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Stuff You Should Know

Selects: How Extinction Works

Published November 1, 2025
View Show Notes

About This Episode

Josh and Chuck explain how extinction works, distinguishing between slow background extinctions and rare but catastrophic mass extinction events. They walk through the history of scientific ideas about extinction, the Big Five mass extinctions in Earth's history, and evidence that we are likely entering a human-driven sixth mass extinction. The episode also touches on de-extinction efforts, ecological cascades from species loss, and a listener letter about how interrogation settings can make innocent people appear guilty.

Topics Covered

Biodiversity loss Comets and asteroids Existential risk Mass extinction Fossil record Background extinction rate Anthropocene De-extinction Megafauna Ocean acidification

Disclaimer: We provide independent summaries of podcasts and are not affiliated with or endorsed in any way by any podcast or creator. All podcast names and content are the property of their respective owners. The views and opinions expressed within the podcasts belong solely to the original hosts and guests and do not reflect the views or positions of Summapod.

Quick Takeaways

  • Extinction is a natural process, with most species lasting around 10 million years, but current extinction rates are estimated to be 100-1,000 times higher than the historical background rate.
  • Scientific acceptance of sudden, catastrophic mass extinctions only became mainstream in the late 20th century, largely due to evidence that an asteroid impact wiped out the dinosaurs 65 million years ago.
  • The fossil record is highly incomplete and biased, making it difficult to know exactly when and how many species went extinct, so scientists rely heavily on mathematical models and inference.
  • Human activities-habitat destruction, overhunting, introducing competition, and contaminating environments-can drive rapid extinctions and may be triggering a sixth mass extinction.
  • Efforts at de-extinction, such as briefly reviving the Bucardo mountain goat or proposed passenger pigeon projects, raise ethical questions and the risk that people will treat biodiversity as having an "undo" button.
  • Mass extinctions don't end life on Earth but radically reorganize which species dominate; what is catastrophic for one group can be a boom time for others, like jellyfish in increasingly acidic oceans.
  • Geologists have proposed naming the current epoch the Anthropocene because human impacts will likely be clearly visible in the geological record millions of years from now.
  • A listener's experience as a witness and mock trial defendant shows how interrogation-like environments can make innocent people appear guilty simply through nervous body language.

Podcast Notes

Episode framing and overview of extinction topic

Josh introduces this as a replay of a 2014 episode on extinction

He says the episode covers all the big extinctions and likely causes, including the one we are probably in now that is likely human-caused[1:23]
He notes listeners can hear early versions of themes he later explored in a separate show about existential risks[1:48]

Hosts introduction and banter leading into extinction theme

Stuff You Should Know intro and studio setup

Josh introduces himself and Chuck (Charles W. "Chuckers" Bryant) and mentions producer Jerry[2:05]
They joke about nearly saying Jerry's last name and introduce "Scent" as a fourth character in the studio[2:09]

Joking about scents and segue to extinction

Chuck smells like patchouli from one of Emily's sugar scrubs, and Jerry contributes an enchilada smell, which together are joked about as forming another person[2:28]
Josh quips that when they leave the studio this "person" will become extinct, using the joke to pivot into the topic of extinction[2:58]

Pop culture reference: Busta Rhymes and extinction imagery

Busta Rhymes' Extinction Level Event album as 1990s extinction anxiety

Chuck mentions thinking about Busta Rhymes because of his album "Extinction Level Event"[3:17]
They connect the album title to late-1990s cultural anxiety about catastrophe: Y2K, the popularity of The X-Files, and disaster movies like Deep Impact and Armageddon[3:37]
They reminisce about Busta Rhymes' features on a Tribe Called Quest song ("Scenario") and his early stance against saggy pants, which he later adopted himself[4:02]

Key reading and sources on extinction

Elizabeth Kolbert and The Sixth Extinction

They highlight journalist Elizabeth Kolbert (or Colbert) as a leading journalistic expert on extinction[4:58]
She wrote a New Yorker article that served as a precursor to her book "The Sixth Extinction"[5:04]

