Convergent evolution is the independent evolution of similar features in species of different lineages. Convergent evolution creates analogous structures that have similar form or function but were not present in the last common ancestor of those groups; the cladistic term for the same phenomenon is homoplasy. The recurrent evolution of flight is a classic example, as flying insects, birds and bats have independently evolved the useful capacity of flight. Functionally similar features that have arisen through convergent evolution are analogous, whereas homologous structures or traits have a common origin but can have dissimilar functions. Bird and pterosaur wings are analogous structures, but their forelimbs are homologous, sharing an ancestral state despite serving different functions; the opposite of convergence is divergent evolution. Convergent evolution is similar to parallel evolution, which occurs when two independent species evolve in the same direction and thus independently acquire similar characteristics.
Many instances of convergent evolution are known in plants, including the repeated development of C4 photosynthesis, seed dispersal by fleshy fruits adapted to be eaten by animals, carnivory. In morphology, analogous traits arise when different species live in similar ways and/or a similar environment, so face the same environmental factors; when occupying similar ecological niches similar problems can lead to similar solutions. The British anatomist Richard Owen was the first to identify the fundamental difference between analogies and homologies. In biochemistry and chemical constraints on mechanisms have caused some active site arrangements such as the catalytic triad to evolve independently in separate enzyme superfamilies. In his 1989 book Wonderful Life, Stephen Jay Gould argued that if one could "rewind the tape of life the same conditions were encountered again, evolution could take a different course". Simon Conway Morris disputes this conclusion, arguing that convergence is a dominant force in evolution, given that the same environmental and physical constraints are at work, life will evolve toward an "optimum" body plan, at some point, evolution is bound to stumble upon intelligence, a trait presently identified with at least primates and cetaceans.
In cladistics, a homoplasy is a trait shared by two or more taxa for any reason other than that they share a common ancestry. Taxa which do share ancestry are part of the same clade. Homoplastic traits caused by convergence are therefore, from the point of view of cladistics, confounding factors which could lead to an incorrect analysis. In some cases, it is difficult to tell whether a trait has been lost and re-evolved convergently, or whether a gene has been switched off and re-enabled later; such a re-emerged trait is called an atavism. From a mathematical standpoint, an unused gene has a decreasing probability of retaining potential functionality over time; the time scale of this process varies in different phylogenies. When two species are similar in a particular character, evolution is defined as parallel if the ancestors were similar, convergent if they were not; some scientists have argued that there is a continuum between parallel and convergent evolution, while others maintain that despite some overlap, there are still important distinctions between the two.
When the ancestral forms are unspecified or unknown, or the range of traits considered is not specified, the distinction between parallel and convergent evolution becomes more subjective. For instance, the striking example of similar placental and marsupial forms is described by Richard Dawkins in The Blind Watchmaker as a case of convergent evolution, because mammals on each continent had a long evolutionary history prior to the extinction of the dinosaurs under which to accumulate relevant differences; the enzymology of proteases provides some of the clearest examples of convergent evolution. These examples reflect the intrinsic chemical constraints on enzymes, leading evolution to converge on equivalent solutions independently and repeatedly. Serine and cysteine proteases use different amino acid functional groups as a nucleophile. In order to activate that nucleophile, they orient an acidic and a basic residue in a catalytic triad; the chemical and physical constraints on enzyme catalysis have caused identical triad arrangements to evolve independently more than 20 times in different enzyme superfamilies.
Threonine proteases use the amino acid threonine as their catalytic nucleophile. Unlike cysteine and serine, threonine is a secondary alcohol; the methyl group of threonine restricts the possible orientations of triad and substrate, as the methyl clashes with either the enzyme backbone or the histidine base. Most threonine proteases use an N-terminal threonine in order to avoid such steric clashes. Several evolutionarily independent enzyme superfamilies with different protein folds use the N-terminal residue as a nucleophile; this commonality of active site but difference of protein fold indicates that the active site evolved convergently in those families. Convergence occurs at the level of DNA and the amino acid sequences produced by translating structural genes into proteins. Studies have found convergence in amino acid sequenc
National Museum of Natural History
The National Museum of Natural History is a natural history museum administered by the Smithsonian Institution, located on the National Mall in Washington, D. C. United States, it is open 364 days a year. In 2016, with 7.1 million visitors, it was the fourth most visited museum in the world and the most visited natural-history museum in the world. Opened in 1910, the museum on the National Mall was one of the first Smithsonian buildings constructed to hold the national collections and research facilities; the main building has an overall area of 1,500,000 square feet with 325,000 square feet of exhibition and public space and houses over 1,000 employees. The museum's collections contain over 126 million specimens of plants, fossils, rocks, human remains, human cultural artifacts, it is home to about 185 professional natural-history scientists—the largest group of scientists dedicated to the study of natural and cultural history in the world. The United States National Museum was founded in 1846 as part of the Smithsonian Institution.
