Taxonomy And Classification

Categorization is a skill every human uses, even for matters that extend beyond zoological nomenclature. A formal system of classification of organisms, or taxonomy, is essential because it provides a language so that people can collaborate, understand one another's results, and test one another's hypotheses.

Not all classification schemes for animals translate across cultures or stand the test of time. For example, the classification of animals in an ancient Chinese encyclopedia includes the following groups: (a) those that belong to the Emperor, (b) embalmed ones, (c) those that are trained, (d) suckling pigs, (e) mermaids, (f) fabulous ones, (g) stray dogs, (h) those that are included in this classification, (i) those that tremble as if they were mad, (j) innumerable ones, (k) those drawn with a very fine camel's hair brush, (l) others, (m) those that have just broken a flower vase, (n) those that resemble flies from a distance (Celestial Emporium of Benevolent Knowledge, referred to by Borges, 1964).

Because of its logical and scientific approach, the classification system created by Carolus Linnaeus translates universally (Table 2.2). In the Linnaean system, the genus and species are always underlined or italicized and the genus can be abbreviated so that Homo sapiens becomes H. sapiens.

At the time Linnaeus named the class that includes humans, he was publicly lobbying for the benefits of breastfeeding, so he chose "Mammalia" to honor mammary glands. His alternative name was the equally appropriate "Pilosa" for "hairy things." So Linnaeus chose to name hairy animals based on a trait that is only functional in half the species and, in them, only functional during a fraction of life. However, what Linnaeus did not know at the time is that mammary glands actually evolved from the same types of follicles in the skin that hairs evolved from, so in the grand scheme of things none of this name game actually mattered.

The taxonomy of organisms implies relationships that can be used to build phylogenies, or trees of relatedness, with the groups in a nested hierarchy. Two species, or branches on a phylogeny, are more closely related to each other than a third, three species are more closely related to each other than a fourth, and so on. Species are considered closely related if there is evidence for their shared ancestry in either their phenotypes or genotypes.

Evidence for evolutionary relatedness comes in the form of shared, derived features that have evolved since the last common ancestor ofthe species under consideration. Lack of a tail is a shared, derived feature linking humans and apes to the exclusion of more distantly related monkeys. Primitive, or ancestral, features like five fingers and toes are not significant and contain no information as to the relatedness of

Table 2.2 Classification of Humans

Kingdom

Animalia

Animals

Eumetazoa

Multicellular animals

Bilateria

Symmetrical animals with right and left sides

Phylum

Chordata

Notochords/spinal columns

Subphylum

Vertebrata

Backbones

Craniata

Having a skull: fishes, amphibians, reptiles, birds, mammals

Superclass

Gnathostomata

Jawed vertebrates

Class

Sarcopterygii

Lobe-finned fishes and terrestrial vertebrates

Tetrapoda

Four limbs

Amniota

Develop in an amniotic sac: reptiles, birds, and mammals

Class

Mammalia

Hair and mammary glands

Infraclass

Eutheria

Placental mammals

Order

Primates

Strepsirhines and haplorhines

Suborder

Haplorhini

Tarsiers, New and Old World monkeys, apes and humans

Infraorder

Catarrhini

Old World monkeys, apes, and humans

Superfamily

Hominoidea

Lesser apes, great apes, and humans

Family

Hominidae

Great apes and humans

Subfamily

Homininae

Chimpanzees and humans

Tribe

Hominini

Humans and their extinct bipedal ancestors, "hominins"

Genus

Homo

"Person"

Species

Homo sapiens

"Wise person"

humans, apes, and monkeys because all groups have five fingers and toes.

Convergent traits evolve in parallel in different lineages from a common ancestor that did not originally have the trait. For example, flight evolved in dinosaur descendents (birds) as well as in small mammals (bats).

Despite the complications that parallel evolution can bring to evolutionary phylogenies, parsimony is always the rule. The principle of parsimony is best summed up by the principle of "Occam's razor" which states that the simplest explanation is usually the correct one. With parsimony, phylogenetic trees are based on the fewest evolutionary changes and the fewest convergences.

Classification is ultimately arbitrary. A major problem with imposing taxonomy onto nature is that nature is not ordered into neat little categories. As one approaches the roots of lineages, it becomes more and more difficult to clearly place fossil species on either of the two or more branches that split around the time of its existence. For instance, the closer you get to the last common ancestor of humans and chimpanzees (LCA) in the fossil record, the harder it is to place fossils with confidence on either the chimpanzee or the human branch.

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