Chordata
Human beings are chordates—of the phylum chordata—and so are all other vertebrates, or animals with a spinal column. In addition, there are two invertebrate groups of chordates: the urochordates and the cephalochordates.
The Urochordata (e.g., tunicates) and Cephalochordata (e.g., lancelets) were the earliest chordates to evolve, and they provide a link between invertebrate and vertebrate animals. However, as different as these organisms arefrom each other and from vertebrate chordates, they all share the following characteristics that identify them as chordates (and distinguish them from all other invertebrate animals): a notochord, a dorsal hollow nerve cord, and pharyngeal gill slits. Many adult vertebrates have no notochord or pharyngeal gill slits, but these structures can nevertheless be found in their embryos.
The notochord is a long, elastic rod that provides structural support to the chordate body. In cephalochordates it prevents the body from shortening when muscle fibers in the body wall draw together, causing a bending from side to side and propulsion of the animal. In most vertebrates (except some fishes), bony vertebrae develop around the nerve cord and the noto-chord, and the vertebral structures largely replace the notochord in most adult vertebrates. However, some adult vertebrates may retain remnants of the notochord (e.g., the gelatinous disks between the vertebrae of humans). The dorsal hollow nerve cord is a key element of the chordate nervous system and is present in all chordates. In vertebrate embryos it develops into the spinal cord and the brain.
The pharyngeal pouches with gill slits originally evolved as filter-feeding devices and can still be found as such in invertebrate chordates. During some point in their development all chordates still exhibit them. However, among the vertebrates only fish retain pharyngeal gill slits as adults. The cartilage-based rods that support the gill bars (the solid areas between the gill slits) in invertebrate chordates gave rise to the vertebrate jaw during vertebrate evolution, completely changing the feeding method in this group of animals. Subsequently, some of the bones in the vertebrate jaw evolved into middle-ear bones in amphibians, reptiles, birds, and mammals; these bones assisted in the transmission of sound and hearing when early vertebrates moved from life in the water onto land.
Vertebrates differ greatly from other chordates in size and activity level, and the evolution of their distinctive characteristics is largely correlated with this difference. Vertebrates actively move around looking for food. This led to the concentration of sense organs at the front end of the body and an accumulation of nerve cells (i.e., a brain) to process all the sensory information. The need for more efficient movement led to the evolution of a stronger support system (vertebral column), a bony skeleton, and four limbs to support the body on land.
Today the vertebrates, with nearly 43,000 living species, are the most diverse group of all chordates. All vertebrate species can be grouped into seven different classes: Agnatha (jawless fishes), Chondrichthyes (cartilaginous fishes), Osteichthyes (bony fishes), Amphibia (amphibians), Reptilia (reptiles), Aves (birds), and Mammalia (mammals).
The first chordates appear as fossils in rocks from the Cambrian period. These rocks are approximately 570 million years old.
Phylogenetic Relationships of Major Groups.
Bibliography
Alexander, R. McNeill. The Chordates. New York: Cambridge University Press, 1975.
Brusca, Richard C., and Gary J. Brusca. Invertebrates. Sunderland, MA: Sinauer Associates, Inc., 1990.
Kluge, Arnold G. Chordate Structure and Function, 2nd ed. New York: Macmillan Publishing, 1977.
Walker, Warren F. Functional Anatomy of the Vertebrates: An Evolutionary Perspective. Philadelphia: Saunders College Publishing, 1987.
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