Timeline of the evolutionary history of life
This timeline of the evolutionary history of life represents the current scientific theory outlining the major events during the development of life on planet Earth. In biology, evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organization, from kingdoms to species, individual organisms and molecules, such as DNA and proteins; the similarities between all present day organisms indicate the presence of a common ancestor from which all known species and extinct, have diverged through the process of evolution. More than 99 percent of all species, amounting to over five billion species, that lived on Earth are estimated to be extinct. Estimates on the number of Earth's current species range from 10 million to 14 million, of which about 1.2 million have been documented and over 86 percent have not yet been described. However, a May 2016 scientific report estimates that 1 trillion species are on Earth, with only one-thousandth of one percent described.
While the dates given in this article are estimates based on scientific evidence, there has been controversy between more traditional views of increased biodiversity through a cone of diversity with the passing of time and the view that the basic pattern on Earth has been one of annihilation and diversification and that in certain past times, such as the Cambrian explosion, there was great diversity. Species go extinct as environments change, as organisms compete for environmental niches, as genetic mutation leads to the rise of new species from older ones. Biodiversity on Earth takes a hit in the form of a mass extinction in which the extinction rate is much higher than usual. A large extinction-event represents an accumulation of smaller extinction- events that take place in a brief period of time; the first known mass extinction in earth's history was the Great Oxygenation Event 2.4 billion years ago. That event led to the loss of most of the planet's obligate anaerobes. Researchers have identified five major extinction events in earth's history since: End of the Ordovician: 440 million years ago, 86% of all species lost, including graptolites Late Devonian: 375 million years ago, 75% of species lost, including most trilobites End of the Permian, "The Great Dying": 251 million years ago, 96% of species lost, including tabulate corals, most extant trees and synapsids End of the Triassic: 200 million years ago, 80% of species lost, including all of the conodonts End of the Cretaceous: 66 million years ago, 76% of species lost, including all of the ammonites, ichthyosaurs, plesiosaurs and nonavian dinosaurs Smaller extinction-events have occurred in the periods between these larger catastrophes, with some standing at the delineation points of the periods and epochs recognized by scientists in geologic time.
The Holocene extinction event is under way. Factors in mass extinctions include continental drift, changes in atmospheric and marine chemistry and other aspects of mountain formation, changes in glaciation, changes in sea level, impact events. In this timeline, Ma means "million years ago," ka means "thousand years ago," and ya means "years ago." 4000 Ma and earlier. 4000 Ma – 2500 Ma 2500 Ma – 542 Ma. Contains the Palaeoproterozoic and Neoproterozoic eras. 542 Ma – present The Phanerozoic Eon the "period of well-displayed life," marks the appearance in the fossil record of abundant, shell-forming and/or trace-making organisms. It is subdivided into three eras, the Paleozoic and Cenozoic, which are divided by major mass extinctions. 542 Ma – 251.0 Ma and contains the Cambrian, Silurian, Devonian and Permian periods. From 251.4 Ma to 66 Ma and containing the Triassic and Cretaceous periods. 66 Ma – present Dawkins, Richard. The Ancestor's Tale: A Pilgrimage to the Dawn of Life. Boston: Houghton Mifflin Company.
ISBN 978-0-618-00583-3. LCCN 2004059864. OCLC 56617123. "Understanding Evolution: your one-stop resource for information on evolution". University of California, Berkeley. Retrieved 2015-03-18. "Life on Earth". Tree of Life Web Project. University of Arizona. January 1, 1997. Retrieved 2015-03-18. Explore complete phylogenetic tree interactively Brandt, Niel. "Evolutionary and Geological Timelines". TalkOrigins Archive. Houston, TX: The TalkOrigins Foundation, Inc. Retrieved 2015-03-18. "Palaeos: Life Through Deep Time". Palaeos. Retrieved 2015-03-18. Kyrk, John. "Evolution". Cell Biology Animation. Retrieved 2015-03-18. Interactive timeline from Big Bang to present "Plant Evolution". Plant and Animal Evolution. University of Waikato. Retrieved 2015-03-18. Sequence of Plant Evolution "The History of Animal Evolution". Plant and Animal Evolution. University of Waikato. Retrieved 2015-03-18. Sequence of Animal Evolution Yeo, Dannel. "History of Life on Earth". Archived from the original on 2015-03-15. Retrieved 2015-03-19.
