Extinction: the story on HearLore

Extinction

The thylacine, also known as the Tasmanian tiger, died in the Hobart Zoo on the 7th of September 1936, marking the final moment of its species. This event is not merely a historical footnote but the precise definition of extinction itself: the termination of an organism by the death of its last member. Before that date, the creature roamed the forests of Tasmania, a marsupial predator with a striped back and a pouch, yet by the time the 20th century arrived, human persecution and habitat loss had reduced its numbers to a handful of captive individuals. The death of Benjamin, the last known thylacine, was the moment the species ceased to exist in any biological sense, transitioning from a living population to a memory preserved in museum specimens and folklore. This specific date anchors the abstract concept of extinction into a tangible reality, proving that the end of a species is not a gradual fading but a singular, irreversible event. While the thylacine is a famous example, it is one of thousands of species that have met this fate, with estimates suggesting that over 99.9% of all species that have ever lived on Earth are now extinct. The difficulty in pinpointing the exact moment of extinction often leads to retrospective declarations, where a species is declared extinct only after years of searching have yielded no results, creating the phenomenon known as Lazarus taxa. These are species thought to be lost to history that suddenly reappear, such as the coelacanth, a fish thought to have vanished 66 million years ago until a living specimen was caught off the coast of South Africa in 1938. The thylacine, however, remains a definitive case of finality, serving as a stark reminder that extinction is a permanent state once the last individual passes away.

Catastrophe And The Great Chain

For centuries, the idea that a species could simply vanish was considered impossible by the prevailing worldview of Western society. Before the 19th century, the theological concept of the great chain of being held sway, suggesting that all life was linked in a continuous, perfect order from the smallest microorganism to God. Under this model, extinction was a logical impossibility because it would create gaps or missing links in the divine chain. Thomas Jefferson, a firm supporter of this view, famously denied the extinction of the woolly mammoth, arguing that nature never allows a race of animals to become extinct. When fossils of extinct creatures began to be discovered in the late 17th century, scientists like Robert Hooke and Sir Thomas Molyneux struggled to explain them without accepting the concept of total loss. Hooke theorized that the nautilus fossils he examined simply lived in the deep ocean and had not yet been discovered, while Molyneux argued that the massive antlers found in Ireland belonged to moose that had once roamed the British Isles but were now hiding in unknown regions. It was not until 1796 that Georges Cuvier, a French paleontologist, established the modern conception of catastrophic extinction. Cuvier presented evidence from mammoth skulls found near Paris, demonstrating that these animals were distinct from any living species and could not be found anywhere on Earth. He argued that the history of life was punctuated by periodic catastrophic events, such as massive flooding, that wiped out huge numbers of species in a short period. This view stood in direct opposition to the gradualist theories of Jean-Baptiste Lamarck and Charles Lyell, who believed that species changed slowly over time and that extinction was a rare exception rather than a rule. The debate between these schools of thought defined the scientific understanding of extinction for nearly two centuries, until the publication of a seminal paper by David Raup and Jack Sepkoski in 1982 vindicated Cuvier's catastrophic model. Today, the understanding of extinction is a synthesis of these historical perspectives, acknowledging both the background extinction that occurs naturally over time and the mass extinction events that have reshaped the planet's biodiversity.

When did the thylacine die and what does that date signify?

The thylacine died on the 7th of September 1936 in the Hobart Zoo, marking the precise moment the species ceased to exist. This event serves as the definitive definition of extinction as the termination of an organism by the death of its last member.

Who established the modern scientific concept of extinction and when?

Georges Cuvier established the modern conception of extinction in 1796 by presenting evidence from mammoth skulls found near Paris. He argued that the history of life was punctuated by periodic catastrophic events that wiped out huge numbers of species in a short period.

What percentage of all species that have ever lived on Earth are now extinct?

Estimates suggest that over 99.9% of all species that have ever lived on Earth are now extinct. This figure includes the thylacine and thousands of other species that have met this fate throughout history.

