Evolutionary Novelties ponders placentas:
For me one of the most visceral confirmations of the common descent of humans and other mammals came while witnessing the birth of my children. Having grown up on a small farm, I have vivid memories of the birth of kittens, lambs, and goats; and after the births of my children, I was struck by the similarity of human placenta and umbilical cord to those of other mammals. Given common descent, how did something as complex as the mammalian placenta originate in the first place? The answer, according to research published last summer in Genome Research, involves the evolutionary mechanisms of co-option and gene duplication.
The post then reviews a recent paper that examined the placenta's genomic evolution by looking at the mouse genome, which has been thoroughly mapped.
The visceral re-enforcement of common ancestry I felt when seeing a human placenta and umbilical cord extends to the genes used in developing placentas, which themselves have ancient origins, and are shared across many organisms.
Definitely worth a read.
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Placentas are themselves a "retrofit" biological mechanism, since our ancient "reptilian" ancestors used to lay eggs. Proto-mammals evolved the trick to first incubate their eggs inside their bodies (as some snakes also do), and later do away with the egg shell, and to transfer nutrients to the now shell-less embryo. Independently, they evolved the trick to provide rich nutrients by way of milk, after the birth.
The study of Monotremes and Marsupials can give us some insight on the transition from egg-layers to true mammals.
The original article is here, if anybody else (but me) is interested.
Arthur, thanks for the head-whack and the link to original. I mean to include that but forgot. Am adding to the post now.