The museum was housed in the Smithsonian Institution Building, better known today as the Smithsonian Castle. A formal exhibit hall opened in 1858; the growing collection led to the construction of the National Museum Building. Covering a then-enormous 2.25 acres, it was built in just 15 months at a cost of $310,000. It opened in March 1881. Congress authorized construction of a new building on June 28, 1902. On January 29, 1903, a special committee composed of members of Congress and representatives from the Smithsonian's board of regents published a report asking Congress to fund a much larger structure than planned; the regents began considering sites for the new building in March, by April 12 settled on a site on the north side of B Street NW between 9th and 12th Streets. The D. C. architectural firm of Hornblower & Marshall was chosen to design the structure. Testing of the soil for the foundations was set for July 1903, with construction expected to take three years; the Natural History Building opened its doors to the public on March 17, 1910, in order to provide the Smithsonian Institution with more space for collections and research.
The building was not completed until June 1911. The structure cost $3.5 million dollars. The Neoclassical style building was the first structure constructed on the north side of the National Mall as part of the 1901 McMillan Commission plan. In addition to the Smithsonian's natural history collection, it housed the American history and cultural collections. Between 1981 and 2003, the National Museum of Natural History had 11 acting directors. There were six directors alone between 1990 and 2002. Turnover was high as the museum's directors were disenchanted by low levels of funding and the Smithsonian's inability to define the museum's mission. Robert W. Fri was named the museum's director in 1996. One of the largest donations in Smithsonian history was made during Fri's tenure. Kenneth E. Behring donated $20 million in 1997 to modernize the museum. Fri resigned in 2001 after disagreeing with Smithsonian leadership over the reorganization of the museum's scientific research programs. J. Dennis O'Connor, Provost of the Smithsonian Institution was named acting director of the museum on July 25, 2001.
Eight months O'Conner resigned to become the vice president of research and dean of the graduate school at the University of Maryland. Douglas Erwin, a paleontologist at the National Museum of Natural History, was appointed interim director in June 2002. In January 2003, the Smithsonian announced that Cristián Samper, a Colombian with an M. Sc. and Ph. D. from Harvard University, would become the museum's permanent director on March 31, 2003. Samper founded the Alexander von Humboldt Biological Resources Research Institute and ran the Smithsonian Tropical Research Institute after 2001. Smithsonian officials said. Under Samper's direction, the museum opened the $100 million Behring Hall of Mammals in November 2003, received $60 million in 2004 for the Sant Hall of Oceans, received a $1 million gift from Tiffany & Co. for the purchase of precious gems for the National Gem Collection. On March 25, 2007, Lawrence M. Small, Secretary of the Smithsonian Institution and the organization's highest-ranking appointed official, resigned abruptly after public reports of lavish spending.
