Australopithecus ( OS-trə-lo-PITH-i-kəs. From paleontological and archaeological evidence, the genus Australopithecus evolved in eastern Africa around 4 million years ago before spreading throughout the continent and becoming extinct two million years ago. Australopithecus is not extinct as the Kenyanthropus and Homo genera emerged as sister of a late Australopithecus species such as A. Africanus and/or A. Sediba. During that time, a number of australopithecine species emerged, including Australopithecus afarensis, A. africanus, A. anamensis, A. bahrelghazali, A. deyiremeda, A. garhi, A. sediba. For some hominid species of this time – A. robustus, A. boisei and A. aethiopicus – some debate exists whether they constitute members of the genus Australopithecus. If so, they would be considered'robust australopiths', while the others would be'gracile australopiths'. However, if these more robust species do constitute their own genus, they would be under the genus name Paranthropus, a genus described by Robert Broom when the first discovery was made in 1938, which makes these species P. robustus, P. boisei and P. aethiopicus.
Australopithecus species played a significant part in human evolution, the genus Homo being derived from Australopithecus at some time after three million years ago. In addition, they were the first hominids to possess certain genes, known as the duplicated SRGAP2, which increased the length and ability of neurons in the brain. One of the australopith species evolved into the genus Homo in Africa around two million years ago, modern humans, H. sapiens sapiens. In January 2019, scientists reported that Australopithecus sediba is distinct from, but shares anatomical similarities to, both the older Australopithecus africanus, the younger Homo habilis. Gracile australopiths shared several traits with modern apes and humans, were widespread throughout Eastern and Northern Africa around 3.5 million years ago. The earliest evidence of fundamentally bipedal hominids can be observed at the site of Laetoli in Tanzania; this site contains hominid footprints that are remarkably similar to those of modern humans and have been dated to as old as 3.6 million years.
The footprints have been classified as australopith, as they are the only form of prehuman hominins known to have existed in that region at that time. Australopithecus anamensis, A. afarensis, A. africanus are among the most famous of the extinct hominins. A. africanus was once considered to be ancestral to the genus Homo. However, fossils assigned to the genus Homo have been found. Thus, the genus Homo either split off from the genus Australopithecus at an earlier date, or both developed from a yet unknown common ancestor independently. According to the Chimpanzee Genome Project, the human and chimpanzee lineages diverged from a common ancestor about five to six million years ago, assuming a constant rate of evolution, it is theoretically more for evolution to happen more as opposed to more from the date suggested by a gene clock However, hominins discovered more are somewhat older than the presumed rate of evolution would suggest. Sahelanthropus tchadensis called "Toumai", is about seven million years old and Orrorin tugenensis lived at least six million years ago.
Since little is known of them, they remain controversial among scientists since the molecular clock in humans has determined that humans and chimpanzees had a genetic split at least a million years later. One theory suggests that the human and chimpanzee lineages diverged somewhat at first some populations interbred around one million years after diverging; the brains of most species of Australopithecus were 35% of the size of a modern human brain. Most species of Australopithecus were diminutive and gracile standing 1.2 to 1.4 m tall. In several variations is a considerable degree of sexual dimorphism, males being larger than females. According to one scholar, A. Zihlman, Australopithecus body proportions resemble those of bonobos, leading evolutionary biologists such as Jeremy Griffith to suggest that bonobos may be phenotypically similar to Australopithecus. Furthermore, thermoregulatory models suggest that Australopithecus species were hair covered, more like chimpanzees and bonobos, unlike humans.
