See what Earth looked like 500 million years ago

As the world celebrates Earth Day, researchers at the University of Puerto Rico's Planetary Habitability Laboratory marked the occasion with a look back - make that a long look back. 500 million years, to be more specific.

In this computer-generated video animation, you can get an idea of how the Earth looked during the Cambrian Period, when our planet was a very different place. It's hard to recognize that world, one in which most of the continents were found in the southern hemisphere and the only green plants of note were fungi, algae, and lichens.

"I think people looking at the whole period will realize how fragile our planet is, how it changes," Abel Mendez, who led the project, told MSNBC.