Mammal Evolution

We humans are the inheritors of a dynasty that has reigned over the planet for nearly 66 million years, through fiery cataclysm and ice ages: the mammals. Our lineage includes saber-toothed tigers, woolly mammoths, armadillos the size of a car, cave bears three times the weight of a grizzly, clever scurriers that outlasted Tyrannosaurus rex, and even other types of humans, like Neanderthals.

For most of us, the story of mammal evolution starts after the asteroid impact that killed the dinosaurs, but over the last 20 years, scientists have uncovered new fossils and used new technologies that have upended this story. In Beasts Before Us, paleontologist Elsa Panciroli charts the emergence of the mammal lineage, Synapsida, beginning at their murky split from the reptiles in the Carboniferous period, over 300 million years ago. They made the world theirs long before the rise of dinosaurs.

The fascinating group of animals called dinosaurs became extinct some 65 million years ago (except for their feathered descendants). In their place evolved an enormous variety of land creatures, especially mammals, which in their way were every bit as remarkable as their Mesozoic cousins. The Age of Mammals, the Cenozoic Era, has never had its Jurassic Park, but it was an amazing time in earth's history, populated by a wonderful assortment of bizarre animals. The rapid evolution of thousands of species of mammals brought forth many incredible creatures—including our own ancestors.

A list of the attributes that define a mammal is a ragbag of things - fur, live birth, three bones in the middle ear, a brain whose two halves are robustly joined together.... But this curious collection of features contain the roots of all the biology that makes us what we are: monkeys with massive brains who parent extensively, enjoy sport and think lots. Which is to say, what makes us mammals makes us human.