Modern humans do not display the same degree of sexual dimorphism as Australopithecus appears to have. In modern populations, males are on average a mere 15% larger than females, while in Australopithecus, males could be up to 50% larger than females. New research suggests, that australopithecines exhibited a lesser degree of sexual dimorphism than these figures suggest, but the issue is not settled. Opinions differ as to whether the species A. aethiopicus, A. boisei, A. robustus should be included within the genus Australopithecus, no current consensus exists as to whether they should be placed in a distinct genus, Paranthropus
Bipedalism is a form of terrestrial locomotion where an organism moves by means of its two rear limbs or legs. An animal or machine that moves in a bipedal manner is known as a biped, meaning "two feet". Types of bipedal movement include running, or hopping. Few modern species are habitual bipeds. Within mammals, habitual bipedalism has evolved multiple times, with the macropods, kangaroo rats and mice, hopping mice and hominin apes as well as various other extinct groups evolving the trait independently. In the Triassic period some groups of archosaurs developed bipedalism. A larger number of modern species intermittently or use a bipedal gait. Several lizard species move bipedally when running to escape from threats. Many primate and bear species will adopt a bipedal gait in order to reach food or explore their environment, though there are a few cases where they walk on their hindlimbs only. Several arboreal primate species, such as gibbons and indriids walk on two legs during the brief periods they spend on the ground.
Many animals rear up on their hind legs whilst copulating. Some animals stand on their hind legs, in order to reach food, to keep watch, to threaten a competitor or predator, or to pose in courtship, but do not move bipedally; the word is derived from the Latin words bi'two' and ped-'foot', as contrasted with quadruped'four feet'. Limited and exclusive bipedalism can offer a species several advantages. Bipedalism raises the head. While upright, non-locomotory limbs become free for other uses, including manipulation, digging, combat or camouflage; the maximum bipedal speed appears less fast than the maximum speed of quadrupedal movement with a flexible backbone – both the ostrich and the red kangaroo can reach speeds of 70 km/h, while the cheetah can exceed 100 km/h. Though bipedalism is slower at first, over long distances, it has allowed humans to outrun most other animals according to the endurance running hypothesis. Bipedality in kangaroo rats has been hypothesized to improve locomotor performance, which could aid in escaping from predators.
Zoologists label behaviors, including bipedalism, as "facultative" or "obligate". This distinction is not clear-cut — for example, humans other than infants walk and run in biped fashion, but all can crawl on hands and knees when necessary. There are reports of humans who walk on all fours with their feet but not their knees on the ground, but these cases are a result of conditions such as Uner Tan syndrome — rare genetic neurological disorders rather than normal behavior. If one ignores exceptions caused by some kind of injury or illness, there are many unclear cases, including the fact that "normal" humans can crawl on hands and knees; this article therefore avoids the terms "facultative" and "obligate", focuses on the range of styles of locomotion used by various groups of animals. Normal humans may be considered "obligate" bipeds because the alternatives are uncomfortable and only resorted to when walking is impossible. There are a number of states of movement associated with bipedalism.
Standing. Staying still on both legs. In most bipeds this is an active process. Walking. One foot in front of another, with at least one foot on the ground at any time. Running. One foot with periods where both feet are off the ground. Jumping/hopping. Moving by a series of jumps with both feet moving together; the great majority of living terrestrial vertebrates are quadrupeds, with bipedalism exhibited by only a handful of living groups. Humans and large birds walk by raising one foot at a time. On the other hand, most macropods, smaller birds and bipedal rodents move by hopping on both legs simultaneously. Tree kangaroos are able to walk or hop, most alternating feet when moving arboreally and hopping on both feet when on the ground. There are fossil bipedal amphibians. Many species of lizards become bipedal during high-speed, sprint locomotion, including the world's fastest lizard, the spiny-tailed iguana; the first known biped is the bolosaurid Eudibamus. Its long hindlegs, short forelegs, distinctive joints all suggest bipedalism.
The species became extinct in the early Permian. All birds are bipeds. Bipedalism evolved more than once in archosaurs, the group that includes both dinosaurs and crocodilians. All dinosaurs are thought to be descended from a bipedal ancestor similar to Eoraptor. Bipedal movement re-evolved in a number of other dinosaur lineages such as the iguanodons; some extinct members of the crocodilian line, a sister group to the dinosaurs and birds evolved bipedal forms - a crocodile relative from the triassic, Effigia okeeffeae, is thought to be
Metal Gear (mecha)
Metal Gear is the name for a series of mecha in the Metal Gear series. In the series, a Metal Gear is defined as a bipedal nuclear weapons-equipped tank; the Metal Gears are autonomous nuclear launch platforms which the player must destroy to save the world and complete the game. Confronting the latest Metal Gear model is one of the final challenges of each game. Hideo Kojima said that the idea of the "Metal Gear" weapon, a nuclear-capable walking tank, was conceived in response to the nuclear war hysteria during the 1980s that resulted from the Cold War as the U. S. military had no mobile land-based nuclear weapons delivery system at the time. The Metal Gear weapon has been described as an all-terrain, nuclear-equipped, walking battle tank capable of launching a nuclear warhead from anywhere on the planet. In the original Metal Gear, the Outer Heaven fortified military state attempts to achieve military dominance over the West and East through the use of the TX-55 Metal Gear model built by Eastern scientist Dr. Drago Pettrovich Madnar after being forced against his will.
