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Xenobiology

Xenobiology

Astrobiology is an interdisciplinary field, combining aspects of astronomy, biology and geology, which considers the question of whether extraterrestrial life exists and if so, then what are its origin, distribution, and evolution. It comes from the Greek words astron = star, bios = life and logos = word/science), and is also known as exobiology (Greek: exo = out) or xenobiology (Greek: xenos = foreign). Some major astrobiological research topics include:
- What is life?
- How did life arise on Earth?
- What kind of environments can life tolerate?
- Can we detect life on other planets? Exobiology and xenobiology are terms also found in science fiction, although typically such terms refer to the speculative biology of an extraterrestrial. A xenobiologist is usually a human doctor or biologist who is expert on the physiology of alien organisms and life forms.

Overview

Although astrobiology is an emerging field, the question of whether life exists elsewhere in the universe is a verifiable hypothesis and thus a valid line of scientific enquiry. Astrobiology does not claim to be a pure scientific discipline in the sense of physics or biology but rather represents an effort by researchers in different disciplines to try and answer questions about life by drawing on knowledge of multiple scientific fields. A particular focus of current astrobiology research is the search for life on Mars. There is a growing body of evidence to suggest that Mars has previously had a considerable amount of water on its surface; water is considered to be an essential precursor to the development of life, although this has not been conclusively proven. Missions specifically designed to search for life include the Viking and Beagle 2 probes, both directed to Mars. The Viking results were inconclusive and Beagle 2 failed to transmit from the surface and is assumed to have crashed. A future mission with a strong astrobiology role would have been the Jupiter Icy Moons Orbiter, designed to study the frozen moons of Jupiter—some of which may have liquid water—had it not been canceled.

Research Outcomes

As of 2005, there is no definite evidence of extraterrestrial life. However examination of meteorites from Antarctica which are thought to have originated from the planet Mars have provided what some scientists believe to be microfossils of extraterrestrial life, although that interpretation of the evidence is still controversial. In 2004, the spectral signature of methane was detected in the martian atmosphere by both Earth-based telescopes as well as by the Mars Express probe. Methane has a relatively short half-life in the martian atmosphere, so there must be a recent source of it. Since one possible source, active volcanism, has thus far not been detected on Mars, this has led scientists to speculate that the source could be (microbial) life. Missions to other planets (such as Beagle 2: Evolution to Mars and Cassini to Saturn's moon Titan) hope to further explore the possibilities of life on other planets in our solar system.

Methodology

Narrowing the task

Main article: Planetary habitability When looking for life in other planets, some simplifying assumptions are useful to reduce the size of the task of astrobiologists. One is to assume that the vast majority of life-forms in our galaxy are based on carbon chemistries, as are all life-forms on Earth. While it is possible that non carbon-based life exists, carbon is well known for the unusually wide variety of molecules that can be formed around it. The presence of liquid water is also a useful assumption, as it is a common molecule and provides an excellent environment for the formation of complicated carbon-based molecules that could eventually lead to the emergence of life. A third assumption is to focus on Sun-like stars. Very big stars have relatively short lifetimes, meaning that life would not likely have time to evolve on planets orbiting them. Very small stars provide so little heat and warmth that only planets in very close orbits around them would not be frozen solid, and in such close orbits these planets would be tidally "locked" to the star, with one side of the planet perpetually baked and the other perpetually frozen. About 10% of the stars in our galaxy are Sun-like, and there are about a thousand such stars within 100 light-years of our Sun. These stars would be useful primary targets for interstellar listening. However, we only know of one planet where life exists, our own. There is no way to know if any of the simplifying assumptions are correct.

The sciences of Astrobiology

Astronomy

Most astronomy-related astrobiological research falls into the category of extrasolar planet (exoplanet) detection, the hypothesis being that if life arose on Earth then it could also arise on other planets with similar characteristics. To that end, a number of instruments designed to detect 'Earth-like' exoplanets are under development, most notably NASA's Terrestrial Planet Finder (TPF) and ESA's Darwin programs. A number of less ambitious ground-based efforts are also underway (see exoplanet). The goal of these missions is not only to detect Earth-sized planets but also to image them spectroscopically. By examining planetary spectra it will be possible to determine the basic composition of an extrasolar planet's atmosphere and surface; given this knowledge, it may be possible to assess the likelihood of life being found on that planet. A NASA research group, the Virtual Planet Laboratory[http://vpl.ipac.caltech.edu] (VPL), is using computer modelling to generate a wide variety of 'virtual' planets to see what they would look like if viewed by TPF or Darwin. It is hoped that once these missions come online, their spectra can be cross-checked with these 'virtual' planetary spectra for features that might indicate the presence of life. An estimate for the number of planets with (intelligent) extraterrestrial life can be gleaned from the Drake equation, essentially an equation expressing the probability of intelligent life as the product of factors such as the fraction of planets that might be habitable and the fraction of planets on which life might arise. However, whilst the rationale behind the equation is sound, it is unlikely that the terms of the equation will be constrained to reasonable error limits any time soon. Another associated topic is the Fermi paradox, which suggests that if intelligent life is common in the universe then there should be obvious signs of it. This is the purpose of project like SETI, which tries to detect signs of radio transmissions from intelligent extraterrestrial civilizations. Another active research area in astrobiology is solar system formation. It has been suggested that the perculiarities of our solar system (for example, the presence of Jupiter as a protective 'shield' or the planetary collision which created the Moon) may have greatly increased the probability of intelligent life arising on our planet. No firm conclusions have been reached so far.

Biology

Extremophiles (organisms able to survive in extreme environments) are a core research element for astrobiologists. Such organisms include biota able to survive kilometers below the ocean's surface near hydrothermal vents and microbes that thrive in highly acidic environments. Characterization of these organisms—their environments and their evolutionary pathways—is considered a crucial component to understanding how life might evolve elsewhere in the universe. The origin of life, as distinct from the evolution of life, is another ongoing field of research. Oparin and Haldane postulated that the conditions on the early Earth were conducive to the formation of organic compounds from inorganic precursors and thus to the formation of many of the chemicals common to all forms of life we see today. The study of this process, known as prebiotic chemistry, has made some progress but it is still unclear whether or not life can have formed in such a manner on Earth. The alternative theory of panspermia is that the first elements of life may have formed on another planet with even more favourable conditions, and then have been carried over to Earth by a variety of means.

Geology

The fossil record provides the oldest known evidence for life on Earth. By examining this evidence, geologists are able to better understand the types of organisms that arose on the early Earth. Some regions on Earth, such as the Pilbara in Western Australia are also considered to be geological analogs to regions of Mars and as such might be able to provide clues to possible Martian life.

Criticisms

Like any emerging scientific field, astrobiology has had to struggle for respectability. While many scientists are enthusiastic about the broad questions involved, there remains concern as to whether astrobiology is distinct enough from its parent disciplines (particularly biology) to constitute a true science. Astrobiology is at present a largely speculative extrapolation of Earth conditions into off-Earth environments. Extremophiles, for example, may reveal how life has evolved in exotic locales through out the universe, yet this remains unverified and the study of extremophiles is adequately covered under the rubric of biology. This ambivalent status is reflected by academic acceptance: those interested may specialize in astrobiology as a subset of an established science. Although some have thought a formal degree program in astrobiology unlikely[http://nai.arc.nasa.gov/astrobio/astrobio_detail.cfm?ID=477], the University of Glamorgan, UK, have started just such a degree this year. [http://case.glam.ac.uk/CASE/Degrees/AstroBio.html] Perhaps a more serious criticism of astrobiology is whether the very act of extrapolating from a single data source is unscientific. At best, this creates tautologies ("Earth-like life will evolve under circumstances which are sufficiently Earth-like for a sufficient period of time") and at worst it may abrogate critical aspects of the scientific method. Characterization of non-Earth life is extraordinarily unsettled; hypotheses and predictions as to its existence and origin vary wildly; true astrobiological experiments (with modest exceptions such as the ALH84001 meteorite) simply cannot occur at present. Finally, astrobiology has been criticized for being unimaginative in the tacit assumption that Earth-like life presents the most likely template for life elsewhere.

