Extinction

• 1. Introduction

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  Over five billion species are estimated to have died out across the history of life. Today, around 8.7 million species of eukaryotes are thought to share the planet, possibly many times more once prokaryotes are counted. Extinction is the termination of a species through the death of its last member. That sounds simple, but the moment is almost impossible to catch. A species' range can be vast, so the end is usually declared only in hindsight. Sometimes a species declared gone abruptly reappears. A small dagger symbol, placed beside a name, is the quiet mark that a lineage has stopped. For much of recorded history, people refused to believe extinction could happen at all. Why was the idea so hard to accept, what actually drives a species into oblivion, and what does it mean that humans now hold the brush that paints those daggers? This is the story of how the world learned that things can vanish for good.

• 2. Declaring The Last One

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  A species is extinct when its last existing member dies, with no surviving individuals able to reproduce and create a new generation. Yet a species can be functionally extinct long before that final death. Only a handful may survive, unable to breed because of poor health, age, sparse distribution, or a lack of both sexes. Pinpointing the end requires a clear definition of the species itself. To declare it gone, the organism must be uniquely distinguishable from any ancestor, daughter species, or close relative. This precision matters to the punctuated equilibrium hypothesis of Stephen Jay Gould and Niles Eldredge, where the replacement of one species by another plays a key role. In ecology, the word is sometimes used loosely for local extinction, where a species vanishes from one study area but persists elsewhere. Wolf reintroduction shows how such local losses can be reversed by bringing in individuals from other locations. Species still living somewhere are termed extant. Those extant but at risk are called threatened or endangered. The International Union for Conservation of Nature maintains a sharper category still, extinct in the wild, for species kept alive only in zoos or other artificial settings, sometimes through carefully planned breeding programs aimed at a future return to the wild.

• 3. The Lazarus Problem

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  The coelacanth was known only from the fossil record and presumed gone since the end of the Cretaceous Period. In 1938, a living specimen turned up off the Chalumna River, now Tyolomnqa, on the east coast of South Africa. A fish related to lungfish and tetrapods had stepped out of deep time. Such a species, thought extinct then rediscovered, is called a Lazarus taxon. The term also covers cases where huge gaps in the fossil record make a creature seem to reappear much later. Calliostoma bullatum, a deepwater sea snail first described from fossils in 1844, became a Lazarus species when living individuals were described in 2019. Attenborough's long-beaked echidna, Zaglossus attenboroughi, from Papua New Guinea, had last been sighted in 1962 and was thought possibly gone, until it was recorded again in November 2023. Other creatures hover between hope and loss. The thylacine, or Tasmanian tiger, last died in Hobart Zoo in Tasmania in 1936. The Japanese wolf was last seen over 100 years ago. The American ivory-billed woodpecker had its last universally accepted sighting in 1944, and the slender-billed curlew has not been seen since 2007.

• 4. Vanishing By Transformation

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  Hyracotherium, an early horse, raises a stranger kind of disappearance. It is sometimes called pseudoextinct rather than extinct, because daughter lineages live on. Pseudoextinction, also called phyletic extinction, describes a parent species vanishing while daughter species or subspecies remain. The old taxon does not die so much as become something else, transformed through anagenesis into a successor or split through cladogenesis into more than one. Several extant species of Equus exist, including the zebra and the donkey, sharing a common ancestor with Hyracotherium. The trouble is proof. Fossil species typically leave no genetic material, so no one can say whether Hyracotherium evolved into modern horses or merely descended from a shared ancestor alongside them. Demonstrating pseudoextinction demands a strong chain of evidence linking a living species to a pre-existing one. It grows easier the larger the taxonomic group in question, where the branching pattern is harder to mistake for a dead end.

• 5. How A Lineage Ends

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  "Just as each species is unique," write Beverly and Stephen C. Stearns, "so is each extinction... the causes for each are varied, some subtle and complex, others obvious and simple." Over 99.9% of all species that ever lived are now extinct, with an average species lifespan of 1 to 10 million years. Extinction can strike suddenly, as when toxic pollution renders an entire habitat unliveable, or unfold gradually as a species loses a contest for food to better adapted competitors. It can even arrive long after its trigger, a delay known as extinction debt. Smaller populations carry their own doom. They gain fewer beneficial mutations each generation, slowing adaptation, while slightly harmful mutations fix more easily, feeding a downward spiral called mutational meltdown. Limited geographic range is the most important determinant of genus extinction at background rates, though it fades in importance once a mass extinction takes hold. Genetic pollution offers a quieter threat, where uncontrolled hybridization and genetic swamping homogenize a rare lineage. The endangered wild water buffalo is most threatened by genetic pollution from the abundant domestic water buffalo, its pure gene pool diluted away. Habitat degradation is currently the main anthropogenic cause of species extinctions, with agriculture the leading culprit, followed by urban sprawl, logging, mining, and some fishing practices. The clearing of tropical rainforest for open pasture removes the very infrastructure a fern in dense shade needs to survive.