On March 27, 2007 Samper was appointed Acting Secretary of the Smithsonian. Paul G. Risser, former chancellor of the University of Oklahoma, was named Acting Director of the Museum of Natural History on March 29. Samper's tenure at the museum was not without controversy. In May 2007, Robert Sullivan, the former associate director in charge of exhibitions at the National Museum of Natural History, charged that Samper and Smithsonian Undersecretary for Science David Evans ordered "last minute" changes in the exhibit "Arctic: A Friend Acting Strangely" to tone down the role of human beings in the discussion of global warming, to make global warming seem more uncertain than depicted. Samper denied that he knew of any scientific objections to the changes, said that no political pressure had been applied to the Smithsonian to make the changes. In November 2007, The Washington Post reported that an interagency group of scientists from the Department of the Interior, NASA, Nati
Moropus is an extinct genus of perissodactyl mammal that belonged to the group called chalicotheres, which were endemic to North America during the Miocene from ~20.4—13.6 Mya, existing for 6.8 million years. The closest extant relatives of Moropus are other perissodactyls: horses and tapirs. Moropus was named by Marsh, its type is Moropus distans. It was synonymized subjectively with Macrotherium by Osborn, it was assigned to Moropodidae by Marsh. Like other chalicotheres, they differed from their modern relatives in having large claws, rather than hooves, on the front feet. Moropus stood about 8 feet tall at the shoulder; the three compressed claw-like hooves on each foot were split down the middle. These claws gave Moropus its name: "slow foot"; this name implies. But the articulation of the phalangeal bones, in addition to the presence of large foot and toe pads, shows that Moropus could raise the claws to enable it to move about quite smoothly; as the hooves curved inward, it had a pigeon-toed gait.
Phillips Ranch, Kern County, California estimated age: ~18.7 Mya. Stewart Spring, Mineral County and Esmeralda County, Nevada estimated age: ~18.7 Mya. Stage Hill I, aka Millennium's End Quarry, Scotts Bluff County, Nebraska estimated age: ~21.6—21.5 Mya. Sucker Creek site, Sucker Creak Formation, Malheur County, Oregon ~16.4 Mya. M. elatus was named by Marsh. Body mass Two specimens were examined by M. Mendoza, C. M. Janis, P. Palmqvist for body mass. Specimen 1: 118.4 kg Specimen 2: 296.8 kg Fossil distribution Granby site, Grand County, Colorado ~23 Mya. Agate Springs Quarries, Sioux County, estimated age: ~20.9—20.8 Mya. American Museum-Cook Quarry, Sioux County, estimated age: ~23.03—5.33 Mya. Cart Trail Quarry, Box Butte County, estimated age: ~23.0—20.3 Mya. Morava Ranch Quarry, Box Butte County, estimated age: ~21.6 Mya. M. hollandi was named by Peterson. Fossil distribution Chugwater, Platte County, estimated age: ~20.7 Mya. Jay Em, Goshen County, estimated age: ~20.7 Mya. Niobrara Canyon, Sioux County, estimated age: ~20.7 Mya.
M. matthewi was named by Holland and Peterson, 1913-1914. M. merrami was named by Peterson. It was recombined as Macrotherium merriami by Stirton. Fossil distribution High Rock Canyon, Humboldt County, estimated age: ~17.2 Mya. Virgin Valley, Humboldt County, estimated age: ~16.3 Mya. Humbug Quarry, Sioux County, Nebraska, ~16.5—16.25 Mya. Echo Quarry, Sioux County, Nebraska, ~16.3—13.6 Mya. M. oregonsis was named by Leidy 1883. It was named by Leidy and recombined as Moropus oregonensis by Holland and Peterson and M. C. Coombs in 1978 and 1998, by M. C. Coombs, R. M. Hunt, E. Stepleton, L. B. Albright, III, T. J. Fremd. Body mass Two specimens were examined by M. Mendoza, C. M. Janis, P. Palmqvist for body mass. Specimen 1: 58.4 kg Specimen 2: 90.3 kg Fossil distribution Johnson Canyon, John Day Formation, Wheeler County, estimated age: ~22.2—21.9 Mya. Rose Creek, John Day Formation, Wheeler County, Oregon Toledo Bend, Fleming Formation, Newton County, estimated age: 21.9 Mya. St. Marks River, Leon County, estimated age: ~23.1—21.9 Mya.
Buda Mine site, Alachua County, estimated age: ~23.1—23 Mya. M. senex was named by Marsh. It was considered a nomen dubium by Coombs. Ancylotherium Cambridge Journals Online, Journal of Zoology
A tooth is a hard, calcified structure found in the jaws of many vertebrates and used to break down food. Some animals carnivores use teeth for hunting or for defensive purposes; the roots of teeth are covered by gums. Teeth hardness; the cellular tissues that become teeth originate from the embryonic germ layer, the ectoderm. The general structure of teeth is similar across the vertebrates, although there is considerable variation in their form and position; the teeth of mammals have deep roots, this pattern is found in some fish, in crocodilians. In most teleost fish, the teeth are attached to the outer surface of the bone, while in lizards they are attached to the inner surface of the jaw by one side. In cartilaginous fish, such as sharks, the teeth are attached by tough ligaments to the hoops of cartilage that form the jaw; some animals develop only one set of teeth. Sharks, for example, grow a new set of teeth. Rodent incisors grow and wear away continually through gnawing, which helps maintain constant length.
