An exciting discovery was reported in Science this week as evidence of plate tectonics occurring 3.8 billion years ago turned up in Greenland. Plate tectonics is the large-scale movement of the Earth's crust. New ocean floor is made by the spreading apart of these plates at ocean ridges. It was previosly thought that during Earth's early years it was too hot for this process to have occurred in the same way as it does today, and some other methods of heat loss have been suggested instead. However, the recent discovery of ophiolite sequences (ocean crust that is exposed on land) in Greenland has confirmed that the same ocean spreading senario thought to happen today also occured 2 billion years earlier than previously thought.
For further info check out: http://www.sciencemag.org/cgi/content/summary/315/5819/1650b