When dinosaurs ruled the world, scampering around their feet were platoons of diminutive insect-eating animals, part reptile, part something new. When the giant reptiles and many other animals were wiped out some 65 million years ago, the shrewlike newcomers prospered. They began evolving into different types of mammals and eventually gave rise to everything from field mice to elephants, whales, and humans. Now, the discovery of the skull of an animal the size of a paper clip pushes back the origin of mammals, including humans, to 195 million years ago. Found in China, the tiny skull shows evidence that the first mammals evolved from reptiles 45 million years earlier than widely believed. Researchers believe the tiny creature, Hadrocodium wui, is the oldest known true mammal and a missing link between reptiles and mammals.