We often think of dinosaurs as all living in a period far in the past. But the Mesozoic Era, as it is called, lasted from 250 million to 66 million years ago. In other words, it lasted almost three times longer than the time that separate us from it now. An early dinosaur like Stegosaurus lived about 150 million years ago. A later specimen like Tyrannosaurus lived around 67 million years ago. As for humans, the earliest australopiths appeared 4 millions years ago.