This article is an excerpt from the Shortform book guide to "The Sixth Extinction" by Elizabeth Kolbert. Shortform has the world's best summaries and analyses of books you should be reading.
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What are the two major theories of extinction? How do they explain the mass extinctions that have happened on planet Earth? Which theory holds more weight?
Theories of extinction can be categorized into two major schools of thought: catastrophism and uniformitarianism. Catastrophists believe extinctions happen as a result of major catastrophes. Uniformitarians, on the other hand, believe in a gradual process of extinction due to evolution.
Keep reading to learn about theories of extinction and how they were developed.
The Theories of Extinction
The current theories of extinction evolved from a series of paradigm shifts.
In the 19th century, those who believed only in gradual extinction as a result of evolution were called uniformitarians. However, during Darwin’s time, humans drove one of Europe’s most unusual species, the great auk, to extinction, contradicting his theories of extinction: that species went extinct only slowly, through competition and natural selection.
In the late nineteen-seventies in Italy, American geologist Walter Alvarez discovered traces of the asteroid that ended the Cretaceous period, causing the fifth mass extinction, which wiped out 75% of all species.
He was studying rock layers in the Gola Del Bottaccione gorge outside the town of Gubbio, Italy, which is north of Rome. The region once lay at the bottom of the sea. The remains of marine animals built up through millennia, eventually creating the Apennine Mountains and elevating limestone cliffs.
Between the diagonal bands of limestone reflecting different time periods, Alvarez saw a thin layer of clay that contained none of the marine lifeforms seen in the limestone layers below or above it. Something had wiped out the foraminifera—tiny creatures with calcite shells that fossilize—below the clay layer; when forams appeared later in the limestone layer above the clay, they were different species and much smaller. Alvarez determined that the larger forams seen below the clay layer had vanished at the time dinosaurs were known to have died off (the End Cretaceous period).
Further, Alvarez and his father Luis, a physicist at UC Berkeley, tested the samples from the clay layer and found it contained a huge amount of iridium, a chemical element rare on the surface of the earth but common in meteorites. Similarly, the Alvarezes found high levels of iridium in samples of late-Cretaceous clay in limestone cliffs in Denmark and from the South Island of New Zealand.
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Like what you just read? Read the rest of the world's best book summary and analysis of Elizabeth Kolbert's "The Sixth Extinction" at Shortform .
Here's what you'll find in our full The Sixth Extinction summary :
- How humans have set in motion a sixth mass extinction
- The 5 mass extinction events that occurred over the last 500 million years
- Why human ingenuity might be able to save the human species from extinction