Additional articles and background reading

They mention a New York Review of Books piece titled "They're Taking Over" about jellyfish population explosions[5:26]
Josh notes a HowStuffWorks article he previously wrote called "Will We Soon Be Extinct?" and another on "Why Is Biodiversity Important?"[5:32]
They also reference an io9 list of animals believed extinct that were later rediscovered[5:43]

Early scientific views on extinction and religious context

Belief that a perfect creation could not include extinction

For a long time, many scientists believed God created all animals and that divine perfection would not allow species to go extinct[6:33]
Large fossil bones were known, but were rationalized as belonging to still-living animals that simply had not yet been found[6:52]

Jefferson and the mastodon

Thomas Jefferson, despite being very smart, thought mastodons might still exist in unexplored parts of North America[7:04]
When he sent Lewis and Clark west, he warned them they might encounter the "Great Mastodon"[7:08]

Georges Cuvier and the birth of extinction science

In 1812, French naturalist Georges Cuvier published an essay titled "Revolutions on the Surface of the Globe"[7:28]
Cuvier argued that extinction is real and that "lost species" were produced, likely through cataclysmic events[7:36]
His ideas implied that major catastrophes had struck Earth, periodically wiping out many species[7:49]

Religious reinterpretation: Noah's flood as catastrophe

Religious thinkers reframed Cuvier's cataclysm ideas as scientific proof of biblical events like Noah's flood[8:02]

Darwin, gradualism, and resistance to catastrophic extinction

Darwin's stance on extinction

Darwin accepted that extinction happens but believed it occurred only gradually, not via sudden catastrophes[8:28]
He held that extinction proceeded more slowly than the emergence of new species (speciation), so diversity should generally increase over time[8:28]

Influence of Darwin's authority

Because Darwin was such a towering figure in biology, his gradualist view dominated scientific thinking until the 1990s[8:28]
Until then, sudden mass extinctions were not widely accepted; apparent abrupt disappearances in the fossil record were often blamed on gaps in the record[9:23]

Alvarez hypothesis and the asteroid that killed the dinosaurs

Walter Alvarez and the iridium anomaly

Geologist Walter Alvarez studied a thin clay layer marking the sudden disappearance of dinosaurs in the fossil record[9:23]
• He measured extremely high levels of iridium, a rare element on Earth but abundant in asteroids
In 1980, Alvarez and colleagues proposed that a large asteroid impact caused a sudden mass extinction 65 million years ago[10:40]

Chicxulub crater and mainstream acceptance

Their hypothesis met strong resistance for over a decade[10:13]
In 1991, a 112-mile-wide crater was identified under the Yucatán Peninsula and dated to the same time as the dinosaur extinction[10:22]
This Chicxulub crater provided strong evidence that an asteroid impact triggered a sudden mass extinction, validating the Alvarez hypothesis in the mainstream[10:40]

K-T event renamed K-Pg

The extinction marking the end of the Cretaceous period in the Mesozoic era is now called the Cretaceous-Paleogene (K-Pg) event, formerly the Cretaceous-Tertiary (K-T) event[10:58]
They note that in geologic notation the Cretaceous is abbreviated with a "K" (from German), hence K-Pg[11:00]
The asteroid impact is seen as ushering in the Cenozoic era, the era humans now inhabit[11:29]

Extinction as a natural process and current acceleration

Natural baseline of extinction

Extinction has a negative connotation today, but it is a natural part of evolution and Earth's history[11:48]
Scientists estimate that as many as 5 billion species have existed on Earth, and more than 99% of them have gone extinct[12:01]
A New Yorker line they quote suggests that all life currently on Earth could be accounted for as a "rounding error" compared to all life that has ever existed[12:10]

Average lifespan of a species and background rate

On average, a species persists for about 10 million years before going extinct[12:50]
Background extinction refers to this slow, ongoing rate of species loss in normal times[13:09]
Researchers estimate a background rate between 1 and 5 species per year, or more precisely 0.023 to 0.135 extinctions per million species per year[12:50]
• They explain the latter statistic means that, for each million species alive, only a tiny fraction of a species would be expected to go extinct in a year under normal conditions

Evidence of accelerated current extinction

Current extinction rates are estimated to be 100 to 1,000 times higher than the background rate[14:04]
They characterize this acceleration as alarming and indicative of a potential mass extinction in progress[14:13]

