The first eukaryotic organisms on Earth are believed to have been protists, which are a diverse group of eukaryotic microorganisms. Protists include a wide range of organisms, such as algae, protozoa, and slime molds, and are thought to have evolved around 2 billion years ago. Protists are characterized by having a nucleus and other membrane-bound organelles, which distinguish them from prokaryotic organisms, such as bacteria. Protists play a vital role in the Earth's ecosystems, as they are primary producers, consumers, and decomposers.