External links

- [http://www.astrobiology.co.uk/ Astrobiology@home]
- [http://aca.mq.edu.au/ Australian Centre for Astrobiology]
- [http://www.astrobiology.com/ The Astrobiology Web]
- [http://www.astrobio.net/ Astrobiology Magazine]
- [http://pokey.arc.nasa.gov/~astrochm/LifeImplications.html Possible Connections Between Interstellar Chemistry and the Origin of Life on the Earth]
- [http://nai.arc.nasa.gov/news_stories/news_detail.cfm?ID=207 Scientists Find Clues That Life Began in Deep Space - NASA Astrobiology Institute]
- [http://www.solstation.com/habitable.htm Stars and Habitable Planets]
- [http://www.emse.fr/~yukna/researchers/reddwarf.htm Life Around a Red Dwarf Reading Exercise]
- [http://www.markelowitz.com/exobiology.htm Mark Elowitz's Web site on Exobiology and SETI]
- [http://www.projectrho.com/rocket/rocket3aa.html Let's Build an Extraterrestrial]
- [http://www.ebicom.net/~rsf1/vel/1918.htm Influenza 1918, A Venus Connection?] Category:Astronomy Category:Protoscience Category:Science fiction themes ja:宇宙生物学 simple:Astrobiology

Astronomy

:This article is about the science branch. For information about the magazine, see Astronomy (magazine). Astronomy (magazine) as they circled the Moon in 1969. Located near the center of the far side of Earth's Moon, its diameter is about 58 miles (93 km).]] Astronomy (Greek: αστρονομία = άστρον + νόμος, astronomia = astron + nomos, literally, "law of the stars") is the science of celestial objects and phenomena that originate outside the Earth's atmosphere, such as stars, planets, comets, galaxies, and the cosmic background radiation. It is concerned with the formation and development of the universe, the evolution and physical and chemical properties of celestial objects and the calculation of their motions. Astronomical observations are not only relevant for astronomy as such, but provide essential information for the verification of fundamental theories in physics, such as general relativity theory. Complementary to observational astronomy, theoretical astrophysics seeks to explain astronomical phenomena. Astronomy is one of the oldest sciences, with a scientific methodology existing at the time of Ancient Greece and advanced observation techniques possibly much earlier (see archaeoastronomy). Historically, amateurs have contributed to many important astronomical discoveries, and astronomy is one of the few sciences where amateurs can still play an active role, especially in the discovery and observation of transient phenomena. Astronomy is not to be confused with astrology, which assumes that people's destiny and human affairs in general are correlated to the apparent positions of astronomical objects in the sky -- although the two fields share a common origin, they are quite different; astronomers embrace the scientific method, while astrologers do not. In other words, there is no proof that the stars predict our future, but there is proof that our planet is 93 million miles from the sun.

Divisions

In ancient Greece and other early civilizations, astronomy consisted largely of astrometry, measuring positions of stars and planets in the sky. Later, the work of Kepler and Newton, whose work led to the development of celestial mechanics, mathematically predicting the motions of celestial bodies interacting under gravity, and solar system objects in particular. Much of the effort in these two areas, once done largely by hand, is highly automated nowadays, to the extent that they are rarely considered as independent disciplines anymore. Motions and positions of objects are now more easily determined, and modern astronomy is more concerned with observing and understanding the actual physical nature of celestial objects. Since the twentieth century, the field of professional astronomy has split into observational astronomy and theoretical astrophysics. Although most astronomers incorporate elements of both into their research, because of the different skills involved, most professional astronomers tend to specialize in one or the other. Observational astronomy is concerned mostly with acquiring data, which involves building and maintaining instruments and processing the results; this branch is at times referred to as "astrometry" or simply as "astronomy". Theoretical astrophysics is concerned mainly with ascertaining the observational implications of different models, and involves working with computer or analytic models. The fields of study can also be categorized in other ways. Categorization by the region of space under study (for example, Galactic astronomy, Planetary Sciences); by subject, such as star formation or cosmology; or by the method used for obtaining information.

By subject or problem addressed

theoretical astrophysics. Photographed by Mars Global Surveyor, the long dark streak is formed by a moving swirling column of Martian atmosphere (with similarities to a terrestrial tornado). The dust devil itself (the black spot) is climbing the crater wall. The streaks on the right are sand dunes on the crater floor.]]
- Astrometry: the study of the position of objects in the sky and their changes of position. Defines the system of coordinates used and the kinematics of objects in our galaxy.
- Astrophysics: the study of physics of the universe, including the physical properties (luminosity, density, temperature, chemical composition) of astronomical objects.
- Cosmology: the study of the origin of the universe and its evolution. The study of cosmology is theoretical astrophysics at its largest scale.
- Galaxy formation and evolution: the study of the formation of the galaxies, and their evolution.
- Galactic astronomy: the study of the structure and components of our galaxy and of other galaxies.
- Extragalactic astronomy: the study of objects (mainly galaxies) outside our galaxy.
- Stellar astronomy: the study of the stars.
- Stellar evolution: the study of the evolution of stars from their formation to their end as a stellar remnant.
- Star formation: the study of the condition and processes that led to the formation of stars in the interior of gas clouds, and the process of formation itself.
- Planetary Sciences: the study of the planets of the Solar System.
- Astrobiology: the study of the advent and evolution of biological systems in the Universe. Other disciplines that may be considered part of astronomy:
- Archaeoastronomy
- Astrochemistry
- Astrosociobiology
- Astrophilosophy See the list of astronomical topics for a more exhaustive list of astronomy-related pages.

Ways of obtaining information

list of astronomical topics :Main article: Observational astronomy. In astronomy, information is mainly received from the detection and analysis of light and other forms of electromagnetic radiation. Other cosmic rays are also observed, and several experiments are designed to detect gravitational waves in the near future. A traditional division of astronomy is given by the region of the electromagnetic spectrum observed:
- Optical astronomy is the part of astronomy that uses optical components (mirrors, lenses, CCD detectors and photographic films) to observe light from near infrared to near ultraviolet wavelengths. Visible light astronomy (using wavelengths that can be detected with the eyes, about 400 - 700 nm) falls in the middle of this range. The most common tool is the telescope, with electronic imagers and spectrographs.
- Infrared astronomy deals with the detection and analysis of infrared radiation (wavelengths longer than red light). The most common tool is the telescope but using a detector which is sensitive to the infrared. Space telescopes are also used to avoid atmospheric thermal emission, atmospheric opacity, and the effects of astronomical seeing at infrared and other wavelengths.
- Radio astronomy detects radiation of millimetre to dekametre wavelength. The receivers are similar to those used in radio broadcast transmission but much more sensitive. See also Radio telescopes.
- High-energy astronomy includes X-ray astronomy, gamma-ray astronomy, and extreme UV (ultraviolet) astronomy, as well as studies of neutrinos and cosmic rays. Optical and radio astronomy can be performed with ground-based observatories, because the atmosphere is transparent at the wavelengths being detected. Infrared light is heavily absorbed by water vapor, so infrared observatories have to be located in high, dry places or in space. The atmosphere is opaque at the wavelengths of X-ray astronomy, gamma-ray astronomy, UV astronomy and (except for a few wavelength "windows") Far infrared astronomy, so observations must be carried out mostly from balloons or space observatories. Powerful gamma rays can, however be detected by the large air showers they produce, and the study of cosmic rays can also be regarded as a branch of astronomy.

