Although they came into their own only after the extinction of the dinosaurs some 65 million years ago, mammals had maintained a low-profile existence for some 150 million years before that. New fossil discoveries reveal more of this early history every year. In 2001, researchers reported that a fossil found in China in 1985 is the remains of a tiny, furry animal that was a relative of the living mammals today, but lived 195 million years ago in the Early Jurassic period. Called Hadrocodium wui, the little creature had certain key mammalian features 40 million years earlier than had previously been known from the fossil record.
Descended from more archaic relatives, the early true mammals were mainly small insect-eating creatures adapted to nighttime activity. They ranged in size from scarcely bigger than a bumblebee to squirrel-sized, keeping out of the way of the predatory dinosaurs. They acquired certain traits that would characterize mammals ever afterward: limbs positioned under the body, an enlarged brain, a more complex physiology, milk-producing glands, and a diverse array of teeth -- incisors, canines, premolars, and molars.
Already present were the ancestors of the three major mammalian groups that exist today -- monotremes (platypus and spiny anteater), which lay eggs externally; marsupials (kangaroos, opossums), which carry their young in a pouch; and placental mammals (humans, cows, horses), which retain the fetus internally during long gestation period.
In the early Cenozoic era, after the dinosaurs became extinct, the number and diversity of mammals exploded. In just 10 million years -- a brief flash of time by geologic standards -- about 130 genera (groups of related species) had evolved, encompassing some 4,000 species. These included the first fully aquatic mammals (whales) and flying mammals (bats), as well as rodents and primates.
This sudden expansion of species diversity into new ways of life is known as adaptive radiation. One way it occurs is in response to events that free up previously occupied environmental zones and roles, making way for many new species that adapt to these vacant living spaces. The extinction of the dinosaurs was one such major event, eliminating a once-dominant group of competitors while some mammals survived. But the mammals did not simply step into ecological roles vacated by the dinosaurs. It took several million years for the mammals to evolve even moderately large body sizes, and the world they inherited was a different place from the one the dinosaurs had dominated. There were new environmental habitats and new food resources to exploit. By the end of the Cretaceous, flowering plants had become dominant, providing food for burgeoning populations of insects, which in turn became another high-quality food source for the mammals, along with fruits and berries. New kinds of forests appeared, offering novel habitats for what would become tree-dwelling mammals -- primates, which first appeared about 50 million years ago, and eventually, some 45 million years later, upright-walking hominids, including us.
The astonishing diversity of mammalian species today stems in part from the continuing breakup of the continents that began some 200 million years ago and sent different landmasses moving apart. Australia and South America were isolated from other continents during much of the Tertiary, and marsupial mammals thrived and diversified there, while placental mammals took over similar roles on the other continents.
The story of the mammals is only one example of adaptive radiation, a process that has happened again and again, at scales both large and small, in evolutionary history.