In Metal Gear 2: Solid Snake, another hostile nation Zanzibar Land uses improved model Metal Gear D to raid nuclear disposal sites. Metal Gear Solid shows new model Metal Gear REX designed by Dr. Hal Emmerich after being misled into believing that Metal Gear would be used as an anti-nuclear device that would be capable of safely shooting down nuclear missiles via a railgun system that shoots nuclear warheads; the weapon was produced by the U. S. Army on Shadow Moses Island, Alaska; the proliferation of Metal Gear-like weapons led to the creation of the Marines' Metal Gear RAY before the events of Metal Gear Solid 2: Sons of Liberty. Metal Gear Solid 3: Snake Eater reveals that Metal Gear's design originates with Soviet weapons designer Dr. Aleksandr Leonovitch Granin comparing the concept to the theory of the missing link between apes and men, evidenced by two design drawings on his desk, with him calling it the missing link between infantry and artillery. Infantry can go anywhere but are destroyed and carry limited firepower.
Artillery are at the mercy of terrain. In short, for a tank to be perfect, it needs legs. Early Metal Gear models are seen thanks to Dr. Granin's colleagues in direct sequels. TX-55 Metal Gear is the Metal Gear model's designation in the original Metal Gear game, although it is referred as "Metal Gear" and no particular meaning is given behind this codename. Within Outer Heaven, Solid Snake learns Metal Gear's weakness after rescuing creator Dr. Drago Pettrovich Madnar and his daughter Ellen Madnar; the player must place a certain number of plastic explosives on Metal Gear's feet in the order stated by Dr. Madnar. Metal Gear is stored in a room guarded by two laser cams and never becomes operational. Metal Gear D in Metal Gear 2: Solid Snake is a revision of the original Metal Gear prototype. Metal Gear D is equipped with a six-missile pod; the improved model was built by Dr. Madnar, willingly this time after his radical ideas were rejected by the Western scientific community; the Metal Gear is piloted by Gray Fox and can only be destroyed by Solid Snake throwing grenades over its feet.
This Metal Gear's mecha design was chosen through an internal contest between Konami's artists, with the primary criterion being realism. The final design is credited to Tomohiro Nishio. In the non-canonical sequel Snake's Revenge, mass-produced versions of the original are stored inside an enemy cargo ship; the wreckage of the original makes a cameo in Metal Gear: Ghost Babel, an alternate sequel to the first game in the Metal Gear Solid series set in a rebuilt Outer Heaven. Both Metal Gear TX-55 and Metal Gear D make cameo appearances during the Metal Gear Solid games: D is visible as Dr. Granin's blueprint shown to Naked Snake in Metal Gear Solid 3: Snake Eater, TX-55 is seen in a psychic vision along with Big Boss and his three "Les Enfants Terribles" sons in Metal Gear Solid: Portable Ops. Metal Gear REX is an different Metal Gear model that appears in Metal Gear Solid designed by Yoji Shinkawa. Unlike previous Metal Gears, its legs are armored and reinforced. Metal Gear REX has near-impenetrable compound armor, a pair of 30 mm gatling guns, anti-tank missiles, a free-electron laser, to protect itself from conventional forces.
