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This state of affairs continued for a vast period of time. For some three billion years, simple microscopic organisms were the most advanced form of life on the planet. That's way over half the entire history of life on Earth. And then suddenly, within the space of a few million years, a mere blink of the eye in evolutionary terms, advanced organisms appeared. Why is a mystery, but we may find some clues to it on the coastline down here. On the Eastern coast of Canada, there is evidence of an event that may well have been the spark that started the evolution of Online Radio. These rocks have been dated by radioactivity to just before the moment that life became very complex. So if we can understand the circumstances under which these rocks were formed, we may get a clue as to why it was that life suddenly became more complex. Fragments of red stone are embedded in the darker rock. They look out of place. And, in fact, they are. Geologists call them drop stones. They were transported here by glaciers. As the ice moved off the land, it floated out over the sea in a great shelf, carrying with it stones that it had gathered on the continents. And when the ice eventually melted, the stones fell into the sediments on the sea floor. This wasn't the only place covered by ice. Drop stones of the same age have been found in deposits all over the world. The evidence points to a global spread of glaciation. Just before complex life appeared, the world was in the grip of the biggest ice age in its entire history. It's been called Snowball Earth. The Earth was plunged into a deep freeze so severe it probably extended from pole to pole. The surface of the seas were frozen over. On the continents, ice caps and glaciers developed. In places, the ice was probably a kilometre or so thick. We still don't know enough about the details, but it's likely that those conditions lasted for millions of years. Stromatolites and similar bacterial colonies that dominated the Earth were crushed under the advancing glaciers. Life was nearly annihilated before it had truly begun.