![EARTH](/uploads/5/1/4/3/51436329/5067754_orig.jpg)
EARTH
Earth, also called the world[n 4] and, less frequently, Gaia[n 5] (and Terra in some works of science fiction[27]) is the third planet from the Sun, the densest planet in the Solar System, the largest of the Solar System's four terrestrial planets, and the only astronomical object known to accommodate life. The earliest life on Earth arose at least 3.5 billion years ago.[28][29][30] Earth's biodiversity has expanded continually except when interrupted by mass extinctions.[31] Although scholars estimate that over 99 percent of all species that ever lived on the planet are extinct,[32][33] Earth is currently home to 10–14 million species of life,[34][35] including over 7.3 billion humans[36] who depend upon its biosphere and minerals. Earth's human population is divided among about two hundred sovereign states which interact through diplomacy, conflict, travel, trade and communication media.
According to evidence from radiometric dating and other sources, Earth was formed around four and a half billion years ago. Within its first billion years,[37] life appeared in its oceans and began to affect its atmosphere and surface, promoting the proliferation of aerobic as well as anaerobic organisms and causing the formation of the atmosphere's ozone layer. This layer and the geomagnetic field blocked the most life-threatening parts of the Sun's radiation, so life was able to flourish on land as well as in water.[38] Since then, the combination of Earth's distance from the Sun, its physical properties and its geological historyhave allowed life to thrive and evolve.
Earth's lithosphere is divided into several rigid tectonic plates that migrate across the surface over periods of many millions of years. Seventy-one percent of Earth's surface is covered with water,[39] with the remainder consisting of continents and islands that together have many lakes and other sources of water that contribute to the hydrosphere. Earth's poles are mostly covered with ice that includes the solid ice of the Antarctic ice sheet and the sea ice of the polar ice packs. Earth's interior remains active with a solid iron inner core, a liquid outer core that generates the magnetic field, and a thick layer of relatively solidmantle.
Earth gravitationally interacts with other objects in space, especially the Sun and the Moon. During one orbit around the Sun, Earth rotates about its own axis 366.26 times, creating 365.26 solar days or onesidereal year.[n 6] Earth's axis of rotation is tilted 23.4° away from the perpendicular of its orbital plane, producing seasonal variations on the planet's surface with a period of one tropical year (365.24 solar days).[40]The Moon is Earth's only natural satellite. It began orbiting Earth about 4.53 billion years ago. The Moon's gravitational interaction with Earth stimulates ocean tides, stabilizes the axial tilt and gradually slows the planet's rotation.
The earliest material found in the Solar System is dated to 4.5672±0.0006 billion years ago (bya);[41] therefore, it is inferred that Earth must have been formed by accretion around this time. By 4.54±0.04 bya[37]the primordial Earth had formed. The formation and evolution of the Solar System bodies occurred in tandem with the Sun. In theory a solar nebula partitions a volume out of a molecular cloud by gravitational collapse, which begins to spin and flatten into a circumstellar disk, and then the planets grow out of that in tandem with the star. A nebula contains gas, ice grains and dust (including primordial nuclides). In nebular theory planetesimals commence forming as particulate accrues by cohesive clumping and then by gravity. The assembly of the primordial Earth proceeded for 10–20 myr.[42] The Moon formed shortly thereafter, about 4.53 bya.[43]
The formation of the Moon remains a topic of debate. The working hypothesis is that it formed by accretion from material loosed from Earth after a Mars-sized object, named Theia, impacted with Earth.[44] This model, however, is not self-consistent. In this scenario, the mass of Theia is 10% of that of Earth,[45] it impacted Earth with a glancing blow,[46] and some of its mass merges with Earth. Between approximately 3.8 and 4.1 bya, numerous asteroid impacts during the Late Heavy Bombardment caused significant changes to the greater surface environment of the Moon, and by inference, to Earth.
Highly energetic chemical reactions are thought to have produced self–replicating molecules around four billion years ago. This was followed a half billion years later by the last common ancestor of all life.[59] The development of photosynthesis allowed the Sun's energy to be harvested directly by life forms; the resultant molecular oxygen (O2) accumulated in the atmosphere and due to interaction with high energy solar radiation, formed a layer of protective ozone (O3) in the upper atmosphere.[60] The incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes.[61] True multicellular organisms formed as cells withincolonies became increasingly specialized. Aided by the absorption of harmful ultraviolet radiation by the ozone layer, life colonized Earth's surface.[62] The earliest fossil evidences for life are graphite found to be biogenic in 3.7 billion-year-old metasedimentary rocks discovered in Western Greenland[63] and microbial mat fossils found in 3.48 billion-year-old sandstone discovered in Western Australia.[64][65]
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.[66]
Following the Cambrian explosion, about 535 mya, there have been five major mass extinctions.[67] 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 asOrrorin tugenensis gained the ability to stand upright.[68] 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 Earth in a short time span as no other life form had,[69] affecting both the nature and quantity of other life forms.
