Further Reading

Dick, Steven J. Life on Other Worlds: The 20th Century Extraterrestrial Life Debate. Cambridge: Cambridge University Press, 2001.

SETI Institute homepage. Available online. URL: http://www.seti.org/Page. aspx?pid=1241. Accessed January 14, 2009.

Burgess Shale Paleozoic landscape showing opabinia, hallucigenia, wiwaxia, and pikaia (Publiphoto/Photo Researchers, Inc.)

these bear signs of early life. This was a time when the Earth's surface environment began a dramatic shift from reducing environments to highly oxidizing conditions. This may be when photosynthesis first developed in sulfur-reducing bacteria. oxygenic photosynthesis first developed in cyanobacteria, later transferred to plants (eukaryotes) through an endo-symbiotic association.

Life continued to have a major role in controlling atmospheric composition and temperature for the next couple of billion years. The first well-documented ice age occurred at the Archean-Proterozoic boundary, although some evidence points to other ice ages in the Archean. The Archean-Proterozoic ice age may have been related to decreasing tectonic activity and to less Co2 in the atmosphere. Decreasing plate tectonic activity resulted in less Co2 released by metamorphism and volcanism. These trends resulted in global levels of atmospheric Co2 falling, and this in turn caused a less effective greenhouse, enhancing cooling, and leading to the ice age.

Prolonged cool periods in the Earth history are called ice houses. Most result from decreased tectonic activity and the formation of supercontinents. Intervening warm periods are called hot houses, or greenhouses. in hot house periods, higher temperatures cause more water vapor to be evaporated and stored in the atmosphere, so more rain falls during hot houses than in normal times, increasing the rates of chemical weathering, especially of calcium silicates. These free Ca and si ions in the ocean combine with atmospheric Co2 and o2, to form limestone and silica that gets deposited in the oceans. This increased removal of CO2 from the atmosphere, in turn, cools the planet in a self-regulating mechanism. The cooling reduces the rate of chemical weathering. Previously deposited calc-silicates are buried and metamorphosed, and they release CO2, which counters the cooling from a runaway ice age effect, warming the planet in another self-regulating step.

The earliest bacteria appear to have been sul-fate-reducing thermophilic organisms that dissolved sulfate by reduction to produce sulfide. In this process the bacteria oxidize organic matter, transferring the electrons to sulfur and leading to the release of C02 into the atmosphere and the deposition of FeS2 (pyrite), which became massive sulfide deposits in the ocean sediments.

BIFs are rocks rich in iron and silica that are common in 2.2 to 1.6-Ga old rock sequences. They are the source of 90 percent of the world's iron ore. BIFs were probably deposited during a hot house interval and require low oxygen in the atmosphere/ hydrosphere system to form. scientists have hypothesized that water with high concentrations of Fe2+ was derived from weathering of crust.

Eukaryotes with membrane-bound cell nuclei emerged at about 2 Ga. Aerobic photosynthetic cells evolved and very effectively generated oxygen. These organisms rapidly built up atmospheric oxygen, to high levels by 1.6 Ga. The eukaryotes evolved into plants and animals. For the next billion years oxygen increased and Co2 fell in the atmosphere until the late Proterozoic, when the explosion of invertebrate metazoans (jellyfish) marked the emergence of complex Phanerozoic styles of life on the Earth. This transition occurred during the formation and breakup of the supercontinent of Gondwana, with associated climate changes from a 700-Ma global ice house (supercontinent), with worldwide glaciations, to equatorial regions. This was followed by warmer climates and rapid diversification of life.

See also Archean; black smoker chimneys; carbon cycle; Cloud, Preston; comet; fossil; historical geology.

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