Protists are a diverse group of single-celled eukaryotes that were some of the first life forms on Earth, appearing as early as 1.5 billion years ago. They are classified based on how they obtain food, either through photosynthesis, as heterotrophs, or as parasites. Some protists move using flagella or cilia, while others do not move on their own. Protists include organisms that glow in the ocean and were formerly classified under the Kingdom Protista.