Anaerobic bacteria still exist today, but back then they had the upper hand. Earth’s atmosphere had little oxygen in it. But then something happened. Some of these anaerobic bacteria evolved to new forms -- a precursor to modern pond scum. And in the normal course of their daily lives, they started producing oxygen.

At first none of the anaerobic bacteria noticed the oxygen their pondscum neighbors produced. For years, the two life forms sat right next to one another. For most of that time, nothing much happened. Whatever oxygen went into Earth’s atmosphere was quickly absorbed by iron in the surrounding rock -- the iron, in other words, rusted. Earth in those days had a lot of pure iron that hadn’t yet oxidized. For hundreds of millions of years, the aerobic bacteria kept producing gas like the flatulent rider on a commuter train, and Earth kept rusting and rusting, until it could rust no more. All the iron had been used up.

Oxygen kept coming -- the pond scum were doing quite well -- but it had nowhere to go. It just stayed in the atmosphere. Eventually the pond scum gassed the anaerobic bacteria to kingdom come -- at least the ones that couldn’t scuttle under5 some crack or hide in the deep-water vent, away from the oxygen rich air. “From the standpoint of a microbe 2.4 billion years ago,” says Erwin, “the onset of the introduction of all this oxygen was a horrible event, because it changed the environment completely. Anaerobic bacteria had to go hide in the mud or something. Oxygen would have been a toxin to a lot of microbes that were adapted to a very different environment. ~Page 42