Complex sentence: The first tetrapods lived in waterside habitats and fed primarily on fish, today the Earth supports a great diversity of tetrapods that live in many habitats and subsist on a variety of diets.
Split into simpler sentences: The first tetrapods lived in waterside habitats and fed primarily on fish. Today, the Earth supports a great diversity of tetrapods that live in many habitats and subsist on a variety of diets.