Causes of the Cambrian Period Extinction Event

Instructor: Christopher Muscato

Chris has a master's degree in history and teaches at the University of Northern Colorado.

Extinction is a fact of geologic history, but not one that we fully understand. In this lesson, we'll explore one of the first major extinction events and see what may have caused it.

Explosion to Extinction

Where did life on Earth come from? Anybody interested in this question has to take some time to learn about the Cambrian Period, the geologic time period lasting from roughly 540-500 million years ago (give or take a few). For a long time, scientists thought that life first evolved in this period. Now we know that life existed earlier, but it was somewhat limited.

For millions of years, Precambrian life consisted of tiny, simple organisms. Then, as the Cambrian began, life exploded in scale and diversity. Nearly all of the major phyla appeared, and complex creatures of various sorts roamed the ancient seas. We call this the Cambrian explosion. For about 40 million years, life grew and changed and diversified at an incredible rate. Then, everything died. Sorry to make that seem sudden, but in geologic time it was nearly instantaneous. In just a few million years, about 40% of all species on the planet went extinct.

Life got pretty interesting after the Cambrian Explosion
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The Cambrian period saw the first major explosion of life on Earth, but also one of its first major extinctions, called the Cambrian extinction. Occurring at the very end of the Cambrian, this event changed evolutionary history forever. But why did it happen? And could it happen again? Scientists may hope that understanding one of these questions could help prevent the other.

Setting the Stage

The Cambrian was a period of general warming on Earth, elevating the global temperatures from an ice age to a nice temperate level. We've observed from geologic records that the seas also had high levels of oxygen at this time, possibly as a bi-product from algae that thrived in the warmer climate.

The temperate weather and high amounts of oxygen may have contributed to the massive explosion of diversity on Earth. It's important for us to remember, however, that all life was still confined to the oceans at this point. Among the most important creatures to evolve in this world were the trilobites, ancient ancestors of arthropods with remarkably complex eyes (for the Cambrian). The many species of trilobites played critical roles in Cambrian ecosystems. They were bottom-feeders that helped speed up decomposition and release that energy back into the food chain. Their behaviors likely encouraged healthy reefs and marine ecosystems. They were also a major food supply that allowed for the evolution of larger predators.

Cambrian trilobite
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Trilobites were the foundation of Cambrian ecosystems, and troubles for them could indicate troubles for all. During the Cambrian period, there were actually four major extinction events. The very first one, in the Early Cambrian, resulted in the deaths of the oldest groups of trilobite species, as well as many reef-building organisms called archaeocyathids. While the marine environments recovered, it was proof that life was not as stable as it seemed. The other three Cambrian extinction events all occurred around the same time in the Late Cambrian, collectively forming the Cambrian extinction. Trilobites were hit hard, with many of the dominant species disappearing forever.

Causes

We know that life was unstable and prone to massive shifts, but what actually caused the extinction at the end of the Cambrian Period? The honest answer is that we don't know for sure. There are two dominant theories, however, both of which are based on geologic evidence of global cooling. We don't know exactly what led to global temperatures cooling down, but it seems to have had a major impact on marine life.

The Glacial Hypothesis

The first theory to explain the Cambrian mass extinction based on evidence of continental glaciers at the boundary between the Cambrian and Ordovician Period. When glaciers form on dry land, that water is being pulled out of the sea. As a result, sea levels can decline rapidly (in the most recent ice age, for example, early humans could likely have walked from Russia to Alaska because sea levels were so low).

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