The industry of the beaver is due in part to this qualification. Many rodents such as voles and guinea pigs, but not mice, as well as leporidae like rabbits, have continuously growing molars in addition to incisors. Teeth are not always attached to the jaw. In many reptiles and fish, teeth are attached to the palate or to the floor of the mouth, forming additional rows inside those on the jaws proper; some teleosts have teeth in the pharynx. While not true teeth in the usual sense, the dermal denticles of sharks are identical in structure and are to have the same evolutionary origin. Indeed, teeth appear to have first evolved in sharks, are not found in the more primitive jawless fish – while lampreys do have tooth-like structures on the tongue, these are in fact, composed of keratin, not of dentine or enamel, bear no relationship to true teeth. Though "modern" teeth-like structures with dentine and enamel have been found in late conodonts, they are now supposed to have evolved independently of vertebrates' teeth.
Living amphibians have small teeth, or none at all, since they feed only on soft foods. In reptiles, teeth are simple and conical in shape, although there is some variation between species, most notably the venom-injecting fangs of snakes; the pattern of incisors, canines and molars is found only in mammals, to varying extents, in their evolutionary ancestors. The numbers of these types of teeth vary between species; the genes governing tooth development in mammals are homologous to those involved in the development of fish scales. Study of a tooth plate of a fossil of the extinct fish Romundina stellina showed that the teeth and scales were made of the same tissues found in mammal teeth, lending support to the theory that teeth evolved as a modification of scales. Teeth are among the most distinctive features of mammal species. Paleontologists use teeth to determine their relationships; the shape of the animal's teeth are related to its diet. For example, plant matter is hard to digest, so herbivores have many molars for chewing and grinding.
Carnivores, on the other hand, have canine teeth to tear meat. Mammals, in general, are diphyodont. In humans, the first set starts to appear at about six months of age, although some babies are born with one or more visible teeth, known as neonatal teeth. Normal tooth eruption at about six months can be painful. Kangaroos and manatees are unusual among mammals because they are polyphyodonts. In Aardvarks, teeth lack enamel and have many pulp tubules, hence the name of the order Tubulidentata. In dogs, the teeth are less than humans to form dental cavities because of the high pH of dog saliva, which prevents enamel from demineralizing. Sometimes called cuspids, these teeth are shaped like points and are used for tearing and grasping food Like human teeth, whale teeth have polyp-like protrusions located on the root surface of the tooth; these polyps are made of cementum in both species, but in human teeth, the protrusions are located on the outside of the root, while in whales the nodule is located on the inside of the pulp chamber.
While the roots of human teeth are made of cementum on the outer surface, whales have cementum on the entire surface of the tooth with a small layer of enamel at the tip. This small enamel layer is only seen in older whales where the cementum has been worn away to show the underlying enamel; the toothed whale is a suborder of the cetaceans characterized by having teeth. The teeth differ among the species, they may be numerous, with some dolphins bearing over 100 teeth in their jaws. On the other hand, the narwhals have a giant unicorn-like tusk, a tooth containing millions of sensory pathways and used for sensing during feeding and mating, it is the most neurologically complex tooth known. Beaked whales are toothless, with only bizarre teeth found in males; these teeth may be used for feeding but for demonstrating aggression and showmanship. In humans there are 20 primary teeth, 28 to 32 of what's known as permanent teeth, in addition to other four being third molars or wisdom teeth, each of which may or may not g
The Silurian is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at 443.8 million years ago, to the beginning of the Devonian Period, 419.2 Mya. The Silurian is the shortest period of the Paleozoic Era; as with other geologic periods, the rock beds that define the period's start and end are well identified, but the exact dates are uncertain by several million years. The base of the Silurian is set at a series of major Ordovician–Silurian extinction events when 60% of marine species were wiped out. A significant evolutionary milestone during the Silurian was the diversification of jawed fish and bony fish. Multi-cellular life began to appear on land in the form of small, bryophyte-like and vascular plants that grew beside lakes and coastlines, terrestrial arthropods are first found on land during the Silurian. However, terrestrial life would not diversify and affect the landscape until the Devonian; the Silurian system was first identified by British geologist Roderick Murchison, examining fossil-bearing sedimentary rock strata in south Wales in the early 1830s.
