A new theory has emerged that almost all of the Earth has been covered with water millions of years ago.

Cin-Ty Lee, a geoscientist from Rice University in Houston, has proposed a new theory that it wasn’t until 700 million years ago that the Earth’s continents didn’t rise above the waves, when the underlying mantle sufficiently cooled.

Lee said during the Dec. 15 meeting of the American Geophysical Union that, despite many scientists being unconvinced, he proposes that continental rise may have contributed to the rapid diversification of life known as the Cambrian explosion.

The Cambrian explosion occurred approximately 541 million years ago in the Cambrian period, during which most major animal phyla appeared.

According to an article in Science News, the Earth’s continental crust formed billions of years ago and slabs of the crust float above the underlying mantle like ice bergs. The relatively cold roots than can extend tens of kilometers into the mantle.

The size of its root and the diversity of the mantle determines the continent’s elevation. In early Earth’s history the mantle was hotter and less dense, which leads Lee to believe that continents sat largely below sea level with only mountains peeking above the water.

According to Lee, the cooling of the mantle over time increased the relative buoyancy of the continents and lifted the landmasses above sea level.

Lee, using mantle cooling rates and the Earth’s topography, proposes that the expansion of Earth’s dry land took place between 1 billion and 500 million years ago and lasted approximately 100 million years.

Lee said when this occurred the new land would have altered carbon and nutrient cycles, which could help explain large shifts in the Earth around that time, which may have nourished the Cambrian explosion.

The major groups of animals, from insects to mammals, also emerged during this time period.