| Summary: | People are currently paying much attention to the ability of present and future human populations to influence Earth's climate through the emission of greenhouse gases (GHGs) such as carbon dioxide. According to some, such a self-induced change in our physical environment qualifies humans for the distinction of a "superspecies" that has passed a threshold of biological evolution. Some 2 to 3 billion years ago, another organism changed the environment in a very pronounced way: a primitive unicellular organism called cyanobacteria (blue- green algae). These cyanobacteria "discovered" a new energy source that enabled them to develop a photosynthetic apparatus that assimilates carbon dioxide through conversion of solar energy to chemical energy. In the process, a "useless" waste product, oxygen, began to accumulate and gradually changed the composition of the atmosphere. As a result, new, more complex oxygen-consuming forms of life evolved, eventually leading to humans. The algae that started it all, while still flourishing, then became the first link in a very elaborate food chain.
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