People’s Place in Nature Chapter Six People’s Place in Nature

Linnaeus’s Classification The science of classifying organisms into different categories is known as taxonomy. The most significant early attempt at taxonomy was by Linnaeus. This system is the basis of the classification system used in modern biology. The basic unit of classification is the species. Each plant or animal is known by a binomen, or two-part name consisting of a generic and a specific name. This is the system of binomial nomenclature. For example, the binomen for humans is Homo sapiens.

The Taxonomic Hierarchy Taxa above the level of genus are referred to as the higher taxa. There are five main higher taxa: family, order, class, phylum, and kingdom. A family contains similar-appearing genera, an order is group of similar- appearing families, and so on. KINGDOM: Animalia PHYLUM: Chordata SUBPHYLUM: Vertebrata CLASS: Mammalia INFRACLASS: Eutheria ORDER: Primates SUBORDER: Anthropoidea SUPERFAMILY: Hominodea FAMILY: Hominidae GENUS: Homo SPECIES: Homo sapiens

Determining Evolutionary Relationships The development of classifications depends on knowledge of evolutionary relationships among taxa. A comparison between two animals may reveal many anatomical correspondences. Structural correspondences that are the result of inheritance from a common ancestor are known as homologies or homologous features.

Determining Evolutionary Relationships Structures that are similar in two species, without being due to inheritance from a common ancestor, are said to be homoplastic. Homoplasy can develop in four different ways: parallelism, convergence, analogy, and chance.

Cladistics In cladistics, homologies that appeared recently and are shared by a relatively small group of closely related taxa are called shared derived features. Homologies that first appeared long ago and are shared by a larger group of species are called shared ancestral features. Uniquely derived features are particular to a specific species within a group of species.

People and the Animal World Animals must obtain their nutrients from consuming other organisms. Animals are composed of specialized kinds of cells. They are highly mobile and have contracting fibers, such as muscles. Most respond quickly to changes in their environment. The animal kingdom is divided into several units known as phyla, with each phylum representing a basic body plan. Humans belong to the phylum Chordata. A distinctive feature of the chordates is the presence of an internal skeleton and a notochord and nerve cord that run along the back of the animal. All chordates also have gill slits at some time in their life history.

The Vertebrates The phylum Chordata includes the subphylum Vertebrata, which is comprised of most animals in the phylum. In place of a notochord, a true vertebral column, or spine, has developed. A major event in vertebrate evolution was the evolution of jaws. The ancestors of land vertebrates were freshwater bony fish capable of coping with drought conditions. These vertebrates had lungs for supplementing their oxygen supply. The origin of land vertebrates also depended on the evolution of limbs.

The Mammals Mammals are homeothermic and can control their body temperature through physiological means. Hair or fur provides a layer of insulation. Mammals are characterized by heterodont dentition and have two sets of teeth, deciduous and permanent. The reptilian jaw is composed of six bones, while the mammalian lower jaw is composed of only two bones, often fused into a single structure. Other anatomical features aid mammals in dealing with terrestrial habitats. These characteristics include the diaphragm, hard palate, and the four-chambered heart.

The Mammals Offspring develop inside the mother. Newborn mammals are nourished by taking milk from their mother’s mammary glands. Care is given to the young by the mother and often by the father and other adults. Much of mammalian behavior is learned. Mammals are characterized by improvements in the nervous system and elaboration of the brain. Mammals are divided into prototherian mammals (the monotremes who lay eggs) and therian mammals. Therian mammals are divided into marsupial and placental mammals.

Evolutionary Relationships among the Mammals From Novacek, M.J., “Mammalian Phylogeny: Shaking the Tree,” Nature, 356 (12 March 1992), 121-125. Used by permission of Nature Publishing Group (New York) and Michael J. Novacek, Ph.D.