Exploring Time. The Science Channel. 2007. Retrieved 2015-03-19. Roberts, Ben. "Plant evolution timeline". University of Cambridge. Archived from the original on 2015-03-13. Retrieved 2015-03-19. Art of the Nature Timelines on Wikipedia
Young stellar object
Young stellar object denotes a star in its early stage of evolution. This class consists of two groups of objects: pre-main-sequence stars. A star forms by accumulation of material that falls in to a protostar from a circumstellar disk or envelope. Material in the disk is cooler than the surface of the protostar, so it radiates at longer wavelengths of light producing excess infrared emission; as material in the disk is depleted, the infrared excess decreases. Thus, YSOs are classified into evolutionary stages based on the slope of their spectral energy distribution in the mid-infrared, using a scheme introduced by Lada, he proposed three classes, based on the values of intervals of spectral index α: α = d log d log . Here λ is wavelength, F λ is flux density; the α is calculated in the wavelength interval of 2.2–20 μ m. Andre et al. discovered a class 0: objects with strong submillimeter emission, but faint at λ < 10 μ m. Greene et al. added a fifth class of "flat spectrum" sources. Class 0 sources – undetectable at λ < 20 μ m Class I sources have α > 0.3 Flat spectrum sources have 0.3 > α > − 0.3 Class II sources have − 0.3 > α > − 1.6 Class III sources have α < − 1.6 This classification schema reflects evolutionary sequence.
It is believed that most embedded Class 0 sources evolve towards Class I stage, dissipating their circumstellar envelopes. They become optically visible on the stellar birthline as pre-main-sequence stars. Class II objects have circumstellar disks and correspond to classical T Tauri stars, while Class III stars have lost their disks and correspond to weak-line T Tauri stars. An intermediate stage where disks can only be detected at longer wavelengths are known as transition-disk objects. YSOs are associated with early star evolution phenomena: jets and bipolar outflows, Herbig–Haro objects, protoplanetary disks; these stars may be differentiated by mass: Massive YSOs, intermediate-mass YSOs, brown dwarfs. Bok globule Media related to Young stellar objects at Wikimedia Commons
A solar eclipse occurs when an observer passes through the shadow cast by the Moon which or blocks the Sun. This can only happen when the Sun and Earth are nearly aligned on a straight line in three dimensions during a new moon when the Moon is close to the ecliptic plane. In a total eclipse, the disk of the Sun is obscured by the Moon. In partial and annular eclipses, only part of the Sun is obscured. If the Moon were in a circular orbit, a little closer to the Earth, in the same orbital plane, there would be total solar eclipses every new moon. However, since the Moon's orbit is tilted at more than 5 degrees to the Earth's orbit around the Sun, its shadow misses Earth. A solar eclipse can only occur when the moon is close enough to the ecliptic plane during a new moon. Special conditions must occur for the two events to coincide because the Moon's orbit crosses the ecliptic at its orbital nodes twice every draconic month while a new moon occurs one every synodic month. Solar eclipses therefore happen only during eclipse seasons resulting in at least two, up to five, solar eclipses each year.
Total eclipses are rare because the timing of the new moon within the eclipse season needs to be more exact for an alignment between the observer and the centers of the Sun and Moon. In addition, the elliptical orbit of the Moon takes it far enough away from Earth that its apparent size is not large enough to block the Sun entirely. Total solar eclipses are rare at any particular location because totality exists only along a narrow path on the Earth's surface traced by the Moon's full shadow or umbra. An eclipse is a natural phenomenon. However, in some ancient and modern cultures, solar eclipses were attributed to supernatural causes or regarded as bad omens. A total solar eclipse can be frightening to people who are unaware of its astronomical explanation, as the Sun seems to disappear during the day and the sky darkens in a matter of minutes. Since looking directly at the Sun can lead to permanent eye damage or blindness, special eye protection or indirect viewing techniques are used when viewing a solar eclipse.
It is technically safe to view only the total phase of a total solar eclipse with the unaided eye and without protection. People referred to as eclipse chasers or umbraphiles will travel to remote locations to observe or witness predicted central solar eclipses. There are four types of solar eclipses: A total eclipse occurs when the dark silhouette of the Moon obscures the intensely bright light of the Sun, allowing the much fainter solar corona to be visible. During any one eclipse, totality occurs at best only in a narrow track on the surface of Earth; this narrow track is called the path of totality. An annular eclipse occurs when the Sun and Moon are in line with the Earth, but the apparent size of the Moon is smaller than that of the Sun. Hence the Sun appears as a bright ring, or annulus, surrounding the dark disk of the Moon. A hybrid eclipse shifts between a annular eclipse. At certain points on the surface of Earth, it appears as a total eclipse, whereas at other points it appears as annular.