History of astronomy

cosmic ray :Main article: History of astronomy. In early times, astronomy only comprised the observation and predictions of the motions of the naked-eye objects. Aristotle said that the Earth was the center of the Universe and everything rotated around it in orbits that were perfect circles. Aristotle had to be right because people thought that Earth had to be in the center with everything rotating around it because the wind would not scatter leaves, and birds would only fly in one direction. For a long time, people thought that Aristotle was right, but it is probable that Aristotle accidentally did more to hinder our knowledge than help it. The Rigveda refers to the 27 constellations associated with the motions of the sun and also the 12 zodiacal divisions of the sky. The ancient Greeks made important contributions to astronomy, among them the definition of the magnitude system. The Bible contains a number of statements on the position of the earth in the universe and the nature of the stars and planets, most of which are poetic rather than literal; see Biblical cosmology. In 500 AD, Aryabhata presented a mathematical system that described the earth as spinning on its axis and considered the motions of the planets with respect to the sun. Observational astronomy was mostly stagnant in medieval Europe, but flourished in the Iranian world and other parts of Islamic realm. The late 9th century Persian astronomer al-Farghani wrote extensively on the motion of celestial bodies. His work was translated into Latin in the 12th century. In the late 10th century, a huge observatory was built near Tehran, Persia (now Iran), by the Persian astronomer al-Khujandi, who observed a series of meridian transits of the Sun, which allowed him to calculate the obliquity of the ecliptic. Also in Persia, Omar Khayyám performed a reformation of the calendar that was more accurate than the Julian and came close to the Gregorian. Abraham Zacuto was responsible in the 15th century for the adaptations of astronomical theory for the practical needs of Portuguese caravel expeditions. During the Renaissance, Copernicus proposed a heliocentric model of the Solar System. His work was defended, expanded upon, and corrected by Galileo Galilei and Johannes Kepler. Galileo added the innovation of using telescopes to enhance his observations. Kepler was the first to devise a system that described correctly the details of the motion of the planets with the Sun at the center. However, Kepler did not succeed in formulating a theory behind the laws he wrote down. It was left to Newton's invention of celestial dynamics and his law of gravitation to finally explain the motions of the planets. Newton also developed the reflecting telescope. Stars were found to be faraway objects. With the advent of spectroscopy it was proved that they were similar to our own sun, but with a wide range of temperatures, masses, and sizes. The existence of our galaxy, the Milky Way, as a separate group of stars was only proven in the 20th century, along with the existence of "external" galaxies, and soon after, the expansion of the universe, seen in the recession of most galaxies from us. Modern astronomy has also discovered many exotic objects such as quasars, pulsars, blazars and radio galaxies, and has used these observations to develop physical theories which describe some of these objects in terms of equally exotic objects such as black holes and neutron stars. Physical cosmology made huge advances during the 20th century, with the model of the Big Bang heavily supported by the evidence provided by astronomy and physics, such as the cosmic microwave background radiation, Hubble's Law, and cosmological abundances of elements.

Timelines in astronomy

cosmological abundances of elements
- Artificial satellites and space probes
- Astronomical maps, catalogs, and surveys
- Big Bang
- Black hole physics
- Cosmic microwave background astronomy
- Cosmology
- Galaxies, clusters of galaxies, and large scale structure
- Interstellar medium and intergalactic medium
- Natural satellites
- Other background radiation fields
- Solar astronomy
- Solar system astronomy
- Stellar astronomy
- Telescopes, observatories, and observing technology
- White dwarfs, neutron stars, and supernovae

External Links

- [http://www.space.com/ Space.com]
- [http://www.Astronomy.com/ Astronomy.com]
- [http://www.AbsoluteAstronomy.com/ AbsoluteAstronomy.com]
- [http://www.badastronomy.com/ Bad Astronomy]
- [http://www.nasa.gov/ Nasa]
- [http://www.run4space.com Run4Space Forum]
- [http://antwrp.gsfc.nasa.gov/apod/astropix.html/ Astronomy Picture of the Day] ko:천문학 ms:Astronomi ja:天文学 simple:Astronomy th:ดาราศาสตร์

Geology

Geology (from Greek γη- (ge-, "the earth") and λογος (logos, "word", "reason")) is the science and study of the Earth, its composition, structure, physical properties, history, and the processes that shape it. It is one of the Earth sciences. Geologists have helped establish the age of the Earth at about 4.5 billion (4.5x10⁹) years, and have determined that the Earth's lithosphere, which includes the crust, is fragmented into tectonic plates that move over a rheic upper mantle (asthenosphere) via processes that are collectively referred to as plate tectonics. Geologists help locate and manage the earth's natural resources, such as petroleum and coal, as well as metals such as iron, copper, and uranium. Additional economic interests include gemstones and many minerals such as asbestos, perlite, mica, phosphates, zeolites, clay, pumice, quartz, and silica, as well as elements such as sulfur, chlorine, and helium. Astrogeology refers to the application of geologic principles to other bodies of the solar system. However, specialised terms such as selenology (studies of the Moon), areology (of Mars), etc., are also in use. The word "geology" was first used by Jean-André Deluc in the year 1778 and introduced as a fixed term by Horace-Bénédict de Saussure in the year 1779. An older meaning of the word was first used by Richard de Bury. He used it to distinguish between earthly and theological jurisprudence.

History

In China, the polymath Shen Kua (1031 - 1095) formulated a hypothesis for the process of land formation: based on his observation of fossil shells in a geological stratum in a mountain hundreds of miles from the ocean, he inferred that the land was formed by erosion of the mountains and by deposition of silt. The work on rocks Peri lithon by Theophrastus, a student of Aristotle, remained authoritative for millennia. However, its interpretation of fossils was not overturned until after the Scientific Revolution. It was translated into Latin and the other languages of Europe such as French. Georg Bauer (Georg Agricola), a physician, summarised the knowledge of mining and metallurgy in 1556. Georg Agricola (1494-1555) wrote the first systematic treatise about mining and smelting works, De re metallica libri XII, with an appendix Buch von den Lebewesen unter Tage (book of the creatures beneath the earth). He covered subjects like wind energy, hydrodynamic power, melting cookers, transport of ores, extraction of soda, sulfur and alum, and administrative issues. The book was published in 1556. By the 1700s Jean-Etienne Guettard and Nicolas Desmarest hiked central France and recorded their observations on geological maps; Guettard recorded the first observation of the volcanic origins of this part of France. James Hutton recorded his Theory of the Earth in the 1788 Transactions of the Royal Society of Edinburgh, later called uniformitarianism. William Smith (1769-1839) drew some of the first geological maps and began the process of ordering rock strata (layers) by examining the fossils contained in them. James Hutton is often viewed as the first modern geologist. In 1785 he presented a paper entitled Theory of the Earth to the Royal Society of Edinburgh. In his paper, he explained his theory that the Earth must be much older than had previously been supposed in order to allow enough time for mountains to be eroded and for sediment to form new rocks at the bottom of the sea, which in turn were raised up to become dry land. Followers of Hutton were known as Plutonists because they believed that some rocks were formed by vulcanism which is the deposition of lava from volcanoes, as opposed to the Neptunists, who believed that all rocks had settled out of a large ocean whose level gradually dropped over time. In 1811 Georges Cuvier and Alexandre Brongniart published their explanation of the antiquity of the Earth, inspired by Cuvier's discovery of fossil elephant bones in Paris. To prove this, they formulated the principle of stratigraphic succession of the layers of the earth. They were independently anticipated by William Smith's stratigraphic studies on England and Scotland. Sir Charles Lyell first published his famous book, Principles of Geology, in 1830 and continued to publish new revisions until he died in 1875. He successfully promoted the doctrine of uniformitarianism. This theory states that slow geological processes have occurred throughout the Earth's history and are still occurring today. In contrast, catastrophism is the theory that Earth's features formed in single, catastrophic events and remained unchanged thereafter. Though Hutton believed in uniformitarianism, the idea was not widely accepted at the time. catastrophism illustrated on relief globe of the Field Museum ]] By 1827 Charles Lyell's Principles of Geology reiterated Hutton's uniformitarianism, which influenced the thought of Charles Darwin. 19th Century geology revolved around the question of the Earth's exact age. Estimates varied from a few 100,000 to billions of years. The most significant advance in 20th century geology has been the development of the theory of plate tectonics in the 1960s. Plate tectonic theory arose out of two separate geological observations: seafloor spreading and continental drift. The theory revolutionised the Earth sciences. The theory of continental drift was proposed by Alfred Wegener in 1912 and by Arthur Holmes, but wasn't broadly accepted until the 1960s when the theory of plate tectonics was developed.

