Earth's atmosphere and oceans formed by volcanic activity and outgassing that included water vapor. The origin of the world's oceans was condensation augmented by water and ice delivered by asteroids, proto-planets, and comets.[40] In this model, atmospheric "greenhouse gases" kept the oceans from freezing while the newly forming Sun was only at 70% luminosity.[41] By 3.5 bya, the Earth's magnetic field was established, which helped prevent the atmosphere from being stripped away by the solar wind.[42]
A crust formed when the molten outer layer of the planet Earth cooled to form a solid as the accumulated water vapor began to act in the atmosphere. The two models[43] that explain land mass propose either a steady growth to the present-day forms[44] or, more likely, a rapid growth[45] early in Earth history[46] followed by a long-term steady continental area.[47][48][49] Continents formed by plate tectonics, a process ultimately driven by the continuous loss of heat from the earth's interior. On time scales lasting hundreds of millions of years, the supercontinents have formed and broken up three times. Roughly 750 mya (million years ago), one of the earliest known supercontinents, Rodinia, began to break apart. The continents later recombined to form Pannotia, 600–540 mya, then finally Pangaea, which also broke apart 180 mya.[50]
The present pattern of ice ages began about 40 mya and then intensified during the Pleistocene about 3 mya. High-latitude regions have since undergone repeated cycles of glaciation and thaw, repeating every 40–100,000 years. The last continental glaciation ended 10,000 years ago.[51]
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Highly energetic chemistry is thought to have produced a self-replicating molecule around 4 bya and half a billion years later the last common ancestor of all life existed.[53] The development of photosynthesis allowed the Sun's energy to be harvested directly by life forms; the resultant oxygen accumulated in the atmosphere and formed a layer of ozone (a form of molecular oxygen [O3]) in the upper atmosphere. The incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes.[54] True multicellular organisms formed as cells within colonies became increasingly specialized. Aided by the absorption of harmful ultraviolet radiation by the ozone layer, life colonized the surface of Earth.[55]
Since the 1960s, it has been hypothesized that severe glacial action between 750 and 580 mya, during the Neoproterozoic, covered much of the planet in a sheet of ice. This hypothesis has been termed "Snowball Earth", and is of particular interest because it preceded the Cambrian explosion, when multicellular life forms began to proliferate.[56]
Following the Cambrian explosion, about 535 mya, there have been five major mass extinctions.[57] The most recent such event was 66 mya, when an asteroid impact triggered the extinction of the (non-avian) dinosaurs and other large reptiles, but spared some small animals such as mammals, which then resembled shrews. Over the past 66 myr, mammalian life has diversified, and several million years ago an African ape-like animal such as Orrorin tugenensis gained the ability to stand upright.[58] This enabled tool use and encouraged communication that provided the nutrition and stimulation needed for a larger brain, which allowed the evolution of the human race. The development of agriculture, and then civilization, allowed humans to influence the Earth in a short time span as no other life form had,[59] affecting both the nature and quantity of other life forms.
Kamis, 15 November 2012
A crust formed when the molten outer layer of the planet Earth cooled to form a solid as
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