Animal Evolution
Animals are multicellular, single-celled, eukaryotic living organisms in the kingdom Animalia. Without many exceptions, animals breathe oxygen, eat organic matter, can move, reproduce sexually, and secrete body heat. They are very complex in their organization and phylogenesis as compared to eukaryotes, which are single-celled and have a prokaryotic origin. A eukaryote is a protoplasm that has developed an organ through an evolutionary developmental process.
Plants are eukaryotic cells. The protoplasm of a plant cell is identical to the cytoplasm of an animal. The cell wall of a plant cell is very thick and consists of a single protein, chloroplast. The cell cycle of a plant cell is completed in about two million years. The cell of an animal is much longer, probably millions of times longer, although the exact number is not known.
The first animals to walk the earth were tiny insects, calledmoons, that are no bigger than a grain of sand or a pea. Insects have been around since before the beginning of recorded history. Over the course of time, animals evolved from these first ancestors of ours. Some evidences suggest that animals such as dinosaurs did not evolve from the same ancestors as humans.
Evidence of animal life spans millions of years, so it is quite possible that there are still animals alive today that belong to a previous extinct species of animals. Evidence of earlier animal life on Earth includes fossils, spores and other marks of animals that have died. Evidence for the pre-human era or the Paleolithic age of the early Earth is also available in the form of artifacts such as bone tools, weapons, and even art.
Plants and animals both derive nourishment from the sun and plants require carbon dioxide to photosynthesize, which involves the conversion of food into starch. Therefore, all living things on Earth are dependent on one another for food and oxygen to function properly. All land animals, including humans, derive their energy from the sun through respiration. In fact, oxygen is the most prevalent element present in the Earth’s atmosphere. Air plants and land animals such as cattle, horses, deer, sheep, goats, chickens, ducks, and frogs all breathe air from their lungs, while land animals such as whales and sharks breathe only from their gills.
Pets are not domesticated because they were tamed by man, but because of certain characteristics that all domestic animals share. All wild animals are accustomed to people and therefore respond to them in a certain way. Domesticated domestic animals do not always behave in the same way, depending upon their original behavior. Nevertheless, they are still domestic animals, belonging to the same family.
