the precambrian era, or Supereon, refers to the geological time comprising the eons that came before the Phanerozoic eon. This time spans from the formation of Earth around 4.5 billion years ago to the evolution of abundant macroscopic hard-shelled animals, which marked the beginning of the Cambrian era, the first period of the first era of the Phanerozoic eon. The Precambrian era encompasses 86 percent of the Earth's history, however very little is known about this time period. In fact, the few fossil discoveries from this period were recently made in the late 20th century. Precambrian time can be further divided into three large eons, the Hadean, Archean, and Proterozoic eons.
The Precambrian's oldest eon, the Hadean (4.5 to 3.9 billion years ago), predates most of the geologic record. During the Hadean, the solar system formed out of gas and dust, the sun began to emit light and heat, and Earth took shape. Meteors and other galactic debris showered the planet over the first half-billion years, making it entirely uninhabitable. Planet Earth was very hot during its initial formation. As the earth began to cool and its mass increased, its gravitational field strengthened. This attracted meteorites and other debris, which continued to bombard the planet for at least another 500 million years, producing enough energy and heat to vaporize any water or melt any rock that may be present. Iron continued to sink to form the Earth's core, while silicon, magnesium, and aluminum gradually rose toward the surface. Gases released from magma
Highlighted in light areas are Precambrian rocks. They formed between 560 million years and 2.6 billion years ago.
inside the Earth escaped through cracks in the surface and began to collect in the early atmosphere. The likely presence of methane and ammonia among the gases made for conditions that would be highly toxic to life as we know it. Because there was little to no free oxygen, no protective ozone layers existed and damaging ultraviolet rays showered the Earth at full strength. As the meteorite bombardment finally slowed, Earth was able to cool, and its surface harden as a crust, rocks and continental plates began to form. Water began to condense in the atmosphere, resulting in torrential rainfall. After several hundred million years of falling rain, great oceans were formed. By about 3,900 billion years ago, Earth's environment had been transformed from a highly unstable state into a more hospitable place. This marked the beginning of the Archean eon (3.9 to 2.5 billion years ago). It was early in the Archaean eon that life first appeared on Earth.
The climate of the late Precambrian time, the Pro-terozoic eon (2.5 billion years ago to 543 million years ago) was typically cold with glaciations spreading over much of the earth. One of the most important events of the Proterozoic was the gathering of oxygen in the Earth's atmosphere. Though oxygen was undoubtedly released by photosynthesis well back in Archean times, it could not build up to any significant degree until chemical sinks unoxidized sulfur and iron had been filled. The first advanced single-celled and multicellular life roughly coincides with the oxygen accumulation. It was also during this period that the first symbiotic relationships between mitochondria and chloroplasts and their hosts evolved. At this time the continents were bunched up in to a single supercontinent known as Rodinia. It broke up starting around 750 million years ago, and as continental fragments reached the North and South poles they likely contributed to the great Ice Ages. In the latest Proterozoic era a new supercontinent called Pannotia came together. A number of glacial periods have been identified going as far back as the Huronian epoch, roughly 2,200 million years ago. The best studied is the Sturtian-Varangian glaciation, around 600 million years ago, which may have brought glacial conditions all the way to the equator, resulting in a "Snowball Earth". This theory states that the continents and oceans were covered in ice approximately 600 million years ago. The Earth may have remained in this frozen form, but it was rescued by the release of volcanic gases. While the Earth was in a deep freeze, chemical cycles were halted; as a result, carbon dioxide accumulated in the atmosphere causing an extreme greenhouse effect. After 10 million years of deep freeze, the Earth thawed in only a few hundred years. These dramatic events may have caused the explosion of life-forms seen in Cambrian fossils.
At this point, at the start of the Cambrian period, Earth had taken on its current form in the life-filled oceans and oxygenated atmosphere. Coevolution of biosphere and lithosphere over billions of years led to this point. Anaerobes and oxygen-breathers had evolved complementary chemical cycles, and bio-genic carbonates entered the plate-tectonic cycle of the crust and upper mantle with new efficiency.
sEE ALsO: Climate Change, Effects; Climate Cycles; Snowball Earth.
bibliography. J.C. Walker, "Precambrian Evolution of the Climate System," Global Planet Change (v.82, 1990); K. Zahnle, "A Constant Daylength during the Precambrian Era?" Precambrian Research (v.37, 1987).
Fernando Herrera University of California, San Diego
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