Its primary weapon, however, is a magnetic railgun capable of delivering an untraceable nuclear warhead anywhere in the world, without the propellant trail or launch flare that gives away the launch position of a traditional ballistic missile. It possessed a radome, a satellite-shaped object on REX's left side that provides information about the environment outside of the cockpit while keeping the pilot safe from harm, increasing the accuracy of all weapons as well. Destroying the radome is crucial to rendering the virtual reality interface useless and forcing the pilot to manually control REX; the pilot is forced to expose himself to enemy fire, by opening the cockpit in order to see outside. The story of Metal Gear Solid shows weapons company ArmsTech president Kenneth Baker bribes DARPA
The Hominidae, whose members are known as great apes or hominids, are a taxonomic family of primates that includes eight extant species in four genera: Pongo, the Bornean and Tapanuli orangutan. Several revisions in classifying the great apes have caused the use of the term "hominid" to vary over time, its original meaning referred only to their closest extinct relatives. That restrictive meaning has now been assumed by the term "hominin", which comprises all members of the human clade after the split from the chimpanzees; the current, 21st-century meaning of "hominid" includes all the great apes including humans. Usage still varies and some scientists and laypersons still use "hominid" in the original restrictive sense. Within the taxon Hominidae, a number of extant and known extinct, that is, genera are grouped with the humans and gorillas in the subfamily Homininae; the most recent common ancestor of all Hominidae lived 14 million years ago, when the ancestors of the orangutans speciated from the ancestral line of the other three genera.
Those ancestors of the family Hominidae had speciated from the family Hylobatidae 15 million to 20 million years ago. In the early Miocene, about 22 million years ago, there were many species of arboreally adapted primitive catarrhines from East Africa. Fossils at 20 million years ago include fragments attributed to Victoriapithecus, the earliest Old World monkey. Among the genera thought to be in the ape lineage leading up to 13 million years ago are Proconsul, Dendropithecus, Nacholapithecus, Nyanzapithecus, Afropithecus and Kenyapithecus, all from East Africa. At sites far distant from East Africa, the presence of other generalized non-cercopithecids, that is, non-monkey primates, of middle Miocene age—Otavipithecus from cave deposits in Namibia, Pierolapithecus and Dryopithecus from France and Austria—is further evidence of a wide diversity of ancestral ape forms across Africa and the Mediterranean basin during the warm and equable climatic regimes of the early and middle Miocene; the most recent of these far-flung Miocene apes is Oreopithecus, from the fossil-rich coal beds in northern Italy and dated to 9 million years ago.
Molecular evidence indicates that the lineage of gibbons, the lesser apes, diverged from that of the great apes some 18–12 million years ago, that of orangutans diverged from the other great apes at about 12 million years. There are no fossils that document the ancestry of gibbons, which may have originated in a still-unknown South East Asian hominoid population. Species close to the last common ancestor of gorillas and humans may be represented by Nakalipithecus fossils found in Kenya and Ouranopithecus found in Greece. Molecular evidence suggests that between 8 and 4 million years ago, first the gorillas, the chimpanzees split off from the line leading to the humans. Human DNA is 98.4% identical to that of chimpanzees when comparing single nucleotide polymorphisms. The fossil record, however, of gorillas and chimpanzees is limited. Other hominins adapted to the drier environments outside the African equatorial belt; the wet equatorial belt contracted after about 8 million years ago, there is little fossil evidence for the divergence of the hominin lineage from that of gorillas and chimpanzees—which split was thought to have occurred around that time.
