| Microbes as the First Organisms |
No one knows for certain where life began. Hot springs and volcanic (hydrothermal) vents on the ocean floor, however, may represent the kinds of environments where cellular life began. Before the ozone layer formed, the surface of Earth was exposed to strong radiation. Thus, most of the Earth's earliest organisms probably developed beneath the terrestrial surface or in the oceans. It's likely that these early microbes adapted to the high temperatures associated with abundant volcanic activity. Geological turmoil resulted in the accumulation of carbon dioxide in the atmosphere.
Sometime later, about 2 or 2.5 billion years ago, gaseous oxygen began to appear. Unlike the carbon dioxide, oxygen almost certainly came about because of microbes. Microbes similar to today's cyanobacteria were present at this time. We know this based on the presence of stromatolites - fossilized microbial mats consisting of layers of filamentous prokaryotes - and trapped sediment that date back to that time. Stromatolite-forming bacteria obtain carbon from carbon dioxide and get their energy by photosynthesis, splitting water to generate oxygen gas in the process. These organisms brought the oxygen level in Earth's atmosphere to about ten percent of what it is today - enough to allow the evolution of oxygen-using organisms. Gaseous oxygen also contributed to the formation of the ozone layer, which blocks UV radiation. New terrestrial habitats were now open for an evolving diversity of microbes.