The fundamental division of forms of life on Earth is not that between plants and animals, as is commonly assumed, but between prokaryotes—organisms … - Lynn Margulis

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 bacteria.

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.

[The smallest bacterium] is so much more like people than Stanley Miller’s mixtures of chemicals, because it already has these system properties. So to go from a bacterium to people is less of a step than to go from a mixture of amino acids to that bacterium.