There are more than 1.5 million animal species. Ranging from simple animals like sponges and jellyfish, the invertebrates include the arthropod phylum with over 1 million species of insects. The higher- level vertebrates include fish with about 35,000 species. Around 350 million years ago, when aquatic animals moved to land, they adapted to the new environment and amphibians were born. They adapted some more and became reptiles. From reptiles came the birds and around 4000 known species of mammals.
