The next step (Chart Four), is truly enormous.  It includes most of the Phanerozoic Æon, where atmospheric oxygen approaches and eventually reaches (during the Devonian) modern levels, and multicellular organisms diversify into familiar forms, including dinosaurs, flowering plants, and the first birds. But it begins one billion years ago, at the top of the Proterozoic Æon, when the atmosphere was very different from now, and most life was still bacterial.