Fish were the first vertebrates, appearing during the Cambrian Period approximately 500 million years ago. Their bodily shapes and lifestyles evolved in diverse ways, and, at present, over 32,000 species are known worldwide. They have advanced into various bodies of water, from rivers and lakes in plains to mountain streams in highlands with an elevation in excess of 5,000 meters, from tropical coral reefs to polar ice seas, and from shorelines to ocean depths of over 7,000 meters.