Plants are critical to other life on this planet because they form the basis of all food webs. Most plants are autotrophic, creating their own food using water, carbon dioxide, and light through a process called photosynthesis. Some of the earliest fossils found have been aged at 3.8 billion years. These fossil deposits show evidence of photosynthesis, so plants, or the plant-like ancestors of plants, have lived on this planet longer that most other groups of organisms. At one time, anything that was green and that wasn’t an animal was considered to be a plant. Now, what were once considered “plants” are divided into several kingdoms: Protista, Fungi, and Plantae. Most aquatic plants occur in the kingdoms Plantae and Protista.