Limitations of the fossil record and challenges in studying extinction

Incompleteness and bias of the fossil record

Fossils form only under specific conditions, so the record is highly incomplete and biased toward certain environments and organisms[14:35]
We do not know how many species have ever lived, because many left no fossils at all[14:28]
The apparent disappearance of a species from the fossil record may simply mean no fossils were preserved after a certain point, not necessarily that it went extinct then[14:43]
Even if a fossil is the last one found for a species, it is extremely unlikely that fossil represents the last individual that ever lived[15:00]

Reliance on mathematical models

Because of gaps in physical evidence, extinction researchers use a lot of math, algorithms, and speculative modeling to infer rates and patterns[15:39]
These models help estimate values like minimum viable population sizes below which extinction risk becomes very high[15:58]

Lazarus species and pseudo-extinction

Coelacanth as a classic Lazarus taxon

The coelacanth, a large, ancient-looking fish, was believed to have gone extinct about 400 million years ago[19:17]
In the 1930s, a live coelacanth was caught off the coast of South Africa, and later another was found near Madagascar or Mauritius[19:21]
Because it reappeared after being presumed extinct, it is called a "Lazarus species" (after the biblical story of Lazarus being raised from the dead)[19:57]

Concept of pseudo-extinction

Pseudo-extinction is when a species disappears as such because it evolves into a new species rather than dying out completely[20:26]
Josh questions why this is labeled pseudo-extinction rather than simply a speciation event, noting the conceptual gray area[20:26]

We still don't know how many living species exist today

Even in the present, scientists continue to discover species thought to be extinct, highlighting our incomplete knowledge[20:26]
The fact that Lazarus species keep popping up underscores that we do not have a full inventory of life on Earth[21:04]

Direct causes of extinction and human responsibility

Main drivers of extinction

Extinction usually follows from changes in a species' habitat that it cannot adapt to[20:26]
Key proximate causes include habitat loss, competition with other species, overhunting, and contamination of the environment with pollutants or pathogens[22:01]

Humans can trigger all four major drivers

Humans are capable of and currently responsible for all four major extinction drivers: clearing habitats, introducing competitors, hunting, and polluting environments[21:48]

Rapid human-driven extinction example: Stellar's sea cow

Tracy Wilson's intro (referenced by Josh) noted the Stellar's sea cow, an Arctic manatee-like animal[22:24]
Stellar's sea cow was first described in 1741 and hunted to extinction by 1768, showing extinction can occur over mere decades when humans are involved[22:16]

Ecological cascades from losing species

They mention concerns over bee declines: loss of bees is not just about bees but also the many plants they pollinate and the animals that rely on those plants[23:18]
At the end of the last Ice Age, many small mammals went extinct, which then contributed to the extinction of larger animals that preyed on them[23:45]
These examples illustrate how species losses can create domino effects throughout ecosystems[23:27]

De-extinction experiments and ethical questions

First de-extinction attempt: the Bucardo

In 2003, scientists cloned the Bucardo, a Spanish mountain goat, from preserved DNA frozen in time[23:57]
The cloned animal survived only a few minutes, but it is considered the first successful de-extinction event[24:22]

Prospects and limits of de-extinction

Josh and Chuck note that while we may not be able to bring back an exact woolly mammoth, we might create something very similar[24:39]
They reference efforts aimed at de-extincting the passenger pigeon[24:59]

Ethical and practical concerns

A core question is whether we should revive species whose original habitats no longer exist; if so, where would they live-only in zoos?[24:59]
Chuck argues attention and resources might be better spent preventing current extinctions instead of resurrecting long-gone species[24:57]
They worry that treating de-extinction as an "undo" button (like hitting Control-Z) could make society less serious about protecting existing biodiversity[25:39]

Case study: passenger pigeon and ecological imbalance

Passenger pigeon overabundance and human roles

When European settlers arrived in North America, an estimated one out of every four birds was a passenger pigeon[26:40]
Flocks were so dense that a single shot into a flock could kill hundreds of birds[26:40]
They refer to Charles C. Mann's books (1491/1493), which argue that passenger pigeons became so numerous because their major predator-Native Americans-had been decimated by Old World diseases a century earlier[27:06]
When European settlers later arrived in large numbers, they heavily hunted passenger pigeons and drove them to extinction[27:42]

