wingless stick insects.
and one more miracle:
Lizards Re-Evolve To Lay Eggs
A family of lizards has achieved something very unexpected, evolving to give birth to live young, before going back to egg laying. Most remarkably, the zoologists who observed this think it is possible they rediscovered laying eggs multiple times.
stuff happens …