The oldest fossils reveal that the first organisms were single bacteria-like cells that fed on dissolved mineral compounds in the oceans. Over the aeons as these simple living forms progressed and diversified, some varieties developed the ability for photosynthesis. The production of oxygen as a by-product of this metabolic process caused one of the greatest changes on our planet. About two billion years ago, atmospheric oxygen began to gradually rise from 1% to 20%. This higher concentration of oxygen was probably a catalyst that allowed complex multicellular life to evolve 700 million years ago.

Scientists can only guess at the total number of today's existing species, maybe as many as 30 million. These are just a fraction of the millions that have come and gone. The 1.4 million identified living species are divided into five major categories called kingdoms. The bacteria and similar simple cells are in one kingdom while more complex protozoans and algae are in a second. These kingdoms are the oldest and most widespread. Food webs begin and end with microorganisms. Some transform inorganic materials into compounds used by higher life forms while some decompose the remains of other organisms after death. The photosynthetic marine algae produce perhaps 70% of the oxygen required by almost every other life form on Earth.

Fungi belong to a third kingdom. Molds, mushrooms, and yeasts absorb recycled nutrients from the environment as the simpler species make it available. The multi-cellular plant kingdom photosynthesizes food for itself then feeds the members of the animal kingdom as well. Herbivores forage for plants then, in turn, become food for carnivores. Each organism within an ecosystem is inseparable from its companion species in the community because of the ecological processes that make life possible.