Hybrid eclipses are comparatively rare. A partial eclipse occurs when the Sun and Moon are not in line with the Earth and the Moon only obscures the Sun; this phenomenon can be seen from a large part of the Earth outside of the track of an annular or total eclipse. However, some eclipses can only be seen as a partial eclipse, because the umbra passes above the Earth's polar regions and never intersects the Earth's surface. Partial eclipses are unnoticeable in terms of the sun's brightness, as it takes well over 90% coverage to notice any darkening at all. At 99%, it would be no darker than civil twilight. Of course, partial eclipses can be observed; the Sun's distance from Earth is about 400 times the Moon's distance, the Sun's diameter is about 400 times the Moon's diameter. Because these ratios are the same, the Sun and the Moon as seen from Earth appear to be the same size: about 0.5 degree of arc in angular measure. A separate category of solar eclipses is that of the Sun being occluded by a body other than the Earth's moon, as can be observed at points in space away from the Earth's surface.
Two examples are when the crew of Apollo 12 observed the Earth eclipse the Sun in 1969 and when the Cassini probe observed Saturn eclipsing the Sun in 2006. The Moon's orbit around the Earth is elliptical, as is the Earth's orbit around the Sun; the apparent sizes of the Sun and Moon therefore vary. The magnitude of an eclipse is the ratio of the apparent size of the Moon to the apparent size of the Sun during an eclipse. An eclipse that occurs when the Moon is near its closest distance to Earth can be a total eclipse because the Moon will appear to be large enough to cover the Sun's bright disk or photosphere. Conversely, an eclipse that occurs when the Moon is near its farthest distance from Earth can only be an annular eclipse because the Moon will appear to be smaller than the Sun. More solar eclipses are
European Southern Observatory
The European Southern Observatory, formally the European Organisation for Astronomical Research in the Southern Hemisphere, is a 16-nation intergovernmental research organization for ground-based astronomy. Created in 1962, ESO has provided astronomers with state-of-the-art research facilities and access to the southern sky; the organisation employs about 730 staff members and receives annual member state contributions of €162 million. Its observatories are located in northern Chile. ESO has operated some of the largest and most technologically advanced telescopes; these include the 3.6 m New Technology Telescope, an early pioneer in the use of active optics, the Very Large Telescope, which consists of four individual 8.2 m telescopes and four smaller auxiliary telescopes which can all work together or separately. The Atacama Large Millimeter Array observes the universe in the millimetre and submillimetre wavelength ranges, is the world's largest ground-based astronomy project to date, it was completed in March 2013 in an international collaboration by Europe, North America, East Asia and Chile.
Under construction is the Extremely Large Telescope. It will use a 39.3-metre-diameter segmented mirror, become the world's largest optical reflecting telescope when operational in 2024. Its light-gathering power will allow detailed studies of planets around other stars, the first objects in the universe, supermassive black holes, the nature and distribution of the dark matter and dark energy which dominate the universe. ESO's observing facilities have made astronomical discoveries and produced several astronomical catalogues, its findings include the discovery of the most distant gamma-ray burst and evidence for a black hole at the centre of the Milky Way. In 2004, the VLT allowed astronomers to obtain the first picture of an extrasolar planet orbiting a brown dwarf 173 light-years away; the High Accuracy Radial Velocity Planet Searcher instrument installed on the older ESO 3.6 m telescope led to the discovery of extrasolar planets, including Gliese 581c—one of the smallest planets seen outside the solar system.
The idea that European astronomers should establish a common large observatory was broached by Walter Baade and Jan Oort at the Leiden Observatory in the Netherlands in spring 1953. It was pursued by Oort, who gathered a group of astronomers in Leiden to consider it on June 21 that year. Thereafter, the subject was further discussed at the Groningen conference in the Netherlands. On January 26, 1954, an ESO declaration was signed by astronomers from six European countries expressing the wish that a joint European observatory be established in the southern hemisphere. At the time, all reflector telescopes with an aperture of 2 metres or more were located in the northern hemisphere; the decision to build the observatory in the southern hemisphere resulted from the necessity of observing the southern sky. Although it was planned to set up telescopes in South Africa, tests from 1955 to 1963 demonstrated that a site in the Andes was preferable. On November 15, 1963 Chile was chosen as the site for ESO's observatory.
The decision was preceded by the ESO Convention, signed 5 October 1962 by Belgium, France, the Netherlands and Sweden. Otto Heckmann was nominated as the organisation's first director general on 1 November 1962. A preliminary proposal for a convention of astronomy organisations in these five countries was drafted in 1954. Although some amendments were made in the initial document, the convention proceeded until 1960 when it was discussed during that year's committee meeting; the new draft was examined in detail, a council member of CERN highlighted the need for a convention between governments. The convention and government involvement became pressing due to rising costs of site-testing expeditions; the final 1962 version was adopted from the CERN convention, due to similarities between the organisations and the dual membership of some members. In 1966, the first ESO telescope at the La Silla site in Chile began operating; because CERN had sophisticated instrumentation, the astronomy organisation turned to the nuclear-research body for advice and a collaborative agreement between ESO and CERN was signed in 1970.
