Early mammals appeared around 200 million years ago during the Mesozoic Era. They were small, shrew-like creatures that lived alongside dinosaurs. These mammals were warm-blooded and had fur, which helped them regulate their body temperature and survive in various environments. After the extinction of the dinosaurs about 66 million years ago, early mammals began to diversify and evolve into many different forms. This period, known as the Cenozoic Era, saw the rise of larger mammals, including whales, elephants, and primates, leading to the rich variety of mammals we see today.