Biology
The Evolution of Placental Diversity and Maternal–Fetal Conflict in Mammals
Quick fact
Placentas vary more than any other mammalian organ: from a diffuse, horseshoe-like shape in pigs to a single round disc in humans. Even more surprising, the number of tissue layers between maternal and fetal blood can range from six down to just one, allowing different species to control how aggressively the fetus draws resources from its mother.
Why this is interesting
Every mammal—including you—was once nourished by a temporary organ that is more varied in shape and function than almost any other body part. Why do some placentas look like a balloon, while others resemble a flat cake?
Read the full explanation
Understanding The Evolution of Placental Diversity and Maternal–Fetal Conflict in Mammals
When a mammal embryo implants in the womb, it forms a placenta—a specialized organ that connects the mother's blood supply to the fetus's. But this organ is not the same in every species. In pigs, the placenta is diffuse, covering the entire fetal sac, while in humans it is a single disc. The invasion level also varies: in some species (like dogs), the placenta stays on the surface of the uterine lining, whereas in humans and rodents, it burrows deep into the mother's tissue. This variation is driven by a fundamental evolutionary tension: the mother and the fetus have different interests. From an evolutionary perspective, a mother's job is to produce many healthy offspring over her lifetime, so she should distribute resources evenly. A fetus, however, benefits by extracting as much as possible from the current pregnancy, even at the cost of the mother's future offspring. This is called maternal-fetal conflict. The placenta is the battlefield where this conflict is fought.
A deeper explanation
The mechanism behind placental diversity is rooted in evolutionary genetics. Genes that are expressed only when inherited from the father (paternally expressed) tend to promote fetal growth, because a father has no investment in the mother's future offspring. In contrast, genes expressed only when inherited from the mother (maternally expressed) tend to restrict growth, because a mother shares genes with all her offspring. This tug-of-war leads to an arms race: fathers push for a more invasive placenta that extracts more nutrients, while mothers evolve defenses to limit invasion. The result is a wide variety of placental structures. For example, the hemochorial placenta of humans and rodents—where fetal tissue directly contacts maternal blood—allows high nutrient transfer but also gives the fetus great control. The epitheliochorial placenta of cows, on the other hand, maintains six layers of tissue, limiting fetal access and protecting the mother. Thus, placental diversity is not random; it reflects the balance of power in the evolutionary conflict between maternal and fetal genomes. Understanding this helps explain why some pregnancies suffer from complications like preeclampsia and gestational diabetes, where this conflict escalates.