The backbone, which is horizontally oriented and perpendicular to the hind limbs in horses and mice and whales and basically all other mammals, rotated to become parallel to the legs, and took on a curved shape.

Volcanoes did it—so-called

Therians and multituberculates were ensconced in the north, from the Asian heartlands to the North American mountains to the European islands. Among them were insectivores that demolished bugs with their tribosphenic teeth (eutherians), herbivores that feasted on flowers and fruits and other angiosperm parts (multituberculates), and the odd

Because these two former jaw bones are now fully detached from the jaw, we can call them by their new names: the hammer and the ring. This small step was revolutionary. Now the jaws could go their own way, and become more efficient at biting and chewing, without worrying about interfering with hearing function.

survive as the forests shrank? What we do know is that these early hominins started walking on two legs before sprouting big brains and learning to shape tools from stone.

chewing their food into mush, these early mammals could do most of the processing in the mouth—essentially beginning digestion before the food hit the stomach. It was yet another way to take in more calories, more efficiently.

They ensure that even our coldest, driest breaths on the most frigid of winter days are warmed and humidified to rain-forest-like conditions within a split second. They also work in reverse, recouping precious water as we breathe out.

Their new molar design was key to their—or, I should say, our—success,

The first group, with their two skull openings, were the diapsids. They would eventually evolve into lizards, snakes, crocodiles, dinosaurs, birds, and turtles (which closed up their holes). The second group, with their single skull opening, were the synapsids. They would diversify into a dazzling array of species, including—more than a hundred