Important principles of geology

There are a number of important principles in geology. Many of these involve the ability to provide the relative ages of strata or the manner in which they were formed. The Principle of Intrusive Relationships concerns crosscutting intrusions. In geology, when an igneous intrusion cuts across a formation of sedimentary rock, it can be determined that the igneous intrusion is younger than the sedimentary rock. There are a number of different types of intrusions, including stocks, laccoliths, batholiths, sills and dikes. The Principle of Cross-cutting Relationships pertains to the formation of faults and the age of the sequences through which they cut. Faults are younger than the rocks they cut; accordingly, if a fault is found that penetrates some formations but not those on top of it, then the formations that were cut are older than the fault, and the ones that are not cut must be younger than the fault. Finding the key bed in these situations may help determine whether the fault is a normal fault or a thrust fault. The Principle of Inclusions and Components states that, with sedimentary rocks, if inclusions (or clasts) are found in a formation, then the inclusions must be older than the formation that contains them. For example, in sedimentary rocks, it is common for gravel from an older formation to be ripped up and included in a newer layer. A similar situation with igneous rocks occurs when xenoliths are found. These foreign bodies are picked up as magma or lava flows, and are incorporated, later to cool in the matrix. As a result, xenoliths are older than the rock which contains them. The Principle of Uniformitarianism states that, the geologic processes observed in operation that modify the Earth's crust at present have worked in much the same way over geologic time. A fundamental principle of geology advanced by the 18th century Scottish physician and geologist James Hutton, is that "The Present is the Key to the Past." In Hutton's words: "the past history of our globe must be explained by what can be seen to be happening now." The Principle of Original Horizontality states that, the deposition of sediments occurs as essentially horizontal beds. Observation of modern marine and nonmarine sediments in a wide variety of environments supports this generalisation (although cross-bedding is inclined, the overall orientation of cross-bedded units is horizontal). The Principle of Superposition states that, a sedimentary rock layer in a tectonically undisturbed sequence is younger than the one beneath it and older than the one above it. Logically a younger layer cannot slip beneath a layer previously deposited. This principle allows sedimentary layers to be viewed as a form of vertical time line, a partial or complete record of the time elapsed from deposition of the lowest layer to deposition of the highest bed. The Principle of Faunal Succession is based on the appearance of fossils in sedimentary rocks. As organisms exist at the same time period throughout the world, their presence or (sometimes) absence may be used to provide a relative age of the formations in which they are found. Based on principles laid out by William Smith almost a hundred years before the publication of Charles Darwin's theory of evolution, the principles of succession were developed independently of evolutionary thought. The principle becomes quite complex, however, given the uncertainties of fossilisation, the localisation of fossil types due to lateral changes in habitat (facies change in sedimentary strata), and that not all fossils may be found globally at the same time.

Fields or related disciplines

- Earth science
- Economic geology
- Mining geology
- Petroleum geology
- Engineering geology
- Environmental geology
- Geoarchaeology
- Geochemistry
- Biogeochemistry
- Isotope geochemistry
- Geochronology
- Geodetics
- Geomicrobiology
- Geomorphology
- Geophysics
- Glaciology
- Historical geology
- Hydrogeology or geohydrology
- Marine geology
- Mineralogy
- Paleoclimatology
- Paleontology
- Micropaleontology
- Palynology
- Petrology
- Plate tectonics
- Sedimentology
- Seismology
- Soil science
- Pedology (soil study)
- Speleology
- Stratigraphy
- Biostratigraphy
- Structural geology
- Volcanology

Regional geology

- Geology of the Alps
- Geology of the Himalaya
- Geology of Victoria (Australia)

United Kingdom

- Geology of Dorset
- Geology of Hampshire
- Geology of Hertfordshire

United States

- Geology of the Bryce Canyon area(Utah)
- Geology of the Canyonlands area (Utah)
- Geology of the Capitol Reef area (Utah)
- Geology of Connecticut
- Geology of the Death Valley area (California)
- Geology of the Grand Canyon area (Arizona)
- Geology of the Grand Teton area (Wyoming)
- Geology of the Lassen area (California)
- Geology of Mount Shasta (California)
- Geology of the Yosemite area (California)
- Geology of the Zion and Kolob canyons area (Utah)
- Glacial geology of the Genesee River (New York, Pennsylvania)

National geology

- Geology of Australia
- Geology of Victoria
- Geology of Iran
- Geology of India
- Geology of Sikkim
- Geology of the United States of America
- Geology of California
- Geology of the Grand Canyon area
- Geology of the United Kingdom
- Geology of Japan

Planetary geology

- Geology of Mars
- Geology of the Moon

External links

- James Hutton's [http://www.mala.bc.ca/~johnstoi/essays/Hutton.htm Theory of the Earth]
- James Hutton's [http://www.uwmc.uwc.edu/geography/hutton/hutton.htm Theory of the Earth & Abstract of the Theory of the Earth] Category:Geology ko:??? ja:??? th:?????????

Origin of life

:This article focuses on modern scientific research on the origin of life. For alternate uses, see origin of life (disambiguation). origin of life (disambiguation) in the Siyeh Formation, Glacier National Park. In 2002, William Schopf of UCLA published a controversial paper in the scientific journal Nature arguing that geological formations such as this possess 3.5 billion year old fossilized algae microbes. [http://www.abc.net.au/science/news/space/SpaceRepublish_497964.htm] If true, they would be the earliest known life on earth.]] Research into the origin of life is a limited field of research despite its profound impact on biology and human understanding of the natural world. Progress in this field is generally slow and sporadic, though it still draws the attention of many due to the gravity of the question being investigated. A few facts give insight into the conditions in which life may have emerged, but the mechanisms by which non-life became life are still elusive. For the observed evolution of life on earth, see the timeline of life.

History of the concept: abiogenesis

Main article: Abiogenesis Research into the origin of life is the modern incarnation of the ancient concept of abiogenesis. Abiogenesis, in its most general sense, is the generation of life from non-living matter. The term is primarily used in the context of biology and the origin of life. Abiogenesis was long considered to be a very common occurrence until the Law of Biogenesis (omne vivum ex ovo or "all life from an egg") became firmly established in modern biology as a result of the work of Louis Pasteur. Charles Darwin in a letter to J.D. Hooker of February 1st 1871, made the suggestion that life may have begun in a "warm little pond, with all sorts of ammonia and phosphoric salts, lights, heat, electricity, etc. present, that a protein compound was chemically formed ready to undergo still more complex changes, at the present day such matter would be instantly devoured or absorbed, which would not have been the case before living creatures were formed." Thus, it is the presence of life itself, operating in an oxygen rich atmosphere, itself a product of life, which prevents "spontaneous generation" from occurring on Earth today. This modern definition of abiogenesis is concerned with the formation of the simplest forms of life from primordial chemicals, in an environment regarded as similar to that at the time shortly after the formation of the Earth. This is significantly different from the concept of Aristotelian abiogenesis, which postulated the formation of complex organisms. This article reviews different hypotheses for modern abiogenetic processes that are currently under debate.

Current models of the origin of life

There is no truly "standard" model of the origin of life, however most currently accepted models build in one way or another upon a number of discoveries concerning the origin of molecular and cellular components for life, which are listed in a rough order of postulated emergence: # Plausible pre-biotic conditions result in the creation of certain basic small molecules (monomers) of life, such as amino acids. This was demonstrated in the Urey-Miller experiment by Stanley L. Miller and Harold C. Urey in 1953. # Phospholipids (of an appropriate length) can spontaneously form lipid bilayers, one of the two basic components of a cell membrane. # The polymerization of nucleotides into random RNA molecules might have resulted in self-replicating ribozymes (RNA world hypothesis). # Selection pressures for catalytic efficiency and diversity result in ribozymes which catalyse peptidyl transfer (hence formation of small proteins), since oligopeptides complex with RNA to form better catalysts. Thus the first ribosome is born, and protein synthesis becomes more prevalent. # Proteins outcompete ribozymes in catalytic ability, and therefore become the dominant biopolymer. Nucleic acids are restricted to predominantly genomic use. The origin (see Origin of organic molecules) of the basic biomolecules, while not settled, is less controversial than the significance and order of steps 2 and 3. The basic inorganic chemicals from which life was formed are methane (CH₄), ammonia (NH₃), water (H₂O), hydrogen sulfide (H₂S), carbon dioxide (CO₂), and phosphate (PO₄³⁺). As of 2004, no one has yet synthesized a "protocell" using basic components which has the necessary properties of life (the so-called "bottom-up-approach"). Without such a proof-of-principle, explanations have tended to be short on specifics. However, some researchers are working in this field, notably Jack Szostak at Harvard. Others have argued that a "top-down approach" is more feasible. One such approach attempted by Craig Venter and others at The Institute for Genomic Research involved engineering existing prokaryotic cells with progressively fewer genes, attempting to discern at which point the most minimal requirements for life were reached. The biologist John Desmond Bernal, in coining the term Biopoesis for this process suggested that there were a number of clearly defined "stages" that could be recognised in explaining the origin of life. Stage 1: The origin of biological monomers
Stage 2: The origin of biological polymers
Stage 3: The evolution from molecules to cell Bernal suggested that Darwinian evolution may have commenced early, some time between Stage 1 and 2.