He named the sequences for a Celtic tribe of Wales, the Silures, inspired by his friend Adam Sedgwick, who had named the period of his study the Cambrian, from the Latin name for Wales. This naming does not indicate any correlation between the occurrence of the Silurian rocks and the land inhabited by the Silures. In 1835 the two men presented a joint paper, under the title On the Silurian and Cambrian Systems, Exhibiting the Order in which the Older Sedimentary Strata Succeed each other in England and Wales, the germ of the modern geological time scale; as it was first identified, the "Silurian" series when traced farther afield came to overlap Sedgwick's "Cambrian" sequence, provoking furious disagreements that ended the friendship. Charles Lapworth resolved the conflict by defining a new Ordovician system including the contested beds. An early alternative name for the Silurian was "Gotlandian" after the strata of the Baltic island of Gotland; the French geologist Joachim Barrande, building on Murchison's work, used the term Silurian in a more comprehensive sense than was justified by subsequent knowledge.
He divided the Silurian rocks of Bohemia into eight stages. His interpretation was questioned in 1854 by Edward Forbes, the stages of Barrande, F, G and H, have since been shown to be Devonian. Despite these modifications in the original groupings of the strata, it is recognized that Barrande established Bohemia as a classic ground for the study of the earliest fossils; the Llandovery Epoch lasted from 443.8 ± 1.5 to 433.4 ± 2.8 mya, is subdivided into three stages: the Rhuddanian, lasting until 440.8 million years ago, the Aeronian, lasting to 438.5 million years ago, the Telychian. The epoch is named for the town of Llandovery in Wales; the Wenlock, which lasted from 433.4 ± 1.5 to 427.4 ± 2.8 mya, is subdivided into the Sheinwoodian and Homerian ages. It is named after Wenlock Edge in England. During the Wenlock, the oldest-known tracheophytes of the genus Cooksonia, appear; the complexity of later Gondwana plants like Baragwanathia, which resembled a modern clubmoss, indicates a much longer history for vascular plants, extending into the early Silurian or Ordovician.
The first terrestrial animals appear in the Wenlock, represented by air-breathing millipedes from Scotland. The Ludlow, lasting from 427.4 ± 1.5 to 423 ± 2.8 mya, comprises the Gorstian stage, lasting until 425.6 million years ago, the Ludfordian stage. It is named for the town of Ludlow in England; the Přídolí, lasting from 423 ± 1.5 to 419.2 ± 2.8 mya, is the final and shortest epoch of the Silurian. It is named after one locality at the Homolka a Přídolí nature reserve near the Prague suburb Slivenec in the Czech Republic. Přídolí is the old name of a cadastral field area. In North America a different suite of regional stages is sometimes used: Cayugan Lockportian Tonawandan Ontarian Alexandrian In Estonia the following suite of regional stages is used: Ohessaare stage Kaugatuma stage Kuressaare stage Paadla stage Rootsiküla stage Jaagarahu stage Jaani stage Adavere stage Raikküla stage Juuru stage With the supercontinent Gondwana covering the equator and much of the southern hemisphere, a large ocean occupied most of the northern half of the globe.
The high sea levels of the Silurian and the flat land resulted in a number of island chains, thus a rich diversity of environmental settings. During the Silurian, Gondwana continued a slow southward drift to high southern latitudes, but there is evidence that the Silurian icecaps were less extensive than those of the late-Ordovician glaciation; the southern continents remained united during this period. The melting of icecaps and glaciers contributed to a rise in sea level, recognizable from the fact that Silurian sediments overlie eroded Ordovician sediments, forming an unconformity; the continents of Avalonia and Laurentia drifted together near the equator, starting the formation of a second supercontinent known as Euramerica. When the proto-Europe coll
The Oligocene is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present. As with other older geologic periods, the rock beds that define the epoch are well identified but the exact dates of the start and end of the epoch are uncertain; the name Oligocene was coined in 1854 by the German paleontologist Heinrich Ernst Beyrich. The Oligocene is followed by the Miocene Epoch; the Oligocene is the final epoch of the Paleogene Period. The Oligocene is considered an important time of transition, a link between the archaic world of the tropical Eocene and the more modern ecosystems of the Miocene. Major changes during the Oligocene included a global expansion of grasslands, a regression of tropical broad leaf forests to the equatorial belt; the start of the Oligocene is marked by a notable extinction event called the Grande Coupure. By contrast, the Oligocene–Miocene boundary is not set at an identified worldwide event but rather at regional boundaries between the warmer late Oligocene and the cooler Miocene.