Buffalo near-extinction as another hunting example

They compare passenger pigeons to American buffalo, which were nearly hunted out of existence, including "sport" shooting from trains and leaving carcasses to rot[28:00]
They underscore how human overhunting can rapidly reduce even massive populations[27:56]

The Big Five mass extinctions

Overview of mass extinction concept

More than 20 extinction events have been identified, but five are so severe they are called the "Big Five" mass extinctions[28:50]
Mass extinctions are different from background extinction in that many species disappear in a geologically short time, often globally[29:37]

Ordovician extinction (~490 million years ago)

About half of all animal families, mostly marine, died out[29:06]
Glaciation formed large ice sheets, lowering sea levels and destroying shallow marine habitats where many organisms lived[29:16]

Late Devonian extinction (~360 million years ago)

Roughly a quarter of marine families and about half of marine genera went extinct[30:05]
Scientists still debate its causes, and no single explanation has been settled on[30:03]

Permian-Triassic extinction ("The Great Dying", ~250 million years ago)

This was the most severe extinction event, killing perhaps 95-96% of all species[30:01]
In the article they reference, it is described as wiping out about 85% of marine genera and 70% of land species[30:11]
Leading hypotheses include massive volcanic activity and associated acid rain[31:19]

End-Triassic extinction (~200 million years ago)

Killed about 20% of marine families and about half of marine genera[30:33]
Causes remain uncertain; multiple hypotheses exist but no consensus[31:09]

Cretaceous-Paleogene (K-Pg) extinction (~65 million years ago)

This event ended the Mesozoic era and wiped out the non-avian dinosaurs[31:32]
They describe a leading mechanism: asteroid impact vaporizing rock that shot into space, reentered the atmosphere, and generated intense heat that "broiled" organisms on Earth's surface[31:38]
An alternative or complementary scenario is that debris and ash blocked sunlight, halting photosynthesis and triggering a "nuclear winter"-like collapse of food chains[32:35]

Defining what counts as a mass extinction

Lack of strict formal definition

Josh notes he could not find a formal, universally agreed definition from any governing body for what qualifies as a mass extinction[32:53]
Because of fossil record gaps and reliance on estimates, scientists use more informal criteria[33:06]

Common features of mass extinctions

They suggest mass extinctions typically involve loss of a large percentage of species (e.g., around 20% of all animal species) in a geologically short time[33:53]
Events are global in scope rather than localized to one region[33:28]
The Big Five are so extreme in scale that there is little debate they are true mass extinctions, even if exact thresholds are fuzzy[33:28]

Perspective on mass extinctions and nature's indifference

Why the current extinction may be more dangerous for humans

They discuss a researcher's point that "one-and-done" catastrophes like asteroid impacts allow the planet to begin recovering relatively quickly afterward[34:09]
By contrast, if humans are driving a prolonged extinction episode, there is no clear end point that allows ecosystems to recuperate[34:13]

Mass extinctions reorganize life rather than ending it

They emphasize that extinction events do not wipe out all life; they dramatically change which species dominate[35:11]
From a detached perspective, extinction can mean boom times for some species even as others vanish[35:16]

No baseline "correct" Earth and nature's lack of concern

The Earth 100 million years ago was radically different from today; there is no fixed baseline that nature is trying to preserve[36:00]
Nature is not invested in maintaining the current human-dominated system; it is indifferent and will continue through cycles of emergence and extinction[36:21]
This perspective does not absolve humans of responsibility, but it reframes extinction as part of long-term planetary change[36:21]

Debate over the Sixth Mass Extinction and early human impacts

Proposal of the Anthropocene epoch

Many geologists argue that human impacts justify naming the current epoch the "Anthropocene"[39:01]
They expect that geologists 10,000 years in the future will be able to see a clear layer marking the beginning of significant human influence[39:11]

Wayback Machine thought experiment: Australia 50,000 years ago

Josh and Chuck imagine traveling 50,000 years into the past to Australia, seeing giant wombat-like creatures as big as hippos, VW Beetle-sized tortoises, and 10-foot-tall short-faced kangaroos[39:33]
They jokingly role-play setting up camp and noticing that as humans spread, these large animals start disappearing[39:13]

