Evolution of the Placenta in Eutherian Mammals
Journal Title
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Issue Date
2026
Authors
Johnston, Drew
Publisher
SUNY Brockport
Keywords
Mammals, Placenta
Abstract
Placenta can be found in nearly every vertebrate line having evolved multiple times in isolation. This suggests convergent evolution resulting in increased maternal invasiveness during fetal development. The degree of invasiveness is influenced by growth-inducing genes and by maternally expressed growth-repressing genes, making the placenta a site of battle for maternal dominance as to avoid premature birth. To support increased invasion into maternal tissue, many species, especially mammals, have evolved diverse solutions for maternal-fetal material exchange. Factors like high extrinsic adult mortality rates within occupied environments drive selection toward investment in offspring earlier in parental lifespan, betting on resource availability as the fetus develops.