Why does everybody agree that atmospheric oxygen... comes from life, but no one speaks about the other gases coming from life? - Lynn Margulis

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.

In the long run, the most vicious predators, like most dread disease-causing microbes, bring about their own ruin by killing their victims. Restrained predation—the attack that doesn't kill or does kill only slowly—is a recurring theme in evolution. The predatory precursors of mitochondria invaded and exploited their hosts, but the prey resisted. Forced to be content with an expendable part of the prey (its waste)... some mitochondria precursors grew but never killed their providers. ...The original prey was probably a larger bacterium like Thermoplasma.

[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.