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Closest calls

Five times the chain almost broke

Over the last 500 million years, most species in the sea, and many on land, died out five times. Each time, a few small animals in your line got through. If one of them had not, you would not be here.

The whole idea in five sentences

Millions of years ago

  • 445End of the OrdovicianAbout 86 of every 100 species died.
  • 372Late DevonianAbout 75 of every 100 species died.
  • 252End of the Permian, “the Great Dying”About 90 of every 100 species died.
  • 201End of the TriassicAbout 80 of every 100 species died.
  • 66End of the CretaceousAbout 76 of every 100 species died.
  1. Five times in the last 500 million years, most animal species died in a short time.
  2. Each time, your ancestors were among the few that got through.
  3. The causes were huge: ice ages, giant volcanoes and, once, an asteroid.
  4. Survivors were often small animals that could shelter underground or sit out the worst years.
  5. After each disaster, empty places opened up for new kinds of life. That is how mammals, and later primates and humans, came to diversify.

The numbers are estimates, mostly for sea animals, which leave the best fossils. Some causes are still debated. The details and the limits are in the full version.

The five closest calls

Pick an event. Each dot is one species out of a hundred. Grey dots died, gold dots lived. The ringed dot stands for your line.

Millions of years ago

End of the Permian, “the Great Dying”

About 252 million years ago

About 90 of every 100 species died.

DiedLivedYour line at the time

Rounded estimates. The exact numbers are uncertain.

What happened

Giant volcanic eruptions in Siberia heated the planet and probably left the oceans sour and short of oxygen.

Your line at the time

Mammal-like animals called cynodonts. Right after the event, at least one kind (Thrinaxodon) is known to have dug burrows.

What came next

The way opened for reptiles such as the ancestors of the dinosaurs. Cynodonts, our own group, went on to give rise to the first mammals.

What helped them survive

The fossils suggest a few patterns. They are patterns, not rules.

Small bodies

Small animals need less food and find shelter more easily. At the end of the Cretaceous, the land animals that survived were mostly small.

A place to hide

Burrows may have protected animals from heat and storms. A fossil burrow about 251 million years old, from just after the end-Permian event, holds the skeleton of a cynodont, so burrowing was already in use by animals in our line.

Waiting it out

Some animals can go dormant or hibernate. That may have helped them sit out the worst years.

Not only skill

Being the best in normal times did not guarantee survival. Where an animal lived, and plain luck, counted too. What helped in one extinction did not always help in the next.

What we do not know

This chapter simplifies a lot. Six things to keep in mind.

  • The numbers are estimates. They count species, mostly sea animals with shells, because those leave the best fossils. The values come from one widely used table (Barnosky 2011); other studies differ. For the Permian, published estimates for marine species run from about 81 to 96 percent, and we show about 90.
  • Some causes are debated. For the Ordovician and the Devonian, scientists do not agree on the main cause. Even for the Permian and the Cretaceous, the exact mix of causes is still studied.
  • We know the group, not the individual. We can say what kind of animal was in your line at the time. We cannot name your actual ancestor.
  • Five is a convention. Scientists call these the “Big Five”. There were many smaller extinctions, and some of the five were really several pulses close together.
  • What “short” means. A mass extinction here means about three quarters or more of species lost in a geologically short time, which can still be tens of thousands to millions of years. Ages come from radioactive dating of volcanic ash layers; losses come from counting fossil species.
  • This chapter is about the past. Whether a sixth mass extinction is under way today is a separate question, and not covered here.

Further reading

Papers and a book behind this chapter.

Review status

  • Language: written with the help of AI. No native speaker has reviewed it yet.
  • Facts: taken from the sources listed above. No expert has reviewed this chapter yet.

At each of the five, some of your ancestors were among the survivors. The chain bent five times and did not break.

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fromLUCA LUCA is the last universal common ancestor: the population of single cells from which all life on Earth today descends. It lived roughly four billion years ago; estimates differ.
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