Our planet gained its nice, oxygen-rich atmosphere about 2.4 billion years ago thanks to early bacteria. One question that has puzzled researchers, however, is why it took at least 300 million years for oxygen to build up to large levels, even though the bacteria had been working madly to produce it. Researchers from the University of Washington have developed a model that shows how volcanic gasses could have sucked up this available oxygen. Not only that, but a large layer of iron from meteorite strikes would have used it for rusting. Not until those sinks were filled could oxygen build up.