The earliest fossils argued by some to belong to the human lineage are Sahelanthropus tchadensis and Orrorin tugenensis, followed by Ardipithecus, with species Ar. kadabba and Ar. ramidus. The classification of the great apes has been revised several times in the last few decades; the original meaning of the term referred to only humans and their closest relatives—what is now the modern meaning of the term "hominin". The meaning of the taxon Hominidae changed leading to a different usage of "hominid" that today includes all the great apes including humans; the term hominid is confused with a number of similar words: A hominoid called an ape, is a member of the superfamily Hominoidea: extant members are the gibbons and the hominids. A hominid is a member of the family Hominidae, the great apes: orangutans, gorillas and humans. A hominine is a member of the subfamily Homininae: gorillas and humans. A hominin is a member of the tribe Hominini: humans. A homininan, following a suggestion by Wood and Richmond, would be a member of the subtribe Homin
The Carboniferous is a geologic period and system that spans 60 million years from the end of the Devonian Period 358.9 million years ago, to the beginning of the Permian Period, 298.9 Mya. The name Carboniferous means "coal-bearing" and derives from the Latin words carbō and ferō, was coined by geologists William Conybeare and William Phillips in 1822. Based on a study of the British rock succession, it was the first of the modern'system' names to be employed, reflects the fact that many coal beds were formed globally during that time; the Carboniferous is treated in North America as two geological periods, the earlier Mississippian and the Pennsylvanian. Terrestrial animal life was well established by the Carboniferous period. Amphibians were the dominant land vertebrates, of which one branch would evolve into amniotes, the first terrestrial vertebrates. Arthropods were very common, many were much larger than those of today. Vast swaths of forest covered the land, which would be laid down and become the coal beds characteristic of the Carboniferous stratigraphy evident today.
The atmospheric content of oxygen reached its highest levels in geological history during the period, 35% compared with 21% today, allowing terrestrial invertebrates to evolve to great size. The half of the period experienced glaciations, low sea level, mountain building as the continents collided to form Pangaea. A minor marine and terrestrial extinction event, the Carboniferous rainforest collapse, occurred at the end of the period, caused by climate change. In the United States the Carboniferous is broken into Mississippian and Pennsylvanian subperiods; the Mississippian is about twice as long as the Pennsylvanian, but due to the large thickness of coal-bearing deposits with Pennsylvanian ages in Europe and North America, the two subperiods were long thought to have been more or less equal in duration. In Europe the Lower Carboniferous sub-system is known as the Dinantian, comprising the Tournaisian and Visean Series, dated at 362.5-332.9 Ma, the Upper Carboniferous sub-system is known as the Silesian, comprising the Namurian and Stephanian Series, dated at 332.9-298.9 Ma.
The Silesian is contemporaneous with the late Mississippian Serpukhovian plus the Pennsylvanian. In Britain the Dinantian is traditionally known as the Carboniferous Limestone, the Namurian as the Millstone Grit, the Westphalian as the Coal Measures and Pennant Sandstone; the International Commission on Stratigraphy faunal stages from youngest to oldest, together with some of their regional subdivisions, are: A global drop in sea level at the end of the Devonian reversed early in the Carboniferous. There was a drop in south polar temperatures; these conditions had little effect in the deep tropics, where lush swamps to become coal, flourished to within 30 degrees of the northernmost glaciers. Mid-Carboniferous, a drop in sea level precipitated a major marine extinction, one that hit crinoids and ammonites hard; this sea level drop and the associated unconformity in North America separate the Mississippian subperiod from the Pennsylvanian subperiod. This happened about 323 million years ago, at the onset of the Permo-Carboniferous Glaciation.
The Carboniferous was a time of active mountain-building as the supercontinent Pangaea came together. The southern continents remained tied together in the supercontinent Gondwana, which collided with North America–Europe along the present line of eastern North America; this continental collision resulted in the Hercynian orogeny in Europe, the Alleghenian orogeny in North America. In the same time frame, much of present eastern Eurasian plate welded itself to Europe along the line of the Ural Mountains. Most of the Mesozoic supercontinent of Pangea was now assembled, although North China, South China continents were still separated from Laurasia; the Late Carboniferous Pangaea was shaped like an "O." There were two major oceans in the Carboniferous—Panthalassa and Paleo-Tethys, inside the "O" in the Carboniferous Pangaea. Other minor oceans were shrinking and closed - Rheic Ocean, the small, shallow Ural Ocean and Proto-Tethys Ocean. Average global temperatures in the Early Carboniferous Period were high: 20 °C.