Earth, also called the world[n 4] and, less frequently, Gaia[n 5] (and Terra in some works of science fiction[27]) is the third planet from the Sun, the densest planet in the Solar System, the largest of the Solar System's four terrestrial planets, and the only astronomical object known to accommodate life. The earliest life on Earth arose at least 3.5 billion years ago.[28][29][30] Earth's biodiversity has expanded continually except when interrupted by mass extinctions.[31] Although scholars estimate that over 99 percent of all species that ever lived on the planet are extinct,[32][33] Earth is currently home to 10–14 million species of life,[34][35] including over 7.3 billion humans[36] who depend upon its biosphere and minerals. Earth's human population is divided among about two hundred sovereign states which interact through diplomacy, conflict, travel, trade and communication media.
According to evidence from radiometric dating and other sources, Earth was formed around four and a half billion years ago. Within its first billion years,[37] life appeared in its oceans and began to affect its atmosphere and surface, promoting the proliferation of aerobic as well as anaerobic organisms and causing the formation of the atmosphere's ozone layer. This layer and the geomagnetic field blocked the most life-threatening parts of the Sun's radiation, so life was able to flourish on land as well as in water.[38] Since then, the combination of Earth's distance from the Sun, its physical properties and its geological historyhave allowed life to thrive and evolve.
Earth's lithosphere is divided into several rigid tectonic plates that migrate across the surface over periods of many millions of years. Seventy-one percent of Earth's surface is covered with water,[39] with the remainder consisting of continents and islands that together have many lakes and other sources of water that contribute to the hydrosphere. Earth's poles are mostly covered with ice that includes the solid ice of the Antarctic ice sheet and the sea ice of the polar ice packs. Earth's interior remains active with a solid iron inner core, a liquid outer core that generates the magnetic field, and a thick layer of relatively solidmantle.
Earth gravitationally interacts with other objects in space, especially the Sun and the Moon. During one orbit around the Sun, Earth rotates about its own axis 366.26 times, creating 365.26 solar days or onesidereal year.[n 6] Earth's axis of rotation is tilted 23.4° away from the perpendicular of its orbital plane, producing seasonal variations on the planet's surface with a period of one tropical year (365.24 solar days).[40]The Moon is Earth's only natural satellite. It began orbiting Earth about 4.53 billion years ago. The Moon's gravitational interaction with Earth stimulates ocean tides, stabilizes the axial tilt and gradually slows the planet's rotation.
The earliest material found in the Solar System is dated to 4.5672±0.0006 billion years ago (bya);[41] therefore, it is inferred that Earth must have been formed by accretion around this time. By 4.54±0.04 bya[37]the primordial Earth had formed. The formation and evolution of the Solar System bodies occurred in tandem with the Sun. In theory a solar nebula partitions a volume out of a molecular cloud by gravitational collapse, which begins to spin and flatten into a circumstellar disk, and then the planets grow out of that in tandem with the star. A nebula contains gas, ice grains and dust (including primordial nuclides). In nebular theory planetesimals commence forming as particulate accrues by cohesive clumping and then by gravity. The assembly of the primordial Earth proceeded for 10–20 myr.[42] The Moon formed shortly thereafter, about 4.53 bya.[43]
The formation of the Moon remains a topic of debate. The working hypothesis is that it formed by accretion from material loosed from Earth after a Mars-sized object, named Theia, impacted with Earth.[44] This model, however, is not self-consistent. In this scenario, the mass of Theia is 10% of that of Earth,[45] it impacted Earth with a glancing blow,[46] and some of its mass merges with Earth. Between approximately 3.8 and 4.1 bya, numerous asteroid impacts during the Late Heavy Bombardment caused significant changes to the greater surface environment of the Moon, and by inference, to Earth.
Highly energetic chemical reactions are thought to have produced self–replicating molecules around four billion years ago. This was followed a half billion years later by the last common ancestor of all life.[59] The development of photosynthesis allowed the Sun's energy to be harvested directly by life forms; the resultant molecular oxygen (O2) accumulated in the atmosphere and due to interaction with high energy solar radiation, formed a layer of protective ozone (O3) in the upper atmosphere.[60] The incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes.[61] True multicellular organisms formed as cells withincolonies became increasingly specialized. Aided by the absorption of harmful ultraviolet radiation by the ozone layer, life colonized Earth's surface.[62] The earliest fossil evidences for life are graphite found to be biogenic in 3.7 billion-year-old metasedimentary rocks discovered in Western Greenland[63] and microbial mat fossils found in 3.48 billion-year-old sandstone discovered in Western Australia.[64][65]
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.[66]
Following the Cambrian explosion, about 535 mya, there have been five major mass extinctions.[67] 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 asOrrorin tugenensis gained the ability to stand upright.[68] 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 Earth in a short time span as no other life form had,[69] affecting both the nature and quantity of other life forms.