The Vertebrate Story - Life on Earth began some 3.5 billion years ago when a series of reactions culminated in a molecule that could reproduce itself. Although life forms may exist elsewhere in our universe or even beyond, life as we know it occurs only on the planet Earth. From this beginning have arisen all of the vast variety of living organisms—viruses, bacteria, fungi, protozoans, plants, and multicellular animals—that inhabit all parts of our planet.
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| The Vertebrate story - fosil |
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The diversity of life and the ability of life forms to adapt to seemingly harsh environments is astounding. Bacteria live in the hot thermal springs in Yellowstone National Park and in the deepest parts of the Pacific Ocean. Plants inhabit the oceans to the lower limit of light penetration and also cover land areas from the tropics to the icepacks in both the Northern and Southern Hemispheres. Unicellular and multicellular animals are found worldwide. Life on Earth is truly amazing! Our knowledge of the processes that create and sustain life has grown over the years and continues to grow steadily as new discoveries are announced by scientists. But much remains to be discovered—new species, new drugs, improved understanding of basic processes, and much more.
All forms of life are classified into five major groups known as kingdoms. The generally recognized kingdoms are Monera (bacteria), Fungi (fungi), Protista (single-celled organisms), Plant (plants), and Animal (multicellular animals). Within each kingdom, each group of organisms with similar characteristics is classified into a category known as a phylum.
Whereas many members of the Animal kingdom possess skeletal, muscular, digestive, respiratory, nervous, and reproductive systems, there is only one group of multicellular animals that possess the following combination of structures: (1) a dorsal, hollow nerve cord; (2) a flexible supportive rod (notochord) running longitudinally through the dorsum just ventral to the nerve cord; (3) pharyngeal slits or pharyngeal pouches; and (4) a postanal tail. These morphological characteristics may be transitory and may be present only during a particular stage of development, or they may be present throughout the animal’s life. This group of animals forms the phylum Chordata. This phylum is divided into three subphyla: Urochordata, Cephalochordata, and Vertebrata. The Urochordata and Cephalochordata consist of small, nonvertebrate marine animals and are often referred to collectively as protochordates. To clearly understand and compare their evolutionary significance in relation to the vertebrates, it is necessary to briefly discuss their characteristics.
Subphylum Urochordata (tunicates): Adult tunicates, also known as sea squirts, are mostly sessile, filter-feeding marine animals whose gill slits function in both gas exchange and feeding. Water is taken in through an incurrent siphon, goes into a chamber known as the pharynx, and then filters through slits into the surrounding atrium. Larval tunicates, which are free-swimming, possess a muscular larval tail that is used for propulsion. This tail contains a well-developed notochord and a dorsal hollow nerve cord. The name Urochordate is derived from the Greek oura, meaning tail, and the Latin chorda, meaning cord; thus, the “tail-chordates.” When the larva transforms or metamorphoses into an adult, the tail, along with its accompanying notochord and most of the nerve cord, is reabsorbed.
Subphylum Cephalochordata (lancelet; amphioxus): Cephalochordates are small (usually less than 5 cm long), fusiform (torpedo-shaped) marine organisms that spend most of their time buried in sand in shallow water. Their bodies are oriented vertically with the tail in the sand and the anterior end exposed. A well-developed notochord and long dorsal hollow nerve cord extend from the head (cephalo means head) to the tail and are retained throughout life. The numerous pharyngeal gill slits are used for both respiration and filterfeeding. Cephalochordates have a superficial resemblance to the larvae of lampreys (ammocoete), which are true vertebrates.
Serially arranged blocks of muscle known as myomeres occur along both sides of the body of the lancelet. Because the notochord is flexible, alternate contraction and relaxation of the myomeres bend the body and propel it. Other similarities to vertebrates include a closed cardiovascular system with a two-chambered heart, similar muscle proteins, and the organization of cranial and spinal nerves. No other group of living animals shows closer structural and developmental affinities with vertebrates. However, even though cephalochordates now are believed to be the closest living relatives of vertebrates, there are some fundamental differences. For example, the functioning units of the excretory system in cephalochordates are known as protonephridia.
They represent a primitive type of kidney design that removes wastes from the coelom. In contrast, the functional units of vertebrate kidneys, which are known as nephrons, are designed to remove wastes by filtering the blood. What long had been thought to be ventral roots of spinal nerves in cephalochordates have now been shown to be muscle fibers (Flood, 1966). Spinal nerves alternate on the two sides of the body in cephalochordates rather than lying in successive pairs as they do in vertebrates (Hildebrand,1995).
SubphylumVertebrata (vertebrates): Vertebrates are chordates with a “backbone”—either a persistent notochord as in lampreys and hagfishes, or a vertebral column of cartilaginous or bony vertebrae that more or less replaces the notochord as the main support of the long axis of the body. All vertebrates possess a cranium, or braincase, of cartilage or bone, or both. The cranium supports and protects the brain and major special sense organs. Many authorities prefer the term Craniata instead of Vertebrata, because it recognizes that hagfish and lampreys have a cranium but no vertebrae. In addition, all vertebrate
embryos pass through a stage when pharyngeal pouches are present. Most living forms of vertebrates also possess
paired appendages and limb girdles.
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