All animals living on our planet today and all those that have become extinct over the geological time constitute the Animal kingdom. The study of the origin, evolution, classification and varied activities of animal species constitute the biological science called Zoology.
Although life is supposed to have originated in a primitive atmosphere in the pre-biological period some 4500 million years ago, the true animals are considered to have originated only about 600 million years ago. In the pre-biological period of the earth's history, molecules of amino acids, nucleic acids and carbohydrates accumulated as organic soups in the surface waters of the earth by the action of solar and electrical energy on ammonia and methane in the atmosphere. Primitive organisms arose from these molecules and utilised them as raw materials for their metabolism. Because the atmosphere lacked oxygen, these organisms probably obtained energy from the fermentation process acting directly on the raw materials. Gradually, the early organisms developed the ability to synthesise complex molecules from simpler compounds. Ultimately, the green plants arose. The photosynthetic activity of plants in which carbon dioxide was utilised as the raw material to produce carbohydrates using solar energy, produced oxygen as a waste product. With the eventual accumulation of excess of plant-produced oxygen in the atmosphere animals became possible. These primitive animals used plants as raw material and oxygen as a source of energy.