There are millions of different kinds of living things on Earth. To study them, scientists called taxonomists classify organisms by their characteristics. The first taxonomist was Carolus Linnaeus (1707–78), a Swedish naturalist who separated all creatures into two extremely large groups, or kingdoms: Plantae (plants) and Animalia (animals). By the middle of the 19th century, these two kingdoms had been joined by the newly designated Protista, the microscopic organisms, and Fungi. When microscopes advanced to the point that taxonomists could differentiate the characteristics of microorganisms, Protista was divided to include the kingdom Monera. By 1969, a five-kingdom classification system made up of Monera (bacteria), Protista (protozoans), Fungi, Animalia, and Plantae was established. The five-kingdom system is still in use today, although most scientists prefer to separate monerans into two groups, the kingdom Archaebacteria and the kingdom Eubacteria.
Monerans are the smallest creatures on Earth, and their cells are much simpler than the cells of other living things. Monerans that cannot make their own food are known as bacteria and include organisms such as Escherichia coli and Bacillus anthracis. Photosynthetic monerans are collectively called cyanobacteria, and include Anabaena affinis and Leptolyngbya fragilis. In the six-kingdom classification system, the most common monerans, those that live in water, soil, and on other living things, are placed in the kingdom Eubacteria. Archaebacteria are the inhabitants of extreme situations, such as hot underwater geothermal vents or extremely salty lakebeds.
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| Archaebacteria and Eubacteria |
Another kingdom of one-celled organisms, Protista, includes amoeba, euglena, and diatoms. Unlike monerans, protists are large, complex cells that are structurally like the cells of multicellular organisms.
Members of the Protista kingdom are a diverse group varying in mobility, size, shape, and feeding strategies. A number are autotrophs, some heterotrophs, and others are mixotrophs, organisms that can make their own food and eat other organisms, depending on the conditions dictated by their environment.
The Fungi kingdom consists primarily of multicelled organisms, like molds and mildews, but there are a few one-celled members, such as the yeasts. Fungi cannot move around, and they are unable to make their own food because they do not contain chlorophyll. They are heterotrophs that feed by secreting digestive enzymes on organic material, then absorbing that material into their bodies.
The other two kingdoms, Plantae and Animalia, are also composed of multicelled organisms. Plants, including seaweeds, trees, and dandelions, do not move around but get their food by converting the Sun’s energy into simple carbon compounds. Therefore, plants are autotrophs. Animals, on the other hand, cannot make their own food. These organisms are heterotrophs, and they include fish, whales, and humans, all of which must actively seek the food they eat.