A Primer on the Evolution of Animals
Animals are multicellular, living, breathing organisms in the Kingdom Animalia. With few exceptions, all animals eat raw organic matter, breathe air, can move, reproduce sexually, and secrete body heat. All vertebrates (izards, snakes, insects, birds) and most mollusks (starfish, snails, oysters, clams) belong to the class Bacteria. The word animal comes from the Greek word that means ‘animal’. Since animals share many characteristics with humans and other mammals, most animal specialists refer to a particular animal’s body system as an animal’s anatomy.
All multicellular organisms are classified into two broad taxonomic groupings: Eutherians and Protists. Both kingdoms possess an animal kingdom, but they differ in regard to the arrangement of their various parts. The arrangements of the organs of an animal are very different from those of a non-anthroploid organism. The arrangement of the organs of the vertebrates, for example, is different from that of any other animal in the kingdom Animalia, while the arrangement of the parts of the bodies of animals such as fishes and mollusks is very similar to that of fishes and snails.
Many animal scientists work on comparative anatomy, concentrating on similarities and differences between species. Comparisons can be drawn between different animals by observing features such as the arrangement of limbs, feet, tails, the presence or absence of eyes, ears, claws, teeth, posture, hair, scales, and tail shape. Some characteristics of animals can also be studied more readily by molecular means; for example, the genetic makeup of an animal (whether it is a man, a woman, a mouse, or a rat) can be studied to determine whether that animal is monogenic (its entire genome is identical), multigenic (identical with its genome), or heterozygous (identical with one copy of its genome). In contrast to comparisons between species, comparisons between different animals can also be made by considering similarities in external appearance (a characteristic not shared by any member of the same species); in this case, genitalia may be compared between different species.
The classification of animals is also based on the extent of their ability to form communities. All living things belong to the animal kingdom because they live as communities of cells called zooids. Cells live together in clumps, which may be loosely connected by thin threads or bundles of DNA called chromosomes. Each animal has organs that function in a way similar to those of its fellow zooids; some are small and enclosed in a fuzzy environment of tissues, while others are large and enclosed in a thick coat of fur. Bacteria and protozoa, which belong to the bacterial and parasitic kingdom, often colonize animal cells.
The classification of animals is also based on the classification of habitats in the animal kingdom. Earthworms, for instance, belong to the phylum chordata, which includes such diverse animals as the mouse, rat, goat, chinchilla, salamander and parrot. Almost all land snails and semi-aquatic snails are placed in the phylum cephalopods. amphibians, such as all fishes and reptiles, are placed in the kingdom Protocoronectal Kingdom Animalia, while mammals, which include all land and aquatic animals, are placed in the kingdom Metaphysia.
Although modern humans have contributed to the extermination of many important animal species, including dinosaurs, some forms of animals have survived the extinction process. Evidence of ancient animal fossils has revealed the evolution of animals over time, including fish-and-land animals, egg-laying animals, placental mammals and arboreal insects. Evidence of animal evolution has been found in various places such as rocks, teeth, feathers, shells, hair, molars and fossilized teeth. The fossils we find in rocks, such as the fossils of Hualalai fossils found in the Hawaiian Island, provide strong evidence of pre-human life.