Eukaryotic cells use O₂ to burn organic fuels—carbohydrate, lipid, and protein—for ATP production (Chapter 7). The increase in atmospheric O₂ with the evolution of photosynthesis was key to the evolution of O₂-based cellular respiration, and that in turn contributed to the diversification of multicellular animal life during the Cambrian Period, about 500 million years ago. In the process of burning fuel by aerobic respiration, CO₂ is produced. Animals have evolved mechanisms to acquire O₂ from their environment and eliminate excess CO₂ from their bodies to the environment. These mechanisms make up the process known as animal respiration—or “breathing.” The transport of O₂ and CO₂ between an animal and its environment is referred to as gas exchange, and is fundamental to all eukaryotic animals, as well as to photosynthetic plants.