Origin of organic molecules: Miller, Eigen and Wächtershäuser's theories

Darwinian evolution The "Miller experiments" (including the original Miller–Urey experiment of 1953, by Harold Urey and his graduate student Stanley Miller) are performed under simulated conditions resembling those thought at the time to have existed shortly after Earth first accreted from the primordial solar nebula. The experiment used a highly reduced mixture of gases (methane, ammonia and hydrogen). However, it should be noted that the composition of the prebiotic atmosphere of earth is currently controversial. Other less reducing gases produce a lower yield and variety. It was once thought that appreciable amounts of molecular oxygen were present in the prebiotic atmosphere, which would have essentially prevented the formation of organic molecules; however, the current scientific consensus is that such was not the case. The experiment showed that some of the basic organic monomers (such as amino acids) that form the polymeric building blocks of modern life can be formed spontaneously. Simple organic molecules are of course a long way from a fully functional self-replicating life form; however, in an environment with no pre-existing life these molecules may have accumulated and provided a rich environment for chemical evolution ("soup theory"). On the other hand, the spontaneous formation of complex polymers from abiotically generated monomers under these conditions is not at all a straightforward process. Besides the necessary basic organic monomers, also compounds that would have prohibited the formation of polymers were formed in high concentration during the experiments. Further, according to Brooks and Shaw (1973), there is no evidence in the geological record that any soup existed. :"If there ever was a primitive soup, then we would expect to find at least somewhere on this planet either massive sediments containing enormous amounts of the various nitrogenous organic compounds, acids, purines, pyrimidines, and the like; or in much metamorphosed sediments we should find vast amounts of nitrogenous cokes. In fact no such materials have been found anywhere on earth." Other sources of complex molecules have been postulated, including sources of extra-terrestrial stellar or interstellar origin. For example, from spectral analyses, organic molecules are known to be present in comets and meteorites. In 2004, a team detected traces of polycyclic aromatic hydrocarbons (PAH's) in a nebula, the most complex molecule, to that date, found in space. It can be argued that the most crucial challenge unanswered by this theory is how the relatively simple organic building blocks polymerise and form more complex structures, interacting in consistent ways to form a protocell. For example, in an aqueous environment hydrolysis of oligomers/polymers into their constituent monomers would be favored over the condensation of individual monomers into polymers. Also, the Miller experiment produces many substances that would undergo cross-reactions with the amino acids or terminate the peptide chain. In the early 1970s a major attack on the problem of the origin of life was organised by a team of scientists gathered around Manfred Eigen of the Max Planck Institute. They tried to examine the transient stages between the molecular chaos in a prebiotic soup and the transient stages of a self replicating hypercycle, between the molecular chaos in a prebiotic soup and simple macromolecular self-reproducing systems. In a hypercycle, the information storing system (possibly RNA) produces an enzyme, which aids catalyse the formation of another information system, in sequence until the product of the last aids in the formation of the first information system. Mathematically treated, hypercycles could create quasispecies, which through natural selection entered into a form of Darwinian evolution. A boost to hypercycle theory was the discovery that RNA, in certain circumstances forms itself into ribozymes, a form of RNA enzyme. Another possible answer to this polymerization conundrum was provided in 1980s by Günter Wächtershäuser, in his iron-sulfur world theory. In this theory, he postulated the evolution of (bio)chemical pathways as fundamentals of the evolution of life. Moreover, he presented a consistent system of tracing today's biochemistry back to ancestral reactions that provide alternative pathways to the synthesis of organic building blocks from simple gaseous compounds. In contrast to the classical Miller experiments, which depend on external sources of energy (e. g. simulated lightning or UV irradiation), "Wächtershäuser systems" come with a built-in source of energy, sulfides of iron and other minerals (e. g. pyrite). The energy released from redox reactions of these metal sulfides is not only available for the synthesis of organic molecules, but also for the formation of oligomers and polymers. It is therefore hypothesized that such systems may be able to evolve into autocatalytic sets of self-replicating, metabolically active entities that would predate the life forms known today. The experiment as performed, produced a relatively small yield of dipeptides (0.4–12.4%) and a smaller yield of tripeptides (0.003%) and the authors note that: "under these same conditions dipeptides hydrolysed rapidly." Another criticism of the result is that the experiment did not include any organomolecules that would most likely cross-react or chain-terminate. (Huber and Wächtershäuser, 1998) The latest modification of the iron-sulfur-hypothesis has been provided by William Martin and Michael Russell in 2002. According to their scenario, the first cellular life forms may have evolved inside so-called black smokers at seafloor spreading zones in the deep sea. These structures consist of microscale caverns that are coated by thin membraneous metal sulfide walls. Therefore, these structures would solve several critical points of the "pure" Wächtershäuser systems at once: # the micro-caverns provide a means of concentrating newly synthesised molecules, thereby increasing the chance of forming oligomers; # the steep temperature gradients inside a black smoker allow for establishing "optimum zones" of partial reactions in different regions of the black smoker (e.g. monomer synthesis in the hotter, oligomerisation in the colder parts); # the flow of hydrothermal water through the structure provides a constant source of building blocks and energy (freshly precipitated metal sulfides); # the model allows for a succession of different steps of cellular evolution (prebiotic chemistry, monomer and oligomer synthesis, peptide and protein synthesis, RNA world, ribonucleoprotein assembly and DNA world) in a single structure, facilitating exchange between all developmental stages; # synthesis of lipids as a means of "closing" the cells against the environment is not necessary, until basically all cellular functions are developed. This model locates the "last universal common ancestor" (LUCA) inside a black smoker, rather than assuming the existence of a free-living form of LUCA. The last evolutionary step would be the synthesis of a lipid membrane that finally allows the organisms to leave the microcavern system of the black smokers and start their independent lives. This postulated late acquisition of lipids is consistent with the presence of completely different types of membrane lipids in archaebacteria and eubacteria (plus eukaryotes) with highly similar cellular physiology of all life forms in most other aspects. Another unsolved issue in chemical evolution is the origin of homochirality, i.e. all monomers having the same "handedness" (amino acids being left handed, and nucleic acid sugars being right handed). Homochirality is essential for the formation of functional ribozymes (and probably proteins too). The origin of homochirality might simply be explained by an initial asymmetry by chance followed by common descent. Work performed in 2003 by scientists at Purdue identified the amino acid serine as being a probable root cause of organic molecules' homochirality. Serine forms particularly strong bonds with amino acids of the same chirality, resulting in a cluster of eight molecules that must be all right-handed or left-handed. This property stands in contrast with other amino acids which are able to form weak bonds with amino acids of opposite chirality. Although the mystery of why left-handed serine became dominant is still unsolved, this result suggests an answer to the question of chiral transmission: how organic molecules of one chirality maintain dominance once asymmetry is established.

From organic molecules to protocells

The question "How do simple organic molecules form a protocell?" is largely unanswered. However, there are many different hypotheses regarding the path that might have been taken. Some of these postulate the early appearance of nucleic acids ("genes-first") whereas others postulate the evolution of biochemical reactions and pathways first ("metabolism-first"). Recently, trends are emerging to create hybrid models that combine aspects of both.