However, cooling during the Middle Carboniferous reduced average global temperatures to about 12 °C. Lack of growth rings of fossilized trees suggest a lack of seasons of a tropical climate. Glaciations in Gondwana, triggered by Gondwana's southward movement, continued into the Permian and because of the lack of clear markers and breaks, the deposits of this glacial period are referred to as Permo-Carboniferous in age; the cooling and drying of the climate led to the Carboniferous Rainforest Collapse during the late Carboniferous. Tropical rainforests fragmented and were devastated by climate change. Carboniferous rocks in Europe and eastern North America consist of a repeated sequence of limestone, sandstone and coal beds. In North America, the early Carboniferous is marine
The Hominini, or hominins, form a taxonomic tribe of the subfamily Homininae. Hominini excludes the genus Gorilla; as of 2019, there is no consensus on whether it should include the genus Pan, the question being tied to the complex speciation process connecting humans and chimpanzees and the development of bipedalism in proto-humans. The tribe was introduced by John Edward Gray, long before any details on the speciation of Pan and Homo were known. Gray's tribe Hominini by definition includes both Homo; this definition is still adhered to in the proposal by Mann and Weiss, which divides Hominini into three subtribes, Panina and Australopithecina. Alternatively, Hominini is taken to exclude Pan. In this case, Panini may be used to refer to the tribe containing Pan as its only genus. Minority dissenting nomenclatures include Gorilla in Hominini and Pan in Homo, or both Pan and Gorilla in Homo. By convention, the adjectival term "hominin" refers to the tribe Hominini, while the members of the Hominina subtribe are referred to as "homininan".
This follows the proposal by Mann and Weiss, which presents tribe Hominini as including both Pan and Homo, placed in separate subtribes. The genus Pan is referred to subtribe Panina, genus Homo is included in the subtribe Hominina. However, there is an alternative convention which uses "hominin" to exclude members of Panina, i.e. either just for Homo or for both human and australopithecine species. This alternative convention is referenced in Dunbar. Potts in addition uses the name Hominini in a different sense, as excluding Pan, uses "hominins" for this, while a separate tribe for chimpanzees is introduced, under the name Panini. In this recent convention, contra Gray, the term "hominin" is applied to Homo, Australopithecus and others that arose after the split from the line that led to chimpanzees; this cladogram shows the clade of superfamily Hominoidea and its descendent clades, focussed on the division of Hominini. The family Hominidae comprises the tribes Ponginae and Hominini, the latter two forming the subfamily of Homininae.
Hominini is divided into Australopithecina. The Hominina are held to have emerged within the Australopithecina. Genetic analysis combined with fossil evidence indicates that hominoids diverged from the Old World monkeys about 25 million years ago, near the Oligocene-Miocene boundary; the most recent common ancestors of the subfamilies Homininae and Ponginae, lived about 15 million years ago. In the following cladogram, the approximate time the clades radiated newer clades indicated in millions of years ago. Both Sahelanthropus and Orrorin existed during the estimated duration of the ancestral chimpanzee-human speciation events, within the range of eight to four million years ago. Few fossil specimens have been found that can be considered directly ancestral to genus Pan. News of the first fossil chimpanzee, found in Kenya, was published in 2005. However, it is dated to recent times—between 545 and 284 thousand years ago; the divergence of a "proto-human" or "pre-human" lineage separate from Pan appears to have been a process of complex speciation-hybridization rather than a clean split, taking place over the period of anywhere between 13 million years ago and some 4 million years ago.
Different chromosomes appear to have split at different times, with broad-scale hybridization activity occurring between the two emerging lineages as late as the period 6.3 to 5.4 Mya, according to Patterson et al. This research group noted that one hypothetical late hybridization period was based in particular on the similarity of X chromosomes in the proto-humans and stem chimpanzees, suggesting the final divergence as recent as 4 Mya. Wakeley rejected these hypotheses. Most DNA studies find that humans and Pan are 99% identical, but one study found only 94% commonality, with some of the difference occurring in noncoding DNA, it is most that the australopithecines, dating from 3 to 4.4 Mya, evolved into the earliest members of genus Homo. In the year 2000, the discovery of Orrorin tugenensis, dated as early as 6.2 Mya challenged critical elements of that hypothesis, as it suggested that Homo did not in fact derive from australopithecine ancestors. All the listed fossil genera are evaluated for: 1) probability of being ancestral to Homo, 2) whether they are more related to Homo than to any other living primate—two traits that could identify them as hominins.
