Learn how bacteria living just beneath the surface of underwater mounds of mud and sand are descendants of Earth’s oldest living things, in this video from NOVA: Making North America: Life. While stromatolites are rare today, fossil evidence suggests the ancestors of the microbes that build them were the dominant form of life for most of Earth’s history. They used the gases in Earth’s early atmosphere—some of which would be toxic to most living things today—to their advantage. They took in carbon dioxide and water and released oxygen into the atmosphere and helped create the conditions that would later support life as we know it.