It is thought that during the Carboniferous period, a group of amphibians gave rise to the reptiles. The first reptiles were small, lizard-sized animals, but they had many differences from their amphibian ancestors, including waterproof skin and thick-shelled eggs. This made it unnecessary for the reptiles to stay near water, to keep moist, or to lay their eggs in water. In fact, the evolution of the reptiles, geologically speaking, occurred very rapidly. Within about 40 million years, the reptiles had produced thousands of different species.