Zoologists have established a classification scheme to organize the structural and evolutionary relationships among living organisms. The highest taxonomic level is the kingdom, and the most specific level is the species. As humans, we are a species belonging to the kingdom Animalia.

We also belong to the phylum Chordata and subphylum Vertebrata, along with fishes, amphibians, reptiles, birds, and other mammals. All chordates have three structures in common: a notochord (no-to-kord), a dorsal hollow nerve cord, and pharyngeal (fa-rin-je-al) pouches. These chordate characteristics are well expressed during our embryonic stage of development and persist, to a certain extent, into adulthood. The notochord is a flexible rod of tissue that extends along the back of an embryo. Traces of the notochord are found in the gelatinous center within each intervertebral disc of the vertebral column. The dorsal hollow nerve cord is positioned above the notochord and develops into the brain and spinal cord. Pharyngeal pouches form gill openings in fish and some amphibians. In other chordates, such as humans, embryonic pharyngeal pouches develop, but persist as chambers within the ear region.

Being members of the class Mammalia, we are vertebrates with hair and mammary glands. Hair is a protective covering for most mammals, and mammary glands serve for sucking the young. Other characteristics of mammals include three small middle-ear bones, a fleshy outer ear, teeth of various shapes (such as incisors and molars), a lower jaw formed by a single bone, usually seven cervical vertebrae (neck bones), an attached placenta, well-developed facial muscles, a muscular diaphragm in the trunk, and a four-chambered heart with a left aortic arch.

The class Mammalia is subdivided into 19 groups called orders. Along with monkeys and apes, we belong to the order Primates. Members of this order have grasping hands and a relatively large, well-developed brain.

Humans are the only living members of the family Hominidae. Homo sapiens is included within this family to which all the ethnic groups of humans belong.

A few of our anatomical characteristics are so specialized that they are diagnostic in separating us from other animals--even from other closely related mammals. Other of our characteristics are equally well developed in other animals, but when these function with the human brain, they provide us with remarkable and unique capabilities. Some of our distinguishing characteristics are as follows:

1. A large, well-developed brain. The adult human brain weighs about 3 pounds (1350 grams). This gives us a large brain-to-body-weight ratio. But more important, the brain has specialized structures that account for emotion, thought, reasoning, and memory.
2. Bipedal Iocomotion. Because we walk on two limbs, our style of locomotion is referred to as bipedal. Our upright posture imposes other structural features, such as the sigmoid (S-shaped) curvature of the spine and arched feet.
3. An opposable thumb. Our thumb joint is adapted for versatility in grasping. Most primates have opposable thumbs.
4. Articulated speech. The structure of our vocal organs and our well-developed brain account for our ability to speak.

We also differ from other mammals in the number and arrangement of our vertebrae, the kinds and number of our teeth, the extent of our facial muscle development, and in certain features of our body organs that are uniquely human.

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