Two bones you hear with were once part of your ancestors' jaw
Every lizard, crocodile and bird hinges its jaw on two small bones, the articular and the quadrate. Mammals don't. During the Jurassic those two bones shrank and migrated into the middle ear, becoming the malleus and incus. That swap is so reliable that palaeontologists use the jaw hinge to decide whether a fossil counts as a mammal.
The story starts in the late Carboniferous, between 320 and 315 million years ago, when early land animals with waterproof eggs split into two lineages. The sauropsids led to reptiles, dinosaurs and birds. The synapsids, identified by a distinctive opening behind each eye, led to us, and mammals are their only surviving members. Dimetrodon, the sail-backed beast often lumped in with dinosaurs, was actually a synapsid. The old label mammal-like reptiles is now discouraged, because these animals were never reptiles; stem mammals is preferred.
Synapsids ruled the Permian, supplying both top carnivores and major plant-eaters. In the Triassic, archosaurs, the group that included dinosaurs' ancestors, took over. Early mammal relatives adapted by going nocturnal, relying on a keen sense of smell backed by a relatively large brain. Life in the dark may have driven the evolution of hair and warm-bloodedness. Even under dinosaur dominance they branched out: Castorocauda swam, Volaticotherium glided and Repenomamus ate baby dinosaurs.
Because soft tissues rarely fossilise, researchers cannot track milk glands directly and lean heavily on teeth, which survive well, plus the ear bones, limb posture and palate. A bony palate separating mouth from airway is often read as allowing eating and breathing at once, supporting a faster metabolism, though some scientists think it mainly helped with chewing. In 1981 Kenneth Kermack and colleagues proposed drawing the mammal line at the new jaw hinge, simply because lower jaws are the bones most likely to be preserved.
The lineages behind placentals and marsupials had separated by about 160 million years ago, when the earliest known placental relative, Juramaia, lived. Since the mid-1990s, discoveries especially in China have filled many gaps. After the extinction that ended the non-avian dinosaurs, mammals diversified rapidly, and by the end of the Neogene every modern order had appeared.
Source: Evolution of mammals
