This was some 700 million years ago. It would take only another 100 Ma for certain organisms to develop a skeleton: hard parts that could be preserved in rock long after the organisms died. What we know of the past forms of Life on Earth is largely based on these fossils: they have given us a far more accurate picture of the past 600 Ma than we have of the billions of years that went before.
Small, hiding in the trees and living on insects, our mammal ancestors were anything but conspicuous. You might almost say they encouraged the world to forget they were there. For this, in fact, was the real beginning of the age of dinosaurs. Recent paleontologic research has given us a whole new perspective on these beasts. Some may have been warm-blooded. The great long-necked, plant-eating sauropods, like the celebrated Diplodocus, gradually gave way to animals sporting horns and duckbills, grazing no longer on the treetops but on grass and bushes. Their predators were those great carnivores, colorful and agile.
Then, 65 Ma ago, a huge catastrophe once again ravaged this world, which had seemed so perfectly adapted and balanced. This was the end of the dinosaurs and many mammals, but also of a great many other terrestrial and marine species, including the well known ammonites and a considerable number of smaller and less familiar organisms that constituted the marine plankton. In all, twothirds of the species then living (and possibly 80% of all individuals) were wiped out. This is the second great mass extinction. Yet again the momentum resumes, and in less than 15 Ma we find the ancestors of most animals that still live on our Earth today.
As the climate turns colder, modern fauna comes into place some 30 Ma ago. The age of dinosaurs has yielded to the age of mammals, delivered at last from their chief rivals. And the Mesozoic is succeeded by the Cenozoic Era.