Several months ESO's telescope division moved into a CERN building in Geneva and ESO's Sky Atlas Laboratory was established on CERN property. ESO's European departments moved into the new ESO headquarters in Garching, Germany in 1980. Although ESO is headquartered in Germany, its telescopes and observatories are in northern Chile, where the organisation operates advanced ground-based astronomical facilities: La Silla, which hosts the New Technology Telescope Paranal, where the Very Large Telescope is located Llano de Chajnantor, which hosts the APEX submillimetre telescope and where ALMA, the Atacama Large Millimeter/submillimeter Array, is locatedThese are among the best locations for astronomical observations in the southern hemisphere. An ESO project is the Extremely Large Telescope, a 40-metre-class telescope based on a five-mirror design and the planned Overwhelmingly Large Telescope; the ELT will be the near-infrared telescope in the world. ESO began its design in early 2006, aimed to begin construction in 2012.
Construction work at the ELT site started in June 2014. As decided by the ESO council on 26 April 2010, a fou
Tidal acceleration is an effect of the tidal forces between an orbiting natural satellite, the primary planet that it orbits. The acceleration causes a gradual recession of a satellite in a prograde orbit away from the primary, a corresponding slowdown of the primary's rotation; the process leads to tidal locking of the smaller first, the larger body. The Earth–Moon system is the best studied case; the similar process of tidal deceleration occurs for satellites that have an orbital period, shorter than the primary's rotational period, or that orbit in a retrograde direction. The naming is somewhat confusing, because the speed of the satellite relative to the body it orbits is decreased as a result of tidal acceleration, increased as a result of tidal deceleration. Edmond Halley was the first to suggest, in 1695, that the mean motion of the Moon was getting faster, by comparison with ancient eclipse observations, but he gave no data. In 1749 Richard Dunthorne confirmed Halley's suspicion after re-examining ancient records, produced the first quantitative estimate for the size of this apparent effect: a centurial rate of +10″ in lunar longitude, a accurate result for its time, not differing from values assessed e.g. in 1786 by de Lalande, to compare with values from about 10″ to nearly 13″ being derived about a century later.
Pierre-Simon Laplace produced in 1786 a theoretical analysis giving a basis on which the Moon's mean motion should accelerate in response to perturbational changes in the eccentricity of the orbit of Earth around the Sun. Laplace's initial computation accounted for the whole effect, thus seeming to tie up the theory neatly with both modern and ancient observations. However, in 1854, John Couch Adams caused the question to be re-opened by finding an error in Laplace's computations: it turned out that only about half of the Moon's apparent acceleration could be accounted for on Laplace's basis by the change in Earth's orbital eccentricity. Adams's finding provoked a sharp astronomical controversy that lasted some years, but the correctness of his result, agreed upon by other mathematical astronomers including C. E. Delaunay, was accepted; the question depended on correct analysis of the lunar motions, received a further complication with another discovery, around the same time, that another significant long-term perturbation, calculated for the Moon was in error, was found on re-examination to be negligible, had to disappear from the theory.
A part of the answer was suggested independently in the 1860s by Delaunay and by William Ferrel: tidal retardation of Earth's rotation rate was lengthening the unit of time and causing a lunar acceleration, only apparent. It took some time for the astronomical community to accept the reality and the scale of tidal effects, but it became clear that three effects are involved, when measured in terms of mean solar time. Beside the effects of perturbational changes in Earth's orbital eccentricity, as found by Laplace and corrected by Adams, there are two tidal effects. First there is a real retardation of the Moon's angular rate of orbital motion, due to tidal exchange of angular momentum between Earth and Moon; this increases the Moon's angular momentum around Earth. Secondly there is an apparent increase in the Moon's angular rate of orbital motion; this arises from the consequent increase in length of day. Because the Moon's mass is a considerable fraction of that of Earth, the two bodies can be regarded as a double planet system, rather than as a planet with a satellite.