"Genes first" models: the RNA world

Main article: RNA world hypothesis The RNA world hypothesis, for example, suggests that relatively short RNA molecules could have spontaneously formed that were capable of catalyzing their own continuing replication. Early cell membranes could have formed spontaneously from proteinoids, protein-like molecules that are produced when amino acid solutions are heated. Other possibilities include systems of chemical reactions taking place within clay substrates or on the surface of pyrite rocks. At this time however, these various hypotheses have incomplete evidence supporting them. Many of them can be simulated and tested in the lab, but a lack of undisturbed sedimentary rock from that early in Earth's history leaves few opportunities to determine what may have actually happened in reality. At this time however, no prebiotically plausible experiment has confirmed this assumption. Further, recent experiments suggest that the original estimates of the size of an RNA molecule capable of self-replication were most probably vast underestimates. Worse, RNA itself does not appear to be a prebiotically plausible molecule; therefore, more-modern forms of the RNA World theory propose that a simpler molecule was capable of self-replication (that other "World" then evolved over time to produce the RNA World).

"Metabolism first" models: iron-sulfur world and others

Several models reject the idea of the self-replication of a "naked-gene" and postulate the emergence of a primitive metabolism which could provide an environment for the later emergence of RNA replication. One of the earliest incarnations of this idea was put forward in 1924 with Alexander Oparin's notion of primitive self-replicating vesicles which predated the discovery of the structure of DNA. More recent variants in the 1980s and 1990s include Günter Wächtershäuser's iron-sulfur world theory and models introduced by Christian de Duve based on the chemistry of thioesters. More abstract and theoretical arguments for the plausibility of the emergence of metabolism without the presence of genes include a mathematical model introduced by Freeman Dyson in the early 1980s and Stuart Kauffman's notion of collectively autocatalytic sets, discussed later in that decade. However, the idea that a closed metabolic cycle, such as the reductive citric acid cycle proposed by Günter Wächtershäuser, could form spontaneously remains unsupported. Further, according to Leslie Orgel, a leader in origin-of-life studies for the past several decades, there is reason to believe the assertion will remain so. In an article entitled "Self-Organizing Biochemical Cycles" (PNAS, vol. 97, no. 23, November 7 2000, p12503-12507), Orgel summarizes his analysis of the proposal by stating, "There is at present no reason to expect that multistep cycles such as the reductive citric acid cycle will self-organize on the surface of FeS/FeS2 or some other mineral."

The Bubble Theory

Waves breaking on the shore create a delicate foam composed of bubbles. Winds sweeping across the ocean have a tendency to drive things to shore, much like driftwood collecting on the beach. It is possible that organic molecules were concentrated on the shorelines in much the same way. Shallow coastal waters also tend to be warmer, further concentrating the molecules through evaporation. While bubbles comprised of mostly water burst quickly, oily bubbles happen to be much more stable, lending more time to the particular bubble to perform these crucial experiments. The phospholipid is a good example of an oily compound believed to have been prevalent in the prebiotic seas. Because phospholipids contain a hydrophilic head on one end, and a hydrophobic tail on the other, they have the tendency to spontaneously form lipid membranes in water. A lipid monolayer bubble can only contain oil, and is therefore not conducive to harbouring water-soluble organic molecules. On the other hand, a lipid bilayer bubble can contain water, and was a likely precursor to the modern cell membrane. If a protein came along that increased the integrity of its parent bubble, then that bubble had an advantage, and was placed at the top of the natural selection waiting list. Primitive reproduction can be envisioned when the bubbles burst, releasing the results of the experiment into the surrounding medium. Once enough of the 'right stuff' was released into the medium, the development of the first prokaryotes, eukaryotes, and multicellular organisms could be achieved. This theory is expanded upon in the book, "The Cell: Evolution of the First Organism" by Joseph Panno Ph.D.

Hybrid models

A growing realization of the inadequacy of either pure "genes-first" or "metabolism-first" models is leading the trend towards models that incorporate aspects of each.

Other models

Clay theory of the origin of life

A hypothesis for the origin of life based on clay was forwarded by Dr A. Graham Cairns-Smith of Glasgow University in 1985 and adopted as a plausible illustration by just a handful of other scientists (including Richard Dawkins). Clay theory postulates complex organic molecules arising gradually on a pre-existing, non-organic replication platform - silicate crystals in solution. Complexity in companion molecules developed as a function of selection pressures on types of clay crystal is then exapted to serve the replication of organic molecules independently of their silicate "launch stage". Cairns-Smith is a staunch critic of other models of chemical evolution (see Genetic Takeover: And the Mineral Origins of Life ISBN 0-52123-312-7). However, he admits, that like many models of the origin of life, his own also has its shortcomings (Horgan 1991).

"Deep-hot biosphere" model of Gold

A controversial theory put forward by Thomas Gold in the 1990s has life first developing not on the surface of the earth, but several kilometers below the surface. It is now known that microbial life is plentiful up to five kilometers below the earth's surface in the form of archaea, which are generally considered to have originated around the same time or earlier than bacteria, most of which live on the surface including the oceans. It is claimed that discovery of microbial life below the surface of another body in our solar system would lend significant credence to this theory. He also noted that a trickle of food from a deep, unreachable, source promotes survival because life arising in a puddle of organic material is likely to consume all of its food and become extinct.

"Primitive" extraterrestrial life

An alternative to Earthly abiogenesis is the hypothesis that primitive life may have originally formed extraterrestrially (note that exogenesis is related to, but is not the same as the notion of panspermia). Organic compounds are relatively common in space, especially in the outer solar system where volatiles are not evaporated by solar heating. Comets are encrusted by outer layers of dark material, thought to be a tar-like substance composed of complex organic material formed from simple carbon compounds after reactions initiated mostly by irradiation by ultraviolet light. It is supposed that a rain of cometary material on the early Earth could have brought significant quantities of complex organic molecules, and that it is possible that primitive life itself may have formed in space was brought to the surface along with it. A related hypothesis holds that life may have formed first on early Mars, and been transported to Earth when crustal material was blasted off of Mars by asteroid and comet impacts to later fall to Earth's surface. Both of these hypotheses are even more difficult to find evidence for, and may have to wait for samples to be taken from comets and Mars for study, and neither of them actually answers the question of how life first originated, merely shifting it to another planet/comet. However, this hypothesis extends tremendously the array of conditions under which life may have have formed, from early Earth plausible conditions to literally any conditions possible in the universe.

Relevant fields

- Astrobiology is a field that may shed light on the nature of life in general, instead of just life as we know it on Earth, and may give clues as to how life originates.
- Complex systems

References

- PMID 11882894
- (Cited on p. 108).
- (Cited on p. 108).
- Brooks, J. and Shaw, G., 1973. Origins and Development of Living Systems. Academic Press, London and New York, p. 359.
- .

