Extinctions: Georges Cuvier (2 of 2)

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Extinction by Catastrophe
Cuvier carefully studied elephant fossils found near Paris. He discovered that their bones were indisputably distinct from those of living elephants in Africa and India. They were distinct even from fossil elephants in Siberia. Cuvier scoffed at the idea that living members of these fossil species were lurking somewhere on Earth, unrecognized—they were simply too big. Instead, Cuvier declared that they were separate species that had vanished. He later studied many other big mammal fossils and demonstrated that they too did not belong to any species alive today. The fossil evidence led him to propose that periodically the Earth went through sudden changes, each of which could wipe out a number of species.

Cuvier established extinctions as a fact that any future scientific theory of life had to explain. In Darwin’s theory, species that did not adapt to changing environments or withstand the competition of other species faced annihilation. Darwin did not, however, accept all of Cuvier’s ideas on extinctions. Like Charles Lyell before him, he doubted that species went extinct in great “catastrophes.” Just as the planet’s geology changed gradually, so did its species become extinct gradually as new species were formed.

 Mammoth jaw comparison A 1798 paper by Cuvier contained this drawing showing the differences between the lower jaws of a mammoth (top) and an Indian elephant. These differences supported the idea that mammoths were indeed extinct.

Background Extinction and Catastrophe
On this score, Cuvier has been somewhat vindicated. Perhaps 99% of all species that ever existed on Earth are now extinct. Most of those extinct species disappeared in a Darwinian trickle—what paleontologists call “background extinctions.” But several times over the past 600 million years, life has experienced mass extinctions, in which half or more of all species alive at the time disappeared in fewer than two million years—a blink of a geological eye. The causes may include asteroids, volcanoes, or relatively fast changes in sea level. These extinctions mark some of the great transitions in life, when new groups of species got the opportunity to take over the niches of old ones. Mammals, for example, only dominated the land after giant dinosaurs vanished 65 million years ago in the Cretaceous-Tertiary extinction. We humans, in other words, are the children of extinctions.

Mass and background extinction Life’s history has been marked by both catastrophic extinction events (red spikes) and constant background extinction (yellow).

Teach this!
Lesson plans for teaching about extinction

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