Attempts to classify plants and animals (taxonomy) date back to Classical Greece. Modern taxonomy was pioneered by Linnaeus in the 18th century. Linnaeus listed several thousand species of plants and animals and introduced the now-standard system of nomenclature: a double-barrelled Latin name comprising the genus (type) and species, as in Homo (genus) sapiens (species). Genera were grouped into families, families into classes, classes into orders, and the orders into one of two kingdoms, plant and animal. Animals were divided into vertebrates (mammals, birds, reptiles, amphibians and fish) and invertebrates (all the rest).
A number of difficulties have become apparent in Linnaeus's system. During the 250 years since it was published the number of known species has grown to millions. The total number of animal species believed to exist is estimated at thirty million. Then there are all the plants, fungi and single-celled eukaryotes; and the prokaryotes, the most ancient and most numerous forms of life on Earth23. When we consider all organisms - animals, plants, fungi, protists and prokaryotes - the number of distinct types in the world today might be around a hundred million. (We use "types" here rather than "species" because of the difficulty of applying the word "species" to prokaryotes; see footnote 23.) That is the world today. It is generally presumed that at least 99% of all species that have ever existed are now extinct; extinction seems to be the ultimate fate of all. This suggests that the number of types of organism that have existed since the origin of life may be around ten thousand million (1010). The figure might be inaccurate but the implication is clear: the variety of ways of being a viable organism is staggering.
Because we know so much more about species, past and present, than was known to Linnaeus, taxonomy has changed dramatically through the intervening years. Late in the 19th century, the German evolutionist Haeckel suggested a third kingdom, monera, distinct from animals or plants and comprising all microscopic organisms. Haeckel also added another level of
23 During the 1970s, Erwin identified 1100 species of beetle inhabiting one species of Amazonian tree and he estimated that 160 of these beetle species were unique to that tree. Extrapolating from these figures, 50,000 species of tropical trees implies 50,000 x 160 = 8 million species of beetles. Given that beetles seem to account for a quarter of all animal species, this suggests that there may be about 30 million species of animals. As for prokaryotes, it is debatable whether we can use the term "species" for them as we can for eukaryotes. They can exchange genes, singly or in small groups, almost at liberty, which makes the concept of a species-related genome inapplicable. However, it is generally assumed that there is more divergence among prokaryotes than there is among eukaryotes as a whole - which would not be surprising, given their relative venerability as tenants of the Earth.
classification, the phylum, intermediate between order and kingdom. Humans are now classified as: kingdom - animal; phylum - chordates (vertebrates); order - mammals; class - primates; family - hominids; genus - Homo; species - sapiens. Early in the 20th century, Copeland divided Haeckel's monera into protists (single-celled eukaryotes) and prokaryotes. In the 1950s, Whittaker suggested that, because fungi are fundamentally different from plants, they should be regarded as a separate fifth kingdom. Most biologists today consider every living organism to belong to one of these five kingdoms.
In the 1980s, Woese divided prokaryotes into (a) what are now called archaea and (b) true bacteria. This distinction is based on a single ribosomal gene, but Woese considers it so dramatic, so fundamental, that archaea and bacteria differ as much from one another as they do from eukaryotes. Not many biologists entirely agree with Woese, but the distinction between archaea and bacteria is generally recognised. These organisms look similar but biochemically they are very different. Nevertheless archaea can exchange genes with bacteria, just as bacteria do with other bacteria.
One effect of the taxonomic revolution has been to marginalise our own species. Once upon a time there were two kingdoms, one of which (animals) was divided into vertebrates and invertebrates. We were a significant example of the vertebrates. This was a hopelessly biased classification. It ignored microscopic life, which is most of life. Moreover, dividing the animal kingdom into vertebrates and invertebrates was like dividing the world's land masses into the Isle of Malta and Not the Isle of Malta. Now there are five kingdoms, of which the animal kingdom by no means the biggest and is divided not into two sub-kingdoms but some thirty-five phyla, of which chordates (vertebrates) are just one. Alternatively, if Woese's taxonomy is accepted, there are three super-kingdoms, and animals in toto make up one subdivision of just one of these.
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