Temporal correlation between human expansion and megafauna loss

Many large animals (megafauna) seem to vanish from the fossil record shortly after humans arrive in new regions[40:30]
In North America, about 11,000 years ago, three-quarters of the largest animals-mastodons, woolly mammoths, giant beavers, saber-toothed tigers-disappeared around when humans crossed the Bering land bridge[41:29]

Overkill hypothesis vs. climate-change explanations

The overkill hypothesis suggests humans arrived with sophisticated toolkits-spears, arrows, axes, clubs-and domesticated dogs and overhunted large animals[42:15]
Humans might have also overhunted smaller prey that large predators depended on, indirectly starving the megafauna[42:03]
Critics argue that human populations were too small to hunt enough animals to cause all the observed extinctions, especially in sparsely populated places like early Australia[43:03]
Alternative theories emphasize climate change: shifts in temperature and environment that megafauna could not adapt to, possibly independent of or only partially influenced by humans[43:21]
Some researchers note repeated ice ages that did not always produce mass extinctions, complicating simple climate-only explanations[44:13]

Sahul case study and timing issues

Sahul was a prehistoric megacontinent including Australia, New Guinea, and Tasmania[43:44]
One study suggests that by the time humans arrived in Sahul, many megafauna were already gone, and archaeological evidence does not show humans yet having tools capable of killing the largest animals[43:59]
This supports the idea that climate change could have played a dominant role in some regions[44:01]

Global mapping of large mammal extinctions

A global analysis mapped distributions of large mammals from 132,000 to 1,000 years ago[44:44]
They found that 177 species of large mammals disappeared in that interval, which the researchers described as a massive loss[44:44]
Human arrival on islands is almost always associated with high extinction rates; the surprising aspect is the severity of losses on large landmasses as well[45:27]

Contemporary biodiversity crisis and ocean acidification

Current threat levels across taxa

They state that roughly one-third of all coral reefs are in danger of extinction[45:36]
About a third of amphibians, a quarter of mammals, and an eighth of birds are categorized as threatened with extinction[45:46]
These patterns are global, fitting the geographic breadth expected in a mass extinction[45:50]

Scale of human environmental modification

They highlight that farming, logging, road building, and construction have drastically altered habitats worldwide[45:57]
Most of the world's waterways have been diverted or dammed; only about 2% of rivers in the United States now run unimpeded[46:09]
Industrial activity, including chemical plants, has increased atmospheric CO2 levels significantly[46:16]

Ocean acidification and jellyfish boom

As CO2 levels rise in the atmosphere, oceans absorb more CO2, creating carbonic acid and lowering ocean pH-"ocean acidification"[46:37]
Ocean acidification has been called global warming's "evil twin", undermining conditions needed by many marine organisms[47:34]
Jellyfish populations are booming under these changing conditions, illustrating how some species benefit while many others struggle[47:45]
They cite an estimate that the drop in ocean pH over the past 50 years may exceed any comparable change in the previous 50 million years[48:49]
Another projection suggests that in the next 50 years, up to half of all species on Earth could go extinct if current trends continue[49:04]

Examples of rediscovered "Lazarus" species

Bermuda petrel

The Bermuda petrel was thought to have disappeared in the 1600s but was rediscovered in 1951[48:30]
There are now about 180 Bermuda petrels alive[48:37]

Cuban solenodon

The Cuban solenodon, a strange rat-like mammal, was discovered in 1861 and has been captured only 37 times[49:06]
It was thought extinct in 1970 but one was found later in the 1970s and another in 2003[48:57]

Gilbert's potoroo

Gilbert's potoroo, a rabbit-sized marsupial in Australia, was last seen in 1879 and presumed extinct[49:29]
It was rediscovered in 1994, but there are currently fewer than 100 individuals known[49:44]

Emotional resonance of rediscoveries

Josh and Chuck describe such rediscoveries as heartwarming stories of species "coming back" from presumed extinction[49:56]
They also wryly note that these species are "welcome back" to an ongoing mass extinction context[51:28]

