The blue whale—190 tonnes in weight and beautifully adapted for swimming—is a placental mammal. The mammal bit means that mothers nourish their babies with milk after they’re born. The placental bit means that mothers nourish their babies via a placenta before they’re born—an organ that allows them to exchange oxygen and nutrients without also swapping blood.
The bumblebee bat—1.5 grams in weight and beautifully adapted for flying—is also a placental mammal. So are you. So is a bear, an anteater, a giraffe and a squirrel. Also: armadillos, rhinos, rabbits, manatees, and pangolins.
All of these creatures, in their wondrous array of shapes and sizes, evolved from a small, unassuming, scurrying insect-eater that lived a few hundred thousand years after the apocalypse that finished off most of the dinosaurs.
After an asteroid killed off the dinosaurs — save for those that evolved into today’s birds — a small, furry animal scurried through the forest in search of insects. Its unassuming looks gave little hint that its descendants would one day rule the planet.
A team of scientists in the United States and Canada has now reconstructed the appearance and anatomy of this creature — the forebear of all ‘placental’ mammals, which give birth to live young at an advanced stage of development — in unprecedented detail, using a record-breaking data set of anatomical traits and genetic sequences.
The critter turned out to be a tree-climbing, furry-tailed insect eater that weighed between 6 and 245 grams. It gave birth to blind, hairless young, one at a time. Its brain was highly folded, and it had three pairs of molars on each jaw.