Internal and external forces on Earth's atmosphere have changed its chemical composition over time. A. Mammals could not have survived without an oxygen-rich atmosphere, and land-based life would not be possible without the ozone layer to filter solar radiation. B. Although they are currently at about 21 percent, oxygen levels will probably not always remain this high. C. The breakdown of organic matter removes free oxygen, but if this process is interrupted, extra oxygen may accumulate in the atmosphere. D. Over the last 4 billion years, outgassing destroyed Earth's primary atmosphere of volatile elements and replaced it with nonvolatile materials including carbon dioxide. E. The small amount of oxygen in Earth's early atmosphere was due to photo dissociation and, later, photosynthesis created free oxygen. F. When oxygen levels in the ocean water reached a critical level about 542 million years ago, life emerged in the oceans, as shown by sedimentary rocks.