Carbon is one of the main building blocks for life on Earth. It's abundant in our planet's atmosphere, where it's found in the form of carbon dioxide. Carbon makes its way into Earthlings' bodies mainly through the process of photosynthesis, which incorporates carbon dioxide into sugars that serve as components for important biomolecules and fuel the global food chain. About a third of this process globally is carried out by single-celled algae that live in the oceans (most of the rest is done by plants).