The Evolution of the Vertebrate Brain

The phylum Chordata is a large and diversified group containing more than 35,000 species, ranging from small wormlike forms to man. All chordates have three characteristics in common at some stage of their life cycle; a human embryo has the same notochord, a single, dorsal, tubular nerve cord, and pharyngeal gill slits as the lowest ancestral chordates. The primitive notochord, the predecessor of the spinal cord, is the feature that has allowed the development of higher and more complexified forms of life. Instead of a widespread system of nerves, the centralization of nerves has allowed specialization and greater control of body areas.