External links

- [http://www.accessexcellence.org/bioforum/bf02/awramik/bf02a1.html Astrobiology and the origins of life]
- [http://genetics.mgh.harvard.edu/szostakweb/publications/Szostak_pdfs/Hanczyc_and_Szostak_2004_COChemBio.pdf Martin M Hanczyc and Jack W Szostak. Replicating vesicles as models of primitive cell growth and division. Current Opinion in Chemical Biology 2004, 8:660–664.]
- [http://www.santafe.edu/sfi/People/kauffman/sak-peptides.html "SELF-REPLICATION: Even peptides do it"] by Stuart A. Kauffman
- [http://originoflife.net/ Cairns Smith illustration of a possible solution using crystalline behaviors of clays]
- [http://www-news.uchicago.edu/releases/98/980331.origin.of.life.shtml Model of origin of life involving zeolite, press release for PNAS paper]
- [http://pokey.arc.nasa.gov/~astrochm/LifeImplications.html Possible Connections Between Interstellar Chemistry and the Origin of Life on the Earth]
- [http://nai.arc.nasa.gov/news_stories/news_detail.cfm?ID=207 Scientists Find Clues That Life Began in Deep Space — NASA Astrobiology Institute]
- [http://people.cornell.edu/pages/tg21/DHB.html The Deep Hot Biosphere Theory (Thomas Gold)]
- [http://www.pnas.org/cgi/content/full/97/23/12503?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=biochemical+cycles&searchid=1119837712082_3423&stored_search=&FIRSTINDEX=0&journalcode=pnas Self-organizing biochemical cycles — by Leslie Orgel]
- [http://www.evowiki.org/index.php/Category:Creationist_claims Evolution wiki] Category:Origin of life Category:Evolution Category:Metabolism ja:生命の起源

Evolution

, based on rRNA gene data, showing the separation of the three domains, bacteria, archaea, and eukaryotes, as described initially by Carl Woese.]] In biology, evolution is the process by which populations of organisms acquire and pass on novel traits from generation to generation, affecting the overall makeup of the population and even leading to the emergence of new species. The terms organic evolution or biological evolution are often used to distinguish this meaning from other usages. The development of the modern theory of evolution began with the introduction of the concept of natural selection in a joint 1858 paper by Charles Darwin and Alfred Russel Wallace. This theory achieved a wider readership in Darwin's 1859 book, The Origin of Species. Darwin and Wallace proposed that evolution occurs because a heritable trait that increases an individual's chance of successfully reproducing will become more common, by inheritance, from one generation to the next, and likewise a heritable trait that decreases an individual's chance of reproducing will become rarer. This work was groundbreaking, and overturned other evolutionary theories, such as that advanced by Jean Baptiste Lamarck. Because of its potential implications for the origins of humankind, the theory has been at the center of many social and religious controversies since its first inception (see Creation-evolution controversy). In the 1930s, scientists combined Darwinian natural selection with the re-discovered theory of Mendelian heredity to create the modern synthesis, now one of the fundamental scientific theories of biology. In the modern synthesis, "evolution" is defined as a change in the frequency of alleles within a population from one generation to the next. The basic mechanisms that produce these changes are natural selection, genetic drift, and genetic variation. The primary sources of genetic variation are mutation, sex, and gene flow.

Overview of evolution

Evidence of evolution

The process of evolution has left behind numerous records which reveal the history of species. While the best-known of these are the fossils, fossils are only a small part of the overall physical record of evolution. Fossils, taken together with the comparative anatomy of present-day plants and animals, constitute the morphological record. By comparing the anatomies of both modern and extinct species, biologists can reconstruct the lineages of those species with some accuracy. Using fossil evidence, for instance, the connection between dinosaurs and birds has been established by way of so-called "transitional" species such as Archaeopteryx. The development of genetics has allowed biologists to study the genetic record of evolution as well. Although we cannot obtain the DNA sequences of most extinct species, the degree of similarity and difference among modern species allows geneticists to reconstruct lineages with greater accuracy. It is from genetic comparisons that claims such as the 98-99% similarity between humans and chimpanzees come from, for instance. Other evidence used to demonstrate evolutionary lineages includes the geographical distribution of species. For instance, monotremes and most marsupials are found only in Australia, showing that their common ancestor with placental mammals lived before the submerging of the ancient land bridge between Australia and Asia. Scientists correlate all of the above evidence – drawn from paleontology, anatomy, genetics, and geography – with other information about the history of the earth. For instance, paleoclimatology attests to periodic ice ages during which the climate was much cooler; and these are found to match up with the spread of species such as the woolly mammoth which are better-equipped to deal with cold.

Morphological evidence

Fossils are important for estimating when various lineages developed. As fossilization on an organism is an uncommon occurrence, usually requiring hard parts (like bone) and death near a site where sediments are being deposited, the fossil record only provides sparse and intermittent information about the evolution of life. Fossil evidence of organisms without hard body parts, such as shell, bone, and teeth, is sparse but exists in the form of ancient microfossils and the fossilization of ancient burrows and a few soft-bodied organisms. Fossil evidence of prehistoric organisms has been found all over the Earth. The age of fossils can often be deduced from the geologic context in which they are found; and their absolute age can be verified with radiometric dating. Some fossils bear a resemblance to organisms alive today, while others are radically different. Fossils have been used to determine at what time a lineage developed, and transitional fossils can be used to demonstrate continuity between two different lineages. Paleontologists investigate evolution largely through analysis of fossils. Phylogeny, the study of the ancestry of species, has revealed that structures with similar internal organization may perform divergent functions. Vertebrate limbs are a common example of such homologous structures. Bat wings, for example, are very similar to hands. A vestigial organ or structure may exist with little or no purpose in one organism, though they have a clear purpose in other species. The human wisdom teeth and appendix are common examples.

Genetic sequence evidence

Comparison of the genetic sequence of organisms reveals that phylogenetically close organisms have a higher degree of sequence similarity than organisms that are phylogenetically distant. For example, neutral human DNA sequences are approximately 1.2% divergent (based on substitutions) from those of their nearest genetic relative, the chimpanzee, 1.6% from gorillas, and 6.6% from baboons. Sequence comparison is considered a measure robust enough to be used to correct erroneous assumptions in the phylogenetic tree in instances where other evidence is scarce. Further evidence for common descent comes from genetic detritus such as pseudogenes, regions of DNA which are orthologous to a gene in a related organism, but are no longer active and appear to be undergoing a steady process of degeneration. Since metabolic processes do not leave fossils, research into the evolution of the basic cellular processes is done largely by comparison of existing organisms. Many lineages diverged when new metabolic processes appeared, and it is theoretically possible to determine when certain metabolic processes appeared by comparing the traits of the descendants of a common ancestor.

History of evolutionary thought

metabolic.]] The idea of biological evolution has existed since ancient times, notably among Hellenists such as Epicurus and Anaximander, but the modern theory was not established until the 18th and 19th centuries, by scientists such as Jean-Baptiste Lamarck and Charles Darwin. While transmutation of species was accepted by a sizeable number of scientists before 1859, it was the publication of Charles Darwin's The Origin of Species by Means of Natural Selection which provided the first cogent mechanism by which evolutionary change could occur: his theory of natural selection. Darwin was motivated to publish his work on evolution after receiving a letter from Alfred Russel Wallace, in which Wallace revealed his own discovery of natural selection. As such, Wallace is sometimes given shared credit for the theory of evolution. Darwin's theory, though it succeeded in profoundly shaking scientific opinion regarding the development of life, could not explain the source of variation in traits within a species, and Darwin's proposal of a hereditary mechanism (pangenesis) was not compelling to most biologists. It was not until the late 19th and early 20th centuries that these mechanisms were established. pangenesis, proposed the theory of punctuated equilibrium in 1972.]] When Gregor Mendel's work regarding the nature of inheritance in the late 19th century was "rediscovered" in 1900, it led to a storm of conflict between Mendelians (Charles Benedict Davenport) and biometricians (Walter Frank Raphael Weldon and Karl Pearson), who insisted that the great majority of traits important to evolution must show continuous variation that was not explainable by Mendelian analysis. Eventually, the two models were reconciled and merged, primarily through the work of the biologist and statistician R.A. Fisher. This combined approach, applying a rigorous statistical model to Mendel's theories of inheritance via genes, became known in the 1930s and 1940s as the modern evolutionary synthesis. In the 1940s, following up on Griffith's experiment, Avery, McCleod and McCarty definitively identified deoxyribonucleic acid (DNA) as the "transforming principle" responsible for transmitting genetic information. In 1953, Francis Crick and James Watson published their famous paper on the structure of DNA, based on the research of Rosalind Franklin and Maurice Wilkins. These developments ignited the era of molecular biology and transformed the understanding of evolution into a molecular process: the mutation of segments of DNA (see molecular evolution). George C. Williams' 1966 Adaptation and natural selection: A Critique of some Current Evolutionary Thought marked a departure from the idea of group selection towards the modern notion of the gene as the unit of selection. In the mid-1970s, Motoo Kimura formulated the neutral theory of molecular evolution, firmly establishing the importance of genetic drift as a major mechanism of evolution. Debates have continued within the field. One of the most prominent public debates was over the theory of punctuated equilibrium, proposed in 1972 by paleontologists Niles Eldredge and Stephen Jay Gould to explain the paucity of transitional forms between phyla in the fossil record.