Some, including Paranthropus and Australopithecus, are broadly thought to be ancestral and related to Homo.
In biology, extinction is the termination of an organism or of a group of organisms a species. The moment of extinction is considered to be the death of the last individual of the species, although the capacity to breed and recover may have been lost before this point; because a species' potential range may be large, determining this moment is difficult, is done retrospectively. This difficulty leads to phenomena such as Lazarus taxa, where a species presumed extinct abruptly "reappears" after a period of apparent absence. More than 99 percent of all species, amounting to over five billion species, that lived on Earth are estimated to have died out. Estimates on the number of Earth's current species range from 10 million to 14 million, of which about 1.2 million have been documented and over 86 percent have not yet been described. In 2016, scientists reported that 1 trillion species are estimated to be on Earth with only one-thousandth of one percent described. Through evolution, species arise through the process of speciation—where new varieties of organisms arise and thrive when they are able to find and exploit an ecological niche—and species become extinct when they are no longer able to survive in changing conditions or against superior competition.
The relationship between animals and their ecological niches has been established. A typical species becomes extinct within 10 million years of its first appearance, although some species, called living fossils, survive with no morphological change for hundreds of millions of years. Mass extinctions are rare events. Only have extinctions been recorded and scientists have become alarmed at the current high rate of extinctions. Most species that become extinct are never scientifically documented; some scientists estimate that up to half of presently existing plant and animal species may become extinct by 2100. A 2018 report indicated that the phylogenetic diversity of 300 mammalian species erased during the human era since the Late Pleistocene would require 5 to 7 million years to recover. A dagger symbol placed next to the name of a species or other taxon indicates its status as extinct. A species is extinct. Extinction therefore becomes a certainty when there are no surviving individuals that can reproduce and create a new generation.
A species may become functionally extinct when only a handful of individuals survive, which cannot reproduce due to poor health, sparse distribution over a large range, a lack of individuals of both sexes, or other reasons. Pinpointing the extinction of a species requires a clear definition of that species. If it is to be declared extinct, the species in question must be uniquely distinguishable from any ancestor or daughter species, from any other related species. Extinction of a species plays a key role in the punctuated equilibrium hypothesis of Stephen Jay Gould and Niles Eldredge. In ecology, extinction is used informally to refer to local extinction, in which a species ceases to exist in the chosen area of study, but may still exist elsewhere; this phenomenon is known as extirpation. Local extinctions may be followed by a replacement of the species taken from other locations. Species which are not extinct are termed extant; those that are extant but threatened by extinction are referred to as threatened or endangered species.
An important aspect of extinction is human attempts to preserve critically endangered species. These are reflected by the creation of the conservation status "extinct in the wild". Species listed under this status by the International Union for Conservation of Nature are not known to have any living specimens in the wild, are maintained only in zoos or other artificial environments; some of these species are functionally extinct, as they are no longer part of their natural habitat and it is unlikely the species will be restored to the wild. When possible, modern zoological institutions try to maintain a viable population for species preservation and possible future reintroduction to the wild, through use of planned breeding programs; the extinction of one species' wild population can have knock-on effects, causing further extinctions. These are called "chains of extinction"; this is common with extinction of keystone species. A 2018 study indicated that the 6th mass extinction started in the Late Pleistocene could take up to 5 to 7 million years to restore 2.5 billion years of unique mammal diversity to what it was before the human era.
Extinction of a parent species where daughter species or subspecies are still extant is called pseudoextinction or phyletic extinction. The old taxon vanishes, transformed into a successor, or split into more than one. Pseudoextinction is difficult to demonstrate unless one has a strong chain of evidence linking a living species to members of a pre-existing species. For example, it is sometimes claimed that the extinct Hyracotherium, an early horse that shares a common ancestor with the modern horse, is pseudoextinct, rather than extinct, because there are several extant species of Equus, including zebra and donkey. However, as fossil species leave no genetic material behind, one cannot say whether Hyracotherium evolved into more modern horse species or evolved from a common ancestor with modern horses. Pseudoextinction is much easier to demonstrate for larger taxonomic groups; the coelacanth, a fish related to lungfish and tetrapods, was consi