An animal (plural: animals) is defined as any of the eukaryotes of the Kingdom Animalia, typically characterized by a plastid exoskeleton, to which many specialized organs and tissues exist. Although all animals share some basic characteristics, such as a mouth, set of teeth, and generally being carnivorous, all animals differ greatly in many details including behavior, size, habitat, age, intelligence, ability to breed, reproductive rate, food preference, etc. To classify an animal accurately one has to consider at least several traits that are usually shared by all members of an animal kingdom. In order to compare two animals or distinguish between species, it is necessary to have a method by which to determine their genotype, in particular where the trait that one desires to observe can be found in only one species. The most common way to test for homologies is through a molecular basis, that is to look for sequences of DNA within an animal’s genome.
Homology is a type of comparison, like a species of cat or chicken, between different organisms. For example, humans and chimpanzees share many similarities, such as their upright posture, small body size, and similarities in facial features, whereas they are also different in several aspects. Humans, chimpanzees, and several other species of animals do not share a complete set of modern human traits, including speech, writing, some features of intelligence, technological development, and even their fossil record. A number of other animals, however, share some modern human traits, such as hand proportions, hand and finger bone anatomy, and some ability to reproduce. Evidence of evolution can be detected through fossils, or through similarities in structures between closely related organisms, such as sharks and whales, or birds and humans.
Many theories to explain the origin and survival of many animal species. It is possible that animals were able to diversify after the emergence of dinosaurs into the Early Cretaceous period because of a sudden influx of new plant and animal species. Evidence of migration and migratory routes for species can also be found. Another theory regarding the origination of species comes from the idea that there were only a few major planetes at the beginning of the universe, which led to planet hopping as continents began to separate and colonize one another.
Evidence of animals having sex lives can be found through fossils and morphological differences among species. Sex determination methods, such as X-rays and tests for hormonal levels, can be used to determine whether animals are asexual reproduction or not. Some animals have an apparent asexual reproduction strategy, such as reptiles and fishes, whereas other animals, such as amphibians and insects, have no discernible sexual function.
Most animals make use of systems to produce and secrete food, shelter, and energy. Animals also secrete chemicals that facilitate the excretion of waste products and regulate metabolism. Cells within the animal body secrete hormones and other chemicals. The development of life on earth began with the appearance of animals that reproduced both sexually and asexually. The first animals to appear on earth were not asexual, but sexual reproduction has been occurring since the beginning of time. Most animals reproduce sexually, to produce a child that will continue the species by producing eggs and a new animal body.
The origin of evil in the world also is tied to a sexual desire among animals. The extermination of animals for entertainment purposes, such as hunting and trophy hunting, along with the illegal trade in wildlife is now widely prohibited throughout the world. Some argue that animals are only a product of nature; e.g., horses are bred for the sole purpose of pulling a sled. Others argue that animals, like birds and fish are merely a product of nurture, e.g., a horse is able to run long distances without breaking down.