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. 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.
- The Vertebrate Story: An Overview
- Systematics and Vertebrate Evolution
- Vertebrate Zoogeography
- Early Chordates and Jawless Fishes
- Gnathostome Fishes
- Evolution of Reptiles
- Reptiles: Morphology, Reproduction, and Development
- Population Dynamics
- Intraspecific Behavior and Ecology
- Interspecific Interactions
- Techniques for Ecological and Behavioral Studies
- Extinction and Extirpation
- Conservation and Management