What Were the First Mammals? Two Hundred Million Years in the Shadows

The mammal line does not come from reptiles. It comes from the synapsids, a group that split from the reptile line over 300 million years ago and dominated the land in the Permian period, when Dimetrodon and the therapsids that followed it were the largest animals alive. The end-Permian extinction 252 million years ago wiped out most of them, and in the Triassic recovery the surviving synapsids were pushed aside by the archosaurs, the group that produced crocodiles and dinosaurs.

The cynodonts, the synapsids that survived, shrank. Fossils from the late Triassic show a sequence of small animals acquiring the mammal package piece by piece: teeth of different kinds for cutting and grinding, a secondary palate that allows breathing while chewing, a jaw hinged on a single bone, and the two bones that had formed the old jaw joint migrating into the ear to become the tiny hammer and anvil that give mammals their sharp hearing. The transition is one of the best documented in the fossil record.

Where exactly to draw the line is a matter of definition, but by around 205 million years ago there were animals such as Morganucodon, found in Wales and China, that most palaeontologists call mammals: about ten centimetres long, weighing less than a mouse, with a mammalian jaw joint, fur suggested by the fine preservation of relatives, and teeth that were replaced only once, which points to a mother feeding milk to young that did not yet need to chew.

They were nocturnal, and that fact shaped everything. Working by night favoured hearing and smell over sight, and the parts of the brain handling those senses grew. Keeping warm through a cold night favoured fur and a fast metabolism. Being small and hunted favoured many offspring and fast growth. The mammal design was an answer to the question of how to make a living while the dinosaurs held the day.

The picture of Mesozoic mammals as an unchanging line of shrews has collapsed in the last twenty-five years, largely because of fossils from northeastern China preserved with fur and stomach contents. Castorocauda, from 165 million years ago, was a beaver-like swimmer with a flat tail. Volaticotherium glided on a skin membrane like a flying squirrel. Fruitafossor dug for termites with mole-like forelimbs. Repenomamus, the size of a badger, had a young dinosaur in its stomach.

The three living groups had also separated by the middle of the age of dinosaurs. The egg-laying monotremes, today's platypus and echidnas, branched off first. The marsupials, which give birth to tiny young that finish developing in a pouch, and the placentals, which carry their young to a much later stage inside the body, had diverged from each other by around 160 million years ago. Juramaia, a 160-million-year-old fossil from China, is the earliest known member of the placental line, the one that leads to us.

The asteroid that ended the Cretaceous 66 million years ago killed every animal larger than a dog on land, and the small, burrowing, seed-and-insect-eating mammals were among the survivors. What followed was the fastest diversification in mammal history. Within a few hundred thousand years mammals were reaching the size of a sheep; within ten million years there were mammals the size of rhinos, along with the first primates, bats, whales and horses.

Body size, which had been capped at a few kilograms for 150 million years, became unlimited once the dinosaurs were gone, and the brain, already large for the body, scaled up with it. The mammal traits that had been adaptations to living small in the dark turned out to work at every size, in the sea and in the air. The main stages:

• •300 million years ago: synapsids split from the reptile line
• •252 million years ago: the Permian extinction leaves a few cynodont survivors
• •205 million years ago: the first true mammals, mouse-sized and nocturnal
• •160 million years ago: monotremes, marsupials and placentals have separated
• •66 million years ago: the dinosaurs go, and mammals fill the empty world

For most of the time palaeontologists have studied early mammals, they have had little to work with beyond teeth. Tooth enamel is the hardest material in the body and survives when fragile skulls and limb bones are crushed or scattered, and tiny animals left tiny remains. Fortunately mammal teeth are highly distinctive. Their cusps and ridges, and the way upper and lower teeth fit together when the jaw closes, differ between groups and reveal diet, so a single molar a few millimetres across can often be assigned to a family and even named as a new species.

Finding such teeth needs a patient method. Rather than searching for bones by eye, researchers collect bags of sediment, wash them through fine sieves, and pick through what is left under a microscope, sometimes sorting huge quantities of material for a handful of specimens. Much of the Mesozoic mammal record was built this way. The exceptional Chinese fossils with complete skeletons and fur changed the picture partly because they showed whole bodies for animals once known only from their teeth, and confirmed that the dental clues had usually pointed in the right direction.

Milk is older than live birth. The platypus and echidnas lay eggs, yet they feed their young on milk, and they have no nipples: the milk oozes from patches of skin on the mother's belly, and the young lap or suck it up. That suggests lactation was already present in the common ancestor of all living mammals, long before marsupials and placentals began carrying their young inside the body. One leading idea, proposed by the biologist Olav Oftedal, is that milk began as a secretion from skin glands that kept thin-shelled eggs moist and helped protect them from microbes, and only later became food.

Genes give an independent check. Egg-laying animals such as birds and reptiles have genes for vitellogenin, the protein that supplies egg yolk. Studies comparing genomes found that the platypus still carries a working copy of one such gene, while in marsupials and placentals they survive only as broken remnants. The genes for casein, the main protein of milk, appear to have arisen in the common ancestor of mammals before the yolk genes decayed, which fits a sequence in which milk took over the job of feeding the young, a large yolk became unnecessary, and live birth followed later.

Mammals hear with three tiny bones in the middle ear, while reptiles have only one, and fossils show where the extra two came from. In the ancestors of mammals, the lower jaw was made of several bones, and the joint with the skull lay between two small bones at the back. Over millions of years a new jaw joint formed, and the old joint bones shrank and moved into the ear.

Those bones became the malleus and incus, which carry sound vibrations from the eardrum to the inner ear. Transitional fossils show intermediate stages, with small bones still attached to the jaw but already involved in hearing. Embryos echo the history too: in a developing mammal, the future ear bones first appear in the region that forms the jaw, before moving to their final place.

The first mammals appeared around 205 million years ago from small synapsid survivors of the Permian extinction, and lived for 140 million years as nocturnal animals in the shadow of the dinosaurs, evolving fur, milk, sharp hearing and large brains in the process. The living groups had already split before the asteroid struck, and when it did, the mammals were ready.