Listener mail: police interrogation and body language

Witness experience in a real investigation

Listener Matt from Victoria, BC, writes about being a witness to a crime and being called to provide a statement[50:57]
He describes being taken into a bleak interrogation-room-like setting, which made him feel very nervous even though he had done nothing wrong[51:08]
He sweated and his voice shook, and he notes that his body language would likely have appeared guilty to someone watching through a one-way mirror[51:18]

Mock trial experience as a defendant

In another story, Matt played the defendant in a mock trial at a public courthouse event where his father (a lawyer) asked him to participate[51:26]
He played someone accused of a minor drug offense and testified in his own defense[51:44]
Afterward, his mother told him he looked really guilty on the stand and worried that in a real trial he would be convicted based on appearance[51:52]

Implications for interpreting nonverbal cues

Matt concludes that simply treating someone like a criminal-putting them in an interrogation context-can make them look guilty regardless of actual innocence[52:02]
Josh connects this to the Stanford Prison Experiment as an example of how environments and roles dramatically shape behavior and appearance[52:15]
They close by hoping listeners (and themselves) never have to experience a real police interrogation and encouraging legal representation if they do[52:46]

Lessons Learned

Actionable insights and wisdom you can apply to your business, career, and personal life.

1

Extinction is a normal part of Earth's history, but when the rate of species loss accelerates far beyond the background rate, it signals a systemic crisis that can destabilize entire ecosystems.

Reflection Questions:

  • • What indicators in my own environment or industry might suggest that change is happening at an unsustainably fast rate rather than at a normal background pace?
  • • How could distinguishing between normal fluctuations and true crises help me respond more appropriately to problems I face?
  • • What is one domain in my life or work where I should measure baseline conditions so I can tell when things are veering into dangerous territory?
2

Small changes-such as the loss of one species or the diversion of one river-can cascade through complex systems and create outsized impacts far from the original disturbance.

Reflection Questions:

  • • Where in my life or organization might I be underestimating the ripple effects of a seemingly localized decision?
  • • How can I better map the dependencies in my projects or relationships so I understand what will be affected if one piece is removed?
  • • What is one current decision I'm making where I should pause and consider second- and third-order consequences before acting?
3

Data sets are often incomplete and biased, so good decision-making requires humility, probabilistic thinking, and a willingness to revise views when new evidence appears.

Reflection Questions:

  • • In what area of my life am I relying on a "fossil record" of partial information and treating it as if it were complete?
  • • How might adopting a mindset of "this is my best model for now"-rather than "this is the truth"-change the way I make decisions?
  • • What is one belief or assumption I hold that I should actively re-examine in light of more recent information or perspectives?
4

Having an apparent "undo" button, like the promise of technological fixes or de-extinction, can tempt people to be more careless about preventing damage in the first place.

Reflection Questions:

  • • Where am I relying on future fixes (technology, money, apologies) instead of designing systems that avoid harm upfront?
  • • How would my choices change if I truly believed there was no way to reverse the consequences of a particular action I'm considering?
  • • What is one area-financial, environmental, relational-where I could remove or limit my personal "undo" button to force more deliberate behavior?
5

Context and power dynamics strongly influence behavior; placing someone in a high-pressure, adversarial setting can make even an innocent person look and act "guilty."

Reflection Questions:

  • • When have I misjudged someone's intentions or honesty because I didn't account for the pressure or context they were in?
  • • How could I redesign difficult conversations (like performance reviews or conflict discussions) to reduce unnecessary intimidation and elicit more honest input?
  • • What is one interaction this week where I can consciously adjust the environment-tone, setting, posture-to make it feel safer and more collaborative?
6

Nature is indifferent to human preferences; planning for the future means recognizing that our current arrangements are temporary and building resilience rather than assuming stability.

Reflection Questions:

  • • Which parts of my life or business am I implicitly treating as permanent, even though history suggests nothing stays the same?
  • • How might I design my plans so they can survive disruptive "extinction events" in my industry or personal circumstances?
  • • What is one resilience-building step-like diversifying skills, income streams, or relationships-that I could start this month?

Episode Summary - Notes by Reese

Selects: How Extinction Works
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