How many years will it take to recover the phylogenetic diversity of 300 mammalian species erased during the human era?

A 2018 study 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. This rapid loss highlights the severity of the current Holocene extinction event.

Which animal was the only one to be brought back from extinction and then go extinct twice?

The Pyrenean ibex is the only animal to have been brought back from extinction and the only one to go extinct twice. A clone born in 2009 died seven minutes later due to physical defects in the lungs.

When was the golden toad last seen and what caused its disappearance?

The golden toad was last seen on the 15th of May 1989 before vanishing from the wild. Its decline is part of an ongoing worldwide reduction in amphibian populations driven by habitat destruction and environmental changes.

The Sixth Mass Extinction

The Earth is currently in the early stages of a human-caused mass extinction event, often referred to as the Holocene extinction, which is proceeding at a rate 100 to 1,000 times higher than the background rates of evolutionary history. A 1998 survey of 400 biologists conducted by the American Museum of Natural History revealed that nearly 70% of respondents believed the Earth was in the midst of the fastest mass extinction in its history, with predictions that up to 20% of all living populations could become extinct within 30 years. The 2019 Global Assessment Report on Biodiversity and Ecosystem Services by IPBES provided grim statistics, indicating that the biomass of wild mammals has fallen by 82%, natural ecosystems have lost about half their area, and one million species are at risk of extinction. This crisis is largely the result of human actions, including the destruction of natural habitats through deforestation, the conversion of land into fields for farming, and the introduction of invasive species. The passenger pigeon, once numbering in the billions, was hunted to extinction over the course of a few decades, serving as a tragic example of how quickly a species can be wiped out by human overharvesting. The current rate of extinction is so rapid that a 2018 study indicated 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. Biologist E. O. Wilson estimated in 2002 that if current rates of human destruction of the biosphere continue, one-half of all plant and animal species of life on Earth will be extinct in 100 years. The drivers of this crisis are multifaceted, ranging from unsustainable fishing and hunting to the pollution of waterways and the alteration of climate patterns. The Holocene extinction is not merely a loss of individual species but a fundamental restructuring of the biosphere, with knock-on effects causing further extinctions through chains of coextinction. When a keystone species goes extinct, the ripple effects can devastate entire ecosystems, leading to the loss of predators, prey, and pollinators that depend on one another for survival.

Genetic Meltdown And Coextinction

The mechanisms driving extinction are often subtle and complex, involving genetic factors that can lead to a population's demise even before environmental pressures become overwhelming. Smaller populations have fewer beneficial mutations entering the population each generation, slowing adaptation and making it easier for slightly deleterious mutations to fix, creating a positive feedback loop known as mutational meltdown. This genetic erosion is exacerbated by population bottlenecks, which dramatically reduce genetic diversity by severely limiting the number of reproducing individuals and making inbreeding more frequent. A diverse or deep gene pool gives a population a higher chance of surviving an adverse change in conditions, but the loss of this diversity increases the chances of extinction. Genetic pollution, or uncontrolled hybridization, also threatens rare species when they come into contact with more abundant ones, swamping the rarer gene pool and creating hybrids that deplete the purebred gene pool. The endangered wild water buffalo, for instance, is most threatened with extinction by genetic pollution from the abundant domestic water buffalo. Coextinction is another insidious form of biodiversity loss, referring to the loss of a species due to the extinction of another. This is especially common when a keystone species goes extinct, triggering a cascade of coextinction across trophic levels. The Haast's eagle and the moa from New Zealand provide a stark example of this phenomenon; the Haast's eagle was a predator that became extinct because its food source, the moa, became extinct. Coextinction can also occur when a species loses its pollinator or when predators lose their prey, leading to a chain reaction that can destabilize entire ecosystems. The interconnectedness of organisms in complex ecosystems means that the loss of one species can have far-reaching consequences, often leading to the extinction of species that were previously stable. This phenomenon is particularly severe in mutualistic and parasitic relationships, where the survival of one species is inextricably linked to the survival of another. The loss of genetic diversity and the cascading effects of coextinction create a precarious situation for many species, making them vulnerable to even minor environmental changes.

