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Living things are classified into three overarching groups: Eubacteria (bacteria), Archaea (archaea), and Eukaryota (plants, animals, and fungi). See the Tree of Life Web Project. Whether viruses are living or not is open to debate. Whence and how they originated is even more debatable.

Eubacteria are the true bacteria, including the blue-green algae, as well as the chloroplasts and mitochodria surviving in todays Eukaryota.

The Archaea are thought by some to have been the ancestors of the modern Eukaryota. Archaea surviving in today's world inhabit unlikely and difficult places. This group includes the thermophiles, halophiles, methanogens, and other "extremophiles." In olden days, the Eubacteria and the Archaea were lumped together in the Prokaryota. DNA studies have shown us that this lumping was mistaken.

The Eukaryota are the modern plants, animals, and fungi. They have complex cells with complex organelles within them. The Eukaryota cells contain within them the remnants of symbiotic bacteria -- the mitochondria in all eukaryotes and the chloroplasts in green plants. The chloroplasts appear to be descended from the blue-green algae in the Eubacteria.

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