Carolus Linnaeus was the first scientist to begin to classify organisms. During Linnaeus's time, the only known differences among living things were the characteristics that separated animals from plants. As biologists learned more about living organisms, they soon realized that only two kingdoms did not reflect the diversity of life. Since the 1700's when Linnaeus began his work, the classification system has continually changed as scientists have learned more and more about the diversity of life on Earth. Today all living organisms are classified into one of six kingdoms: Archaebacteria, Eubacteria, Protista, Fungi, Plantae, or Animalia. The chart below shows how the kingdoms have changed over time.

As scientists began to understand more about DNA, evolution biologists established a new taxonomic category - the domain. A domain is larger and more inclusive category than a kingdom. There are three domains - domain Bacteria, domain Archaea, and domain Eukarya.

Watch the following video to learn more about the three domains.