The plane of the Moon's orbit around Earth lies close to the plane of Earth's orbit around the Sun, rather than in the plane perpendicular to the axis of rotation of Earth as is the case with planetary satellites. The mass of the Moon is sufficiently large, it is sufficiently close, to raise tides in the matter of Earth. In particular, the water of the oceans bulges out away from the Moon; the average tidal bulge is synchronized with the Moon's orbit, Earth rotates under this tidal bulge in just over a day. However, Earth's rotation drags the position of the tidal bulge ahead of the position directly under the Moon; as a consequence, there exists a substantial amount of mass in the bulge, offset from the line through the centers of Earth and the Moon. Because of this offset, a portion of the gravitational pull between Earth's tidal bulges and the Moon is not parallel to the Earth–Moon line, i.e. there exists a torque between Earth and the Moon. Since the bulge nearer the moon pulls more on it than the bulge further away, this torque boosts the Moon in its orbit and slows the rotation of Earth.
As a result of this process, the mean solar day, nominally 86,400 seconds long, is getting longer when measured in SI seconds with stable atomic clocks. The small difference accumulates over time, which leads to an increasing difference between our clock time on the one hand, Atomic Time and Ephemeris Time on
Herbig Ae/Be star
A Herbig Ae/Be star is a pre-main-sequence star – a young star of spectral types A or B. These stars are still embedded in gas-dust envelopes and are sometimes accompanied by circumstellar disks. Hydrogen and calcium emission lines are observed in their spectra, they are 2-8 Solar mass objects, still existing in the star formation stage and approaching the main sequence. In the Hertzsprung–Russell diagram these stars are located to the right of the main sequence, they are named after the American astronomer George Herbig, who first distinguished them from other stars in 1960. The original Herbig criteria were: Spectral type earlier than F0, Balmer emission lines in the stellar spectrum, Projected location within the boundaries of a dark interstellar cloud, Illumination of a nearby bright reflection nebula. There are now several known isolated Herbig Ae/Be stars, thus the most reliable criteria now can be: Spectral type earlier than F0, Balmer emission lines in the stellar spectrum, Infrared radiation excess due to circumstellar dust.
Sometimes Herbig Ae/Be stars show significant brightness variability. They are believed to be due to clumps in the circumstellar disk. In the lowest brightness stage the radiation from the star becomes linearly polarized. Analogs of Herbig Ae/Be stars in the smaller mass range – F, G, K, M spectral type pre-main-sequence stars – are called T Tauri stars. More massive stars in pre-main-sequence stage are not observed, because they evolve quickly: when they become visible, the hydrogen in the center is burning and they are main-sequence objects. Planets around Herbig Ae/Be stars include: HD 95086 b around an A-type star Pérez M. R. Grady C. A. Observational Overview of Young Intermediate-Mass Objects: Herbig Ae/Be Stars, Space Science Reviews, Vol 82, p. 407-450 Waters L. B. F. M. Waelkens, C. HERBIG Ae/Be STARS, Annual Review of Astronomy and Astrophysics, Vol. 36, p. 233-266 Herbig Ae/Be stars"Molecular Hydrogen In The Circumstellar Environment Of Herbig Ae/Be Stars". Mpia-hd.mpg.de. Retrieved 2008-10-16
In astronomy, Bok globules are isolated and small dark nebulae, containing dense cosmic dust and gas from which star formation may take place. Bok globules are found within H II regions, have a mass of about 2 to 50 solar masses contained within a region about a light year or so across, they contain molecular hydrogen, carbon oxides and helium, around 1% silicate dust. Bok globules most result in the formation of double- or multiple-star systems. Bok globules were first observed by astronomer Bart Bok in the 1940s. In an article published in 1947, he and Edith Reilly hypothesized that these clouds were "similar to insect's cocoons" that were undergoing gravitational collapse to form new stars, from which stars and star clusters were born; this hypothesis was difficult to verify due to the observational difficulties of establishing what was happening inside a dense dark cloud that obscured all visible light emitted from within it. An analysis of near-infrared observations published in 1990 confirmed that stars were being born inside Bok globules.
Further observations have revealed that some Bok globules contain embedded warm sources, some contain Herbig–Haro objects, some show outflows of molecular gas. Millimeter-wave emission line studies have provided evidence for the infall of material onto an accreting protostar, it is now thought that a typical Bok globule contains about 10 solar masses of material in a region about a light-year or so across, that Bok globules most result in the formation of double- or multiple-star systems. Bok globules are still a subject of intense research. Known to be some of the coldest objects in the natural universe, their structure and density remains somewhat a mystery. Methods applied so far have relied on column density derived from near-infrared extinction and star counting in a bid to probe these objects further. Bok globules that are irradiated by ultraviolet light from hot nearby stars exhibit stripping of materials to produce a tail; these types are called "cometary globules". Molecular cloud Barnard 68 CG 4 NGC 281 IC 2944 A Star in the Making