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Eukaryotes could now get a great deal of energy which they could use in new ways. They could feed by pulling in sacs of membrane and so swallowing and digesting bacteria. Eukaryotes which ate bacteria are called protozoa, meaning first animals. One of these is a protozoan.

An amoeba is a typical protozoan

Reproduction was much more complex for eukaryotes than for bacteria. Mostly they still reproduced by dividing in two, but they evolved some machinery to make sure that each daughter cell got one copy of each chromosome and at least one mitochondrion. Later sexual reproduction evolved.

Biologists classify protozoa as belonging to the Protista.

The protozoa are single-celled eukaryotes, mostly too small to be seen with the naked eye but visible in the average microscope. Although they are the simplest eukaryotes, they are still far more complex than bacteria. Protozoa are a very diverse group, not closely related to one another.

There are more than 65,000 known species, over half of which are fossils. They are found in almost all soil, all water and their spores fill the air. Every time you drink a cup of water you swallow millions of live protozoa! Many are symbionts of other organisms, and about one-third of the living species are parasites.

Protozoan Diseases

Protozoa cause several diseases in plants and animals. Among those caused to humans are African sleeping sickness, amoebic dysentery, Chagas' disease, giardiasis, leishmaniasis, malaria, primary amoebic meningoencephalitis (PAM) and vaginitis.

Protozoan Benefits

Protozoa are useful in the food chain. Many eat bacteria, and so are important for passing on the molecules they make to other animals. This is vital since bacteria are the only living things able to fix nitrogen.

Size of Protozoa

Protozoa vary enormously in size. If you imagine a soccer ball inflated to the size of the Earth (Soccearth) then a protozoan would be anywhere between 500 meters and 100 km across. Typically they would be about 3 km across, about the size of a large factory.

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