Stuff You Should Know

Selects: How Extinction Works

November 1, 2025

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  • Extinction is a natural process, with scientists estimating that over 99% of all species that have ever lived on Earth are now extinct, often with a typical species lifespan of about 10 million years. 
  • The understanding of mass extinctions evolved significantly, moving from the belief that extinction was impossible to the acceptance of sudden, cataclysmic events like the K-Pg event (dinosaur extinction) following the discovery of the iridium layer and the Chicxulub crater. 
  • The current era is widely considered by scientists to be the beginning of a Sixth Mass Extinction, characterized by rates of species loss potentially 100 to 1,000 times the background rate, largely driven by human activity. 

Segments

Introduction and Episode Context
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(00:00:00)
  • Key Takeaway: The hosts introduce this classic episode of Stuff You Should Know, Selects: How Extinction Works, which covers major extinctions and the current human-caused one.
  • Summary: The episode is a selection from 2014 focusing on extinction events. The discussion promises to cover historical mass extinctions and the extinction event currently believed to be caused by humans. Josh Clark notes that the episode contains early glimmers of his side podcast, The End of the World.
Early Views on Extinction
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(00:06:43)
  • Key Takeaway: Early scientific thought, influenced by religious views, resisted the concept of extinction until George Cuvier asserted the reality of lost species due to cataclysmic events in 1812.
  • Summary: For a long time, scientists believed God’s creation was too perfect to allow extinction, rationalizing large fossil bones as undiscovered animals, a view held by figures like Thomas Jefferson. George Cuvier’s 1812 essay asserted that extinction occurred via cataclysmic events, which religious thinkers reconciled with Noah’s flood. Darwin, however, favored gradual extinction, a view that dominated until the 1990s.
The Alvarez Hypothesis and Mass Extinction
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(00:09:00)
  • Key Takeaway: The Alvarez hypothesis, proposing an asteroid impact caused the dinosaur extinction, gained wide acceptance in 1991 after the discovery and dating of the Yucatan Peninsula crater.
  • Summary: The view that extinction happens slowly persisted until 1991, when evidence supported Walter Alvarez’s hypothesis that an asteroid caused the dinosaur extinction 65 million years ago. Alvarez found iridium levels off the charts in the clay layer marking the end of the Cretaceous period. The discovery of a matching crater under the Yucatan Peninsula confirmed the impact theory, establishing that extinction can occur on a sudden, mass scale.
Defining Background vs. Mass Extinction
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(00:11:46)
  • Key Takeaway: While extinction is a natural process with an average background rate of one to five species per year, current rates are estimated to be 100 to 1,000 times higher.
  • Summary: Scientists estimate that over 99% of all species that have ever lived are gone, but extinction is a natural part of life, with species typically lasting about 10 million years. Background extinction is slow, but current rates are alarmingly accelerated due to human impact. The fossil record is incomplete, making precise calculations difficult, but researchers use mathematical speculation to estimate these rates.
Causes of Gradual Extinction
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(00:21:44)
  • Key Takeaway: The four main causes of gradual extinction—habitat loss, competition, human hunting, and environmental contamination—are all factors humans can influence.
  • Summary: Extinction typically results from a species’ inability to adapt to changes in its habitat, competition, human hunting, or environmental contamination from humans, bacteria, or viruses. The stellar sea cow illustrates rapid extinction, disappearing between 1741 and 1768 due to human introduction. Losing one species can cause a domino effect throughout an ecosystem, as seen with the passenger pigeon’s population explosion following the decline of its Native American predators.
The Big Five Mass Extinctions
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(00:28:51)
  • Key Takeaway: The Permian-Triassic extinction, or ‘The Great Dying,’ was the largest mass extinction, wiping out up to 96% of all life 250 million years ago, possibly due to volcanic activity.
  • Summary: There have been more than 20 mass extinction events, with the ‘Big Five’ being the most significant, though there is no strict definition for what constitutes one. The Ordovician extinction (490 million years ago) was likely caused by glacier formation and sea-level drops. The Permian-Triassic extinction is the largest, potentially caused by volcanic activity leading to acid rain, while the K-Pg event involved an asteroid impact that either broiled the Earth or caused a nuclear winter-like blackout.
The Proposed Sixth Extinction
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(00:36:36)
  • Key Takeaway: Evidence strongly suggests humanity is causing a Sixth Mass Extinction, evidenced by the rapid decline of amphibians, mammals, and coral reefs globally.
  • Summary: The current mass extinction is considered more dangerous than past events because there is no stress relief or recuperation period, as human impact is constant. Human expansion, including farming, logging, and damming waterways (only 2% of US rivers run unimpeded), drives this crisis. Ocean acidification, described as global warming’s evil twin, is changing ocean chemistry faster than in the last 50 million years, leading to booming jellyfish populations while other life declines.
Human Impact on Megafauna
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(00:40:09)
  • Key Takeaway: The spread of humans approximately 50,000 years ago correlates strongly with the extinction of megafauna across continents like Australia and North America (e.g., mammoths, giant beavers).
  • Summary: The ’theory of overkill’ suggests that early human expansion, armed with tools and domesticated dogs, led to the mass extinction of large animals. In North America, three-quarters of the largest animals disappeared around the time humans crossed the Bering Land Bridge. While some argue climate change was the primary factor, the correlation between human arrival and megafauna loss is a major point of debate regarding the Sixth Extinction’s cause.
Rediscovered Species and Conclusion
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(00:49:10)
  • Key Takeaway: Despite the grim outlook, some species once thought extinct, like the coelacanth and the Bermuda petrel, have been rediscovered, offering small glimmers of hope.
  • Summary: The existence of ‘Lazarus species’ like the coelacanth, which reappeared after being presumed extinct for millions of years, highlights the gaps in our knowledge of life on Earth. The Bermuda petrel was rediscovered in 1951 after being thought gone since the 1600s, and the Cuban solenodon reappeared after being declared extinct in 1970. These rediscoveries contrast with the ongoing, human-accelerated mass extinction event.