Oligocene faunal stages from youngest to oldest are: The Paleogene Period general temperature decline is interrupted by an Oligocene 7-million-year stepwise climate change. A deeper 8.2 °C, 400,000-year temperature depression leads the 2 °C, seven-million-year stepwise climate change 33.5 Ma. The stepwise climate change began 32.5 Ma and lasted through to 25.5 Ma, as depicted in the PaleoTemps chart. The Oligocene climate change was a global increase in ice volume and a 55 m decrease in sea level with a related temperature depression; the 7-million-year depression abruptly terminated within 1–2 million years of the La Garita Caldera eruption at 28–26 Ma. A deep 400,000-year glaciated Oligocene Miocene boundary event is recorded at McMurdo Sound and King George Island. During this epoch, the continents continued to drift toward their present positions. Antarctica became more isolated and developed an ice cap. Mountain building in western North America continued, the Alps started to rise in Europe as the African plate continued to push north into the Eurasian plate, isolating the remnants of the Tethys Sea.
A brief marine incursion marks the early Oligocene in Europe. Marine fossils from the Oligocene are rare in North America. There appears to have been a land bridge in the early Oligocene between North America and Europe, since the faunas of the two regions are similar. Sometime during the Oligocene, South America was detached from Antarctica and drifted north towards North America, it allowed the Antarctic Circumpolar Current to flow cooling the Antarctic continent. Angiosperms continued their expansion throughout the world as tropical and sub-tropical forests were replaced by temperate deciduous forests. Open plains and deserts became more common and grasses expanded from their water-bank habitat in the Eocene moving out into open tracts; however at the end of the period, grass was not quite common enough for modern savannas. In North America, subtropical species dominated with cashews and lychee trees present, temperate trees such as roses and pines were common; the legumes spread, while sedges and ferns continued their ascent.
More open landscapes allowed animals to grow to larger sizes than they had earlier in the Paleocene epoch 30 million years earlier. Marine faunas became modern, as did terrestrial vertebrate fauna on the northern continents; this was more as a result of older forms dying out than as a result of more modern forms evolving. Many groups, such as equids, rhinos and camelids, became more able to run during this time, adapting to the plains that were spreading as the Eocene rainforests receded; the first felid, originated in Asia during the late Oligocene and spread to Europe. South America was isolated from the other continents and evolved a quite distinct fauna during the Oligocene; the South American continent became home to strange animals such as pyrotheres and astrapotheres, as well as litopterns and notoungulates. Sebecosuchians, terror birds, carnivorous metatheres, like the borhyaenids remained the dominant predators. Brontotheres died out in the Earliest Oligocene, creodonts died out outside Africa and the Middle East at the end of the period.
Multituberculates, an ancient lineage of primitive mammals that originated back in the Jurassic became extinct in the Oligocene, aside from the gondwanatheres. The Oligocene was home to a wide variety of strange mammals. A good example of this would be the White River Fauna of central North America, which were a semiarid prairie home to many different types of endemic mammals, including entelodonts like Archaeotherium, running rhinoceratoids, three-toed equids, nimravids and early canids like Hesperocyon. Merycoidodonts, an endemic American group, were diverse during this time. In Asia during the Oligocene, a group of running rhinoceratoids gave rise to the indricotheres, like Paraceratherium, which were the largest land mammals to walk the Earth; the marine animals of Oligocene oceans resembled today's fauna, such as the bivalves. Calcareous cirratulids appeared in the Oligocene; the fossil record of marine mammals is a little spotty during this time, not as well known as the Eocene o