How big the continent of America is(it’s big) is not an indication of how difficult it is to find an American. Merely pointing to it’s total size tells you nothing about the average density, connections, or geographic distribution. And hey, if you find an American, s/he can usually tell you where to find more.
Regardless of how you think the genes involved in development of the placenta themselves originated, given that ancestral lizards do not have placenta, if those placenta evolved by as much as a single nucleotide substitution in a specific pre-existing gene which then went on to regulate some whole host of other processes, then that placenta evolved. Trans-generational genetic and phenotypic change is evolution by definition.