'I have never written of a stranger organ': The rise of the placenta and how it helped make us human

In this adapted excerpt from "Infinite Life: The Story of Eggs, Evolution, and Life on Earth," (Pegasus Books, 2024) author Jules Howard examines the invasiveness of the placenta — how far it permeates into the wall of the uterus and the maternal tissue — in maMMAls after the dinosaur-killing asteroid struck.

Although it has not been preserved in the fossil record, the diversity of placentae among modern-day mammals suggests that, about 10 or 20 million years after the end-Cretaceous, at around the time that the animals of the Messel Pit were alive, the mammal placenta was changing. Natural selection was tweaking this organ.

In many cases, it was selecting the individual placentae best able to extract as much energy from the maternal host as possible. Yet, surprisingly, in some lineages the placenta appeared to take a step back, becoming less, rather than more, invasive. Looking at data across 60 mammal species, a trend becomes apparent.

Plotting the invasiveness of each placenta (judged partly by how many blood- gathering, finger-like projections the placenta has) against important life-history details, such as how long a species takes to mature and how many offspring each year a species might produce, the least invasive mammal placentae are those associated with a more rapid pace of life.

Species that live fast and die young, in other words, appear to end up evolving a less invasive placenta.

Brain size is another marker that tracks closely with how invasive a placenta evolves to be. Not just how large the brain is in relation to the body, but also how quickly the brain grows before birth. Both factors correlate with especially invasive placentae. How it works is simple: The bigger a mammal’s brain evolves to be, the greater selective force is placed on the placenta to acquire energy for the growth of the embryo, which, naturally, drives the evolution of an ever-hungrier placenta.

MaMMAls are, as a group, more brainy than other similar sized organisms, but this wasn’t always as key a feature of our kind. It seemed to happen gradually, after the demise of dinosaurs and as the Cenozoic Era (66 million years ago to the present) began to progress. Scientists had originally thought that this relative increase in brain size was simply a byproduct of the evolution of larger body size in maMMAls, but recently (using three-dimensional models of fossilized maMMAl skulls) this assumption has been more rigorously tested.