Precambrian
In geologic time, Precambrian time encompasses the time from Earth's formation, approximately 4.5 billion years ago, until the start of the Cambrian approximately 540 million years ago (mya). Because the Precambrian is not a true geologic eon, era, period, or epoch, geologists often refer to it as Precambrian time (or simply, Precambrian). Precambrian time represents the vast bulk of Earth's geologic history and covers nearly 90% of Earth's history.
Although scientists do not yet know all the exact steps by which the earth formed, cooled, and took on its approximate shape and physical characteristics, a good deal of reliable evidence can be inferred from studies that concentrate on the formation of landmass, oceans, and atmosphere. Astrophysical data—and theories of physics that explain the evolution of physical law and nucelosynthesis—make these studies of Earth's formation both possible and reliable because the same laws of physics and chemistry that exist now operated during the formation of Earth's solar system.
Radiological dating provides overwhelming evidence that dates known terrestrial (Earth origin) rock specimens to more than 3.6 billion years old. Earth and lunar meteorites date to 4.5 billion years.
Precambrian time is subdivided into Hadean time (4.5 billion years ago to 3.8 billion years ago); Archean time (3.8 billion years ago to 2.5 billion years ago; Paleoproterozoic time (2.5 billion years ago to 1.6 billion years ago); Mesoproterozoic time (1.6 billion years ago to 900 million years ago); and Neoproterozoic time (900 million years ago to 540 mya).
Hadean time represents the time during which the solar system formed. During the subsequent course of Precambrian time, Earth's lithospheric plates formed and the mechanisms of geologic change described by modern plate tectonic theory began to occur. During Precambrian time, life arose on Earth. The oldest known fossil evidence (fig tree group fossils in what is now Africa) dates to early in Archaean time. During the Paleoproterozoic, Earth's primitive atmosphere made a transition to an oxygen rich atmosphere. Soon thereafter in geologic time, i.e. within a few hundred million years, there is evidence of the earliest appearance of eukaryotes (organisms with a true nucleus containing DNA). Evidence of the oldest fossilized animal remains dates to the end of Neoproterozic time.
The extensive debris field that existed in the early solar system assured frequent bombardment of Earth's primitive atmosphere by asteroids and comets. Despite the consuming effects of geological weathering and erosion, evidence of Precambrian time impacts dating almost 2.0 billion years ago have been found in what are now South Africa and Canada.
Cambrian Period; Cenozoic Era; Cretaceous Period; Dating Methods; Devonian Period; Eocene Epoch; Evolution, Evidence Of; Fossil Record; Fossils and Fossilization; Historical Geology; Holocene Epoch; Jurassic Period; Mesozoic Era; Miller-Urey Experiment; Miocene Epoch; Mississippian Period; Oligocene Epoch; Ordovician Period; Paleocene Epoch; Paleozoic Era; Pennsylvanian Period; Phanerozoic Eon; Pleistocene Epoch; Pliocene Epoch; Precambrian; Proterozoic Era; Quaternary Period; Silurian Period; Tertiary Period; Triassic Period
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