Ediacaran biota Precambrian cholesterol Dickinsonia anaerobic respiration living
Animals are found as long ago as the Ediacaran biota, towards the end of the Precambrian, and possibly somewhat earlier. It had long been doubted whether these life-forms included animals, but the discovery of the animal lipid cholesterol in fossils of Dickinsonia establishes their nature. Animals are thought to have originated under low-oxygen conditions, suggesting that they were capable of living entirely by anaerobic respiration, but as they became specialized for aerobic metabolism they became fully dependent on oxygen in their environments.
