Not only did life originate on earth very early in its history as a planet, but for the first two billion years, Earth was inhabited solely by bacter… - Lynn Margulis

From the paramecium to the human race, all life forms are meticulously organized, sophisticated aggregates of evolving microbial life. Far from leaving microorganisms behind on an evolutionary "ladder," we are both surrounded by them and composed of them. Having survived in an unbroken line from the beginnings of life, all organisms today are equally evolved.

The question "What is Life?" is... a linguistic trap. To answer according to the rules of grammar, we must supply a noun, a thing. But life on Earth is more like a verb. It repairs, maintains, re-creates, and outdoes itself.

The oxygen crisis that began only two billion years ago prompted the evolution of respiring bacteria. These microbes that used oxygen to derive biochemical energy more efficiently than ever before eventually took over most of the world. Some of the oxygen-breathing bacteria became symbiotic, merging with different (oxygen-eschewing) bacteria to form nucleated cells, which, after becoming sexual, evolved into fungi, plants, and animals.