• 6. Tangled Fates

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  The Haast's eagle hunted the moa, several species of flightless birds, and when the moa vanished, so did the eagle that fed on them. This is coextinction, the loss of a species caused by the extinction of another. It strikes parasitic insects that lose their hosts, pollinators that lose their flowers, and predators that lose their prey. "Species coextinction is a manifestation of one of the interconnectednesses of organisms in complex ecosystems," the source notes, calling it an insidious cause of loss even if not the most important one. Models suggest coextinction is the most common form of biodiversity loss, cascading across trophic levels and hitting mutualistic and parasitic relationships hardest. Climate change writes its own chapter, confirmed in fossil studies such as the extinction of amphibians during the Carboniferous Rainforest Collapse, 305 million years ago. A 2003 review across 14 biodiversity research centers predicted that 15 to 37% of land species could be committed to extinction by 2050, with the Cape Floristic Region and the Caribbean Basin among the hardest hit, threatening 56,000 plant and 3,700 animal species. Sexual selection adds a surprising risk. Fossil studies show that highly sexually dimorphic species, especially those where males carry costly ornaments to compete for mates, die out within mere thousands of years, while the least dimorphic survive for millions.

• 7. The Five Great Dyings

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  Roughly 250 million years ago, the Permian-Triassic extinction event is estimated to have killed 90% of species then existing, likely driven by a massive eruptive event that filled the atmosphere with tephra particles. Evidence suggests it was preceded by another, known as Olson's Extinction. There have been at least five mass extinctions in the history of life on earth, and four in the last 350 million years. The Cretaceous-Paleogene extinction event struck 66 million years ago at the end of the Cretaceous period, best known for wiping out the non-avian dinosaurs. For a long time, science could not even agree that such cataclysms were real. Charles Darwin's On the Origin of Species treated extinction as a steady side effect of competition, with less fit lineages disappearing gradually and evenly, a process now called background extinction. That view held until 1982, when David Raup and Jack Sepkoski published their seminal paper on mass extinctions. Only then was Georges Cuvier vindicated and catastrophic extinction accepted as an important mechanism. The current understanding is a synthesis, joining Cuvier's cataclysms with the background extinction proposed by Lyell and Darwin.

• 8. Learning To Believe It

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  Thomas Jefferson denied the extinction of the woolly mammoth, insisting that nature never allows a race of animals to become extinct. He was a firm supporter of the great chain of being, a theological concept that peaked in popularity in the 1700s, linking all life from the tiniest microorganism to God in a continuous chain. Under that model extinction was impossible, since a missing link would shatter the natural order. The fossils kept coming. In October 1686, Robert Hooke presented to the Royal Society an impression of a nautilus more than two feet in diameter, unlike any living species, and guessed it merely lived undiscovered in the deep ocean. In 1695, Sir Thomas Molyneux described enormous antlers from Ireland and argued they came from a North American moose still hiding somewhere, though they were later confirmed to belong to the extinct deer Megaloceros. Georges Cuvier broke the deadlock, credited with establishing the modern conception of catastrophic extinction in an 1796 lecture to the French Institute. A paleontologist famed for rebuilding a whole animal from a few fragments of bone, he used mammoth skulls found near Paris as his primary evidence. He named the extinct mammal genus Palaeotherium in 1804, and in 1812, with Alexandre Brongniart and Geoffroy Saint-Hilaire, mapped the strata of the Paris basin to infer historic cycles of flooding, extinction, and repopulation. Today humanity is no longer just the doubter but the driver. Human-driven extinction began as people migrated out of Africa more than 60,000 years ago, and the IUCN has recorded 784 extinctions since the year 1500. The smallpox virus is now extinct in the wild, deliberately destroyed yet kept in laboratory samples, the rare case where a species ends not by accident but by human design.