Kingdoms of Life

Life on Earth originated between 3.5 and 4 billion years ago. Since then millions of different organisms have evolved (and most have gone extinct). Faced with such a multitude and diversity of organisms, biologists have looked for ways to classify all these organisms into different groups to make it easier to study them. But what criteria should they use for grouping? And how big or small should the groups be?

The most meaningful way of grouping organisms is based on how they are related. In this way, group membership will reflect the organisms' common evolutionary history. This means that scientists want to group organisms that share a common ancestor. In addition, they want to show characteristics that unite all group members with each other but that also distinguish group members from nongroup members. For example, all mammals have hair and milk glands, characteristics unique to mammals. The number of groups and the number of organisms within a group depends on the grouping criteria and whether the emphasis is on how organisms are different or how they are the same (this is often referred to as the "splitters" vs. "lumpers" approach). There is really no incorrect way of grouping organisms as long as the grouping criteria are clearly stated and strictly followed.

Systematists (scientists that study the classification of organisms) have traditionally considered the kingdom to be the highest and most inclusive category. However, deciding exactly how many kingdoms to recognize had been a source of controversy until 1969, when Robert H. Whittaker of Cornell University introduced a five-kingdom system that became popular with most biologists. Whittaker's five kingdoms are Monera, Protista, Plantae, Fungi, and Animalia. One kingdom, Monera, contains all the prokaryotes , and the other kingdoms contain different groups of eukaryotes. However, work done in the 1980s by Carl R. Woese of the University of Illinois on the genetic makeup of cells seems to favor a six-kingdom system that divides Monera into two kingdoms, Bacteria (Eubacteria) and Archae (Archaebacteria).

Bacteria and Archae are both prokaryotes, but when researchers compared their ribosomal RNA sequences, they found that Archae are more closely related to eukaryotes than they are to other prokaryotes. It appears that eukaryotes evolved from an Archae-like ancestor and subsequently took up genes from Bacteria (e.g., purple bacteria, cyanobacteria), thus acquiring mitochondria and chloroplasts.

Bacteria live in many different environments. This kingdom includes many pathogens , including Salmonella, which causes food poisoning, but also many economically important species such as Lactobacillus, which is used to make yogurt; Rhizobium, which "fixes" atmospheric nitrogen for plants to use; and Streptomyces, which is a source of many antibiotics. In contrast, Archae, including the methanogens, halophils, and thermophiles, live in extreme environments that are hot, salty, or acidic such as hot springs or deep sea vents. One Archae, a thermophil, is the source of a heat-resistant enzyme that is widely used in molecular biology.

The eukaryotic Plantae, Fungi, and Animalia kingdoms contain mostly multicellular eukaryotes that differ in their structures, modes of nutrition, and life cycles (before reproduction). For example, Plantae (mosses, ferns, conifers, flowering plants) have cell walls containing cellulose and are autotrophs , which means they make their own food from carbon dioxide and an energy source such as sunlight. In contrast, both Fungi (yeasts, mushrooms, truffles, bread molds) and Animalia (jellyfishes, sponges, worms,

snails, insects, fishes, amphibians, reptiles, birds, mammals) are heterotrophs , which means they get their energy by eating other organisms. Fungi and Animalia differ in that Fungi secrete digestive enzymes and then absorb the digestive juices, while Animalia ingest other organisms. In addition, Fungi have cell walls, Animalia do not.

The kingdom Protista contains a wide variety of organisms, ranging from single-celled autotrophs (diatoms, dinoflagellates) and heterotrophs (amoebas, ciliates) to colonially living heterotrophs (slime molds) and large multicellular autotrophs (algae such as kelp and seaweed). Protists were the first eukaryotes to evolve, and members of this group gave rise to the other eukaryotic kingdoms. The diversity that we find among the organisms in the kingdom Protista stems partly from the fact that Protista is what systematists call a "wastebasket" kingdom. This means that any organism the systematists cannot assign to one of the other kingdoms (using the above grouping criteria) is assigned to the Protista. This also means that with further work and the establishment of new grouping criteria, we may see a further division of the Protista into several new kingdoms in the not-too-distant future. Science is an ongoing process!

see also Phylogenetic Relationships of Major Groups.

Kathrin F. Stanger-Hall

Bibliography

Campbell, Neil A., Jane B. Reece, and Lawrence G. Mitchell. Biology, 5th ed. Menlo Park, CA: Addison Wesley Longman Inc., 1999.

Purves, William K., Gordon H. Orains, and H. Craig Heller. Life: The Science of Biology, 4th ed. Sunderland, MA: Sinauer Associates Inc., 1995.

Doolittle, W. Ford. "Uprooting the Tree of Life." Scientific American 282, no. 2 (2000):90-95.