Forming Fossils

Time Periods

What theories attempt to explain why the Cambrian Explosion occurred?

In the past, paleontologists have offered many theories as to why the great blossoming of life, known as the Cambrian Explosion, occurred. Some point to the mass extinction of Ediacara organisms (the earliest known complex multi-cellular organisms); another theory states that the development of eyes in animals changed the predator-prey dynamics; and yet another theory suggests that an increase in size of many organisms accelerated diversity.

There is also one study that places the blame, or credit, for this evolutionary explosion on our planet itself—and although not everyone agrees, it is an interesting theory. More than 500 million years ago, shifting masses in the interior mantle of Earth essentially unbalanced the planet, or “tipped” it, causing the entire surface to reorient itself in an effort to become balanced again. In a process called “true polar wander,” the ancestral North America moved from near the South Pole up to the region of the equator; the large continent of Gondwanaland (made up of South America, Antarctica, Australia, India, and Africa) traveled all the way across the Southern Hemisphere. This movement all happened at more than twice the rate of continental drift found in the normal process of plate tectonics today.

Evidence for this theory comes from Earth itself. During the formation of rocks, the minerals inside naturally align themselves with the existing magnetic field of the planet. By studying the orientation of grains in the minerals, scientists can determine the position of ancient continents relative to the magnetic north pole, which almost always lies close to Earth’s axis of rotation. When the positions of the continents were plotted using this data, scientists found that there was a major movement of the continents within a relatively short period of time around the Cambrian period. The data showed that ancestral North America moved to the equator between 540 and 515 million years ago, while Gondwanaland shifted between 535 and 500 million years ago.


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