Summary: The fossil record contains evidence of large animals only for the most recent 15 percent of Earth's history. Before then, life on our planet consisted primarily of microbes, which have left microfossil and chemical evidence of their existence. Microbes had a profound effect in shaping Earth's environment in the past. For example, when cyanobacteria evolved oxygen-generating photosynthesis, the event led to the oxygenation of the atmosphere and the evolution of eukaryotes and animals. Today many of Earth's ecosystems continue to depend on microbes. The same methods used to study the history of life on Earth are also being used to determine whether Mars ever supported life.
