EVOLUTION AND DIVERSITY
Protistans are a diverse group of unicellular and multicellular eukaryotic organisms that inhabit aquatic or moist habitats. Over 65,000 species of protistans have been identified, and the relationships between these organisms are constantly being revised and updated.

While most protistans are free living, some protistans enter into mutualistic, commensal, or parasitic relations with other organisms. Additionally, many protistans are autotrophs, but there are a substantial number of heterotrophic protistans as well. Protistans are important in ecosystems as primary producers and primary consumers, and their activities help to maintain the natural balance of the biosphere.