Social and religious controversies

Stephen Jay Gould from 1871 reflects part of the social controversy over whether humans and apes share a common lineage.]] There has been constant controversy surrounding the ideas presented by The Origin of Species since it was first printed in 1859. Since the early twentieth century, however, the idea that biological evolution of some form occurred and is responsible for speciation has been almost completely uncontested within the scientific community. Most controversy over the theory has come because of its philosophical, cosmological, and religious implications, and supporters as well as detractors have interpreted it as generally indicating that human beings are, like all animals, evolved, and that this account of the origins of humankind is squarely at odds with many religious interpretations. The idea that humans are "merely" animals, and are genetically very closely related to primates, have been independently argued as repellent notions by generations of detractors. Others also intepreted the truth of the theory to imply varying types of social changes — one prominent example is the idea of eugenics, formulated by Darwin's cousin Francis Galton, which argues for the improvement of human heredity by means of political policies. Others have found different political interpretations which have been used as arguments both for and against the theory. The questions raised about the relation of evolution to the origins of humans has made it an especially tenacious issue with religious traditions. It has prominently been seen as opposing a "literal" interpretation of the account of the origins of humankind as described in Genesis, the first book of the Bible. In many countries — notably in the United States — this has led to what has been called the Creation-evolution controversy, which has focused primarily on struggles over teaching curriculum.

Science of evolution

Science: fact and theory

The word "evolution" has been used to refer both to a fact and a theory, and it is important to understand both these different meanings of evolution, and the relationship between fact and theory in science.

Evolution as fact and theory

When "evolution" is used to describe a fact, it refers to the observations that populations of one species of organism do, over time change into new, or several new, species. In this sense, evolution occurs whenever a new strain of bacterium evolves that is resistant to antibodies that had been lethal to prior strains. Another clear case of evolution as fact involves the hawthorn fly, Rhagoletis pomonella. Different populations of hawthorn fly feed on different fruits. A new population spontaneously emerged in North America in the 19th century some time after apples, a non-native species, were introduced. The apple feeding population normally feeds only on apples and not on the historically preferred fruit of hawthorns. Likewise the current hawthorn feeding population does not normally feed on apples. A current area of scientific research is the investigation of whether or not the apple feeding race may further evolve into a new species. Some evidence, such as the fact that six out of thirteen alozyme loci are different, that hawthorn flies mature later in the season, and take longer to mature, than apple flies, and that there is little evidence of interbreeding (researchers have documented a 4-6%hybridization rate) suggests that this is indeed ocurring. (see Berlocher and Bush 1982, Berlocher and Feder 2002, Bush 1969, McPheron et. al. 1988, Prokopy et. al. 1988, Smith 1988) When "evolution" is used to describe a theory, it refers to an explanation for why and how evolution (for example, in the sense of "speciation") occurs. An example of evolution as theory is the modern synthesis of Darwin and Wallace's theory of natural selection and Mendel's principles of genetics. This theory has three major aspects: # Common descent of all organisms from a single ancestor or ancestral gene pool. # Manifestation of novel traits in a lineage. # Mechanisms that cause some traits to persist while others perish. When people provide evidence for evolution, in some cases they are providing evidence that evolution occurs; in other cases they are providing evidence that a given theory is the best explanation yet as to why and how evolution occurs.

The meaning of, and relationship between, fact and theory in science

:Main article: Theory The modern synthesis, like its Mendelian and Darwinian antecedents, is a scientific theory. In plain English, people use the word "theory" to signify "conjecture", "speculation", or "opinion". In this popular sense, "theories" are opposed to "facts" — parts of the world, or claims about the world, that are real or true regardless of what people think. In scientific terminology however, a theory is a model of the world (or some portion of it) from which falsifiable hypotheses can be generated and tested through controlled experiments, or be verified through empirical observation. In this scientific sense, "facts" are parts of theories – they are things, or relationships between things, that theories must take for granted in order to make predictions, or that theories predict. In other words, for scientists "theory" and "fact" do not stand in opposition, but rather exist in a reciprocal relationship – for example, it is a "fact" that every apple ever dropped on earth (under normal, controlled conditions) has been observed to fall towards the center of the planet in a straight line, and the "theory" which explains these observations is the current theory of gravitation. In this same sense evolution is a fact and modern synthesis is currently the most powerful theory explaining evolution, variation and speciation. Within the science of biology, modern synthesis has completely replaced earlier accepted explanations for the origin of species, including Lamarckism and creationism.
Who studies evolution?
Scholars in a number of academic disciplines and subdisciplines document the fact of evolution, and contribute to the theory of evolution.
Physical anthropology
Physical anthropology emerged in the late 1800s as the study of human osteology, and the fossilized skeletal remains of other hominids. At that time anthropologists debated whether their evidence supported Darwin's claims, because skeletal remains revelaed temporal and spacial variation among hominids, but Darwin had not offered an explanation of the mechanisms that produce variation. With the recognition of Mendelian genetics and the rise of the modern synthesis, however, evolution became both the fundamental conceptual framework for, and object of study of, physical anthropologists. In addition to studying skeletal remains, they began to study genetic variation among human populations (i.e. population genetics; thus, some physical anthropologists began calling themselves biological anthropologists.
Evolutionary biology
Evolutionary biology is a subfield of biology concerned with the origin and descent of species, as well as their change over time. At first it was an interdisciplinarity field including scientists from many traditional taxonomically oriented disciplines. For example, it generally includes scientists who may have a specialist training in particular organisms such as mammalogy, ornithology, or herpetology but use those organisms as systems to answer general questions in evolution. Evolutionary biology as an academic discipline in its own right emerged as a result of the modern evolutionary synthesis in the 1930s and 1940s. It was not until the 1970s and 1980s, however, that a significant number of universities had departments that specifically included the term evolutionary biology in their titles.
Evolutionary developmental biology
Evolutionary developmental biology is an emergent subfield of evolutionary biology that looks at genes of related and unrelated organisms. By comparing the explicit nucleotide sequences of DNA/RNA, it is possible to experimentally determine and trace timelines of species development. For example, gene sequences support the conclusion that chimpanzees are the closest primate ancestor to humans, and that arthropods (e.g., insects) and vertebrates (e.g., humans) have a common biological ancestor.
Ancestry of organisms
vertebrates In biology, the theory of universal common descent proposes that all organisms on Earth are descended from a common ancestor or ancestral gene pool (which is called having "common descent"). Evidence for common descent may be found in traits shared between all living organisms. In Darwin's day, the evidence of shared traits was based solely on visible observation of morphologic similarities, such as the fact that all birds — even those which do not fly — have wings. Today, the theory of evolution has been strongly confirmed by genetics. For example, every

Xenobiology

Overview

Research Outcomes

Methodology

Narrowing the task

The sciences of Astrobiology

Astronomy

Biology

Geology

Criticisms

See also

External links

Astronomy

Divisions

By subject or problem addressed

Ways of obtaining information

History of astronomy

Timelines in astronomy

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Astronomy tools

External Links

Geology

History

Important principles of geology

Fields or related disciplines

Regional geology

United Kingdom

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Origin of life

History of the concept: abiogenesis

Current models of the origin of life

Origin of organic molecules: Miller, Eigen and Wächtershäuser's theories

From organic molecules to protocells

"Genes first" models: the RNA world

"Metabolism first" models: iron-sulfur world and others

The Bubble Theory

Hybrid models

Other models

Clay theory of the origin of life

"Deep-hot biosphere" model of Gold

"Primitive" extraterrestrial life

Relevant fields

See also

References

External links

Evolution

Overview of evolution

Evidence of evolution

Morphological evidence

Genetic sequence evidence

History of evolutionary thought

Social and religious controversies

Science of evolution

Science: fact and theory

Evolution as fact and theory

The meaning of, and relationship between, fact and theory in science

Who studies evolution?

Physical anthropology

Evolutionary biology

Evolutionary developmental biology

Ancestry of organisms