CHAPTER 6: PEOPLE’S PLACE IN NATURE
Central to the scientific study of the diversity of life is a system of ordering data known as classification; the science of classifying organisms into different categories is known as taxonomy. The classification system used today was originally developed by Linnaeus. Linnaeus gave all living species a binomial name, or binomen, and placed species in genera, genera in families, and so on up the taxonomic hierarchy. Species were seen as unchanging, divinely created units, each of which had an archetype, or divine blueprint.
Modern taxonomists think of the species as a dynamic unit defined in terms of reproductive success. Evolutionary relationships between species can be deduced on the basis of structural similarities that are the result of inheritance from a common ancestor; such similarities are known as homologies. Structures in two different animals can also be similar without being homologies; such similarities are said to be homoplastic. Homoplasy can come about in several different ways. Independent evolution of similarities in related species is referred to as parallelism. Convergence refers to developments that arise in divergent evolutionary lines when similar selective pressures cause similar adaptations. In cladistics, a distinction is made between homologies that have appeared recently and are shared by a relatively small group of species (shared derived features) and homologies that first appeared a much longer time ago and are shared by a relatively large group of species (shared ancestral features).
People belong to the animal kingdom. This large group of organisms is divided into several phyla that represent basic body plans. The phylum Chordata is characterized by a notochord, dorsal hollow verve cord, and gill slits. In the largest subphylum of chordates, the vertebrates, the notochord is replaced by a vertebral column in the embryo. One group of early vertebrates gave rise - through the refinement of lungs and limbs - to the first land vertebrates, the amphibians. With the evolution of the amniote egg, reproduction was no longer tied to water. This evolutionary development resulted in the great reptilian radiation. Through a long evolutionary history, these reptiles ultimately gave rise to the mammals.
The mammals are a class of vertebrates. They are characterized by homeothermy and endothermy, heterodont dentition, mammary glands, and complex patterns of learned behavior. The mammals have radiated into 20 living orders. Included in one of these orders, the order Primates, are people.
