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About a billion years ago, the continents emerged relatively suddenly from an ocean that covered 95 percent of the Earth's surface, according to a new theory by Eldridge Moores, a geologist at the University of California, Davis. The appearance of large masses of dry land would have caused more extreme weather, changes in ocean currents and the emergence of proper seasons. In turn, these environmental changes may have led to rise in atmospheric oxygen that enabled the explosion of new life forms around 500 million years ago.

Since the 1960s, geologists have accepted that the continents sit on tectonic plates floating above a layer of hot, partly molten rock. Between the continental plates, the crust of the ocean floor is continually created or pushed under and destroyed as the plates drift around and bump into each other. The crust of the modern ocean floor is much thinner than that below the continents.

In the early Earth, the ocean crust and the continental crust were much closer together in thickness, Moores said. That means that the difference between the height of the ocean floor and the height of the continents was much less. The oceans would have been much shallower, and the water would therefore have spread much further across the continents -- covering 90 to 95 percent of the planet's surface, instead of the present 70 percent.

Many geologists agree with this scenario, Moores said. What is controversial is how quickly the Earth changed from a planet covered in water with a few mountainous islands to one with large continental landmasses. According to Moores' theory, the continents emerged quite suddenly, over about 200 million years, at the same time that the supercontinent Rodinia was forming.

Moores' theory, published in the January issue of the , also implies that over time, the way plate tectonics works has changed.

The period around 800 million years ago is also important for the evolution of life. Complex, multicellular plants and animals, including the precursors of all the major groups of animals found today, appear at the beginning of the Cambrian era around 550 million years ago. That suggests that something, perhaps a drastic change in the environment, triggered a burst of evolutionary activity.

To reach his conclusions, Moores studies rocks called ophiolites. Ophiolites are remnants of ocean crust that are preserved in continents when plates move. Unfortunately, many of the world's most interesting ophiolites are located in areas that are remote and inaccessible due to geography or politics, such as central Africa.

Moores' theories also have implications for studying other planets, such as Mars, where plate tectonics seems to have started but then stopped. Venus is thought to have active volcanoes and wrinkled, broken crust, but does not seem to have tectonic plates in the same way that the Earth does. The crust of Venus might be more like the early Earth, with all the crust about the same thickness. A crucial difference between the Earth and Venus might be the presence of water on Earth, which could help to "lubricate" the continental plates, Moores said.