The abundance of molecular oxygen (O₂) in the atmosphere has varied enormously during Earth's history. O₂ first appeared as a chemical by-product of photosynthesis over 2 billion years ago and reached its zenith of about 30% during the Carboniferous period approximately 300 million years ago. Many geological and biological processes regulate atmospheric O₂ levels, and it is interesting to note that increased metazoan body sizes, as well as the appearance of placental mammals, correlate with a rise in O₂ levels from 10 to 20% over the past 200 million years. (Note that metazoans are the multicellular animals that make up the major portion of the animal kingdom, whose cells are organized in layers or groups as specialized tissues or organ systems.)