0.35 Bis2a 10.0 prokaryotes: bacteria and archaea  (Page 2/7)

The first microbial mats likely obtained their energy from chemicals found near hydrothermal vents. A hydrothermal vent is a breakage or fissure in the Earth’s surface that releases geothermally heated water. With the evolution of photosynthesis about 3 billion years ago, some prokaryotes in microbial mats came to use a more widely available energy source—sunlight—whereas others were still dependent on chemicals from hydrothermal vents for energy and food.

Stromatolites

Fossilized microbial mats represent the earliest record of life on Earth. A stromatolite is a sedimentary structure formed when minerals precipitate out of water by prokaryotes in a microbial mat ( [link] ). Stromatolites form layered rocks made of carbonate or silicate. Although most stromatolites are artifacts from the past, there are places on Earth where stromatolites are still forming. For example, growing stromatolites have been found in the Anza-Borrego Desert State Park in San Diego County, California.

The ancient atmosphere

Evidence indicates that during the first two billion years of Earth’s existence, the atmosphere was anoxic , meaning that there was no molecular oxygen. Therefore, only those organisms that can grow without oxygen— anaerobic    organisms—were able to live. Autotrophic organisms that convert solar energy into chemical energy are called phototrophs , and they appeared within one billion years of the formation of Earth. Then, cyanobacteria , also known as blue-green algae, evolved from these simple phototrophs one billion years later. Cyanobacteria ( [link] ) began the oxygenation of the atmosphere. Increased atmospheric oxygen allowed the development of more efficient O ₂ -utilizing catabolic pathways. It also opened up the land to increased colonization, because some O ₂ is converted into O ₃ (ozone) and ozone effectively absorbs the ultraviolet light that would otherwise cause lethal mutations in DNA. Ultimately, the increase in O ₂ concentrations allowed the evolution of other life forms.