The Human Predator And The Wild

Humans have become an additional cause of extinction of some species, acting as a new mega-predator by transporting animals and plants from one part of the world to another. The overkill hypothesis suggests that the swift extinction of the megafauna in areas such as Australia, North and South America, Madagascar, Hawaii, and New Zealand resulted from the sudden introduction of human beings to environments full of animals that had never seen them before and were therefore completely unadapted to their predation techniques. In most cases, the introductions are unsuccessful, but when an invasive alien species does become established, the consequences can be catastrophic. Invasive alien species can affect native species directly by eating them, competing with them, and introducing pathogens or parasites that sicken or kill them. Human populations may themselves act as invasive predators, driving species to extinction through overhunting, habitat destruction, and the introduction of toxic pollutants. The American bison, once numbering in the millions, was nearly wiped out by mass hunts sanctioned by the United States government, to force the removal of Native Americans, many of whom relied on the bison for food. The golden toad, last seen on the 15th of May 1989, is another example of a species that has vanished, with the decline in amphibian populations ongoing worldwide. The destruction of tropical rainforests and the replacement with open pastureland has eliminated the infrastructure needed by many species to survive, while the destruction of ocean floors by bottom trawling has devastated marine ecosystems. The loss of native species is not just an ecological tragedy but also an economic one, as governments sometimes see the loss of native species as a loss to ecotourism. However, people who live close to nature can be dependent on the survival of all the species in their environment, leaving them highly exposed to extinction risks. The prioritization of day-to-day survival over species conservation, combined with human overpopulation in tropical developing countries, has placed enormous pressure on forests due to subsistence agriculture, including slash-and-burn agricultural techniques that can reduce endangered species's habitats.

Cloning And The Ethics Of Loss

The possibility of bringing extinct species back to life through cloning has emerged as a controversial and theoretically possible solution to the extinction crisis. Harvard geneticist George M. Church believes that ongoing technological advances will let us bring back to life an extinct species by cloning, using DNA from the remains of that species. Proposed targets for cloning include the mammoth, the thylacine, and the Pyrenean ibex. For this to succeed, enough individuals would have to be cloned, from the DNA of different individuals, to create a viable population. In 2003, scientists tried to clone the extinct Pyrenean ibex, but the attempt failed; of the 285 embryos reconstructed, 54 were transferred to 12 Spanish ibexes and ibex-domestic goat hybrids, but only two survived the initial two months of gestation before they, too, died. In 2009, a second attempt was made to clone the Pyrenean ibex; one clone was born alive, but died seven minutes later, due to physical defects in the lungs. This makes the Pyrenean ibex the only animal to have been brought back from extinction and the only one to go extinct twice. Despite the technical challenges, the cloning of extinct creatures seems theoretically possible, though it raises significant bioethical and philosophical objections. The debate extends beyond the technical feasibility to the moral implications of playing God with the genetic code of extinct species. Some scientists argue that the loss of a species is not necessarily a disaster, as removing one species sometimes causes shifts in the populations of other species, but different need not mean worse. Biologist Olivia Judson has advocated the deliberate extinction of certain disease-carrying mosquito species, arguing that eradicating these would save at least one million human lives per year, at a cost of reducing the genetic diversity of the family Culicidae by only 1%. The concept of planned extinction, whether for disease vectors or invasive species, challenges the traditional view that all extinction is a tragedy. The smallpox virus is now extinct in the wild, and the rinderpest virus, which infected domestic cattle, is now extinct in the wild, demonstrating that extinction can be a positive outcome in specific contexts. However, the cloning of extinct creatures remains a distant dream, with the ethical and ecological implications yet to be fully understood.