Researchers analysis the chemistry of ancient deposits in China and Japan concluded that a space rock three to seven miles across smashed into the earth about 251 million years ago, the time of the Permian-triassic extinction event. The study appears Friday in the hournal Science. In what has been called "The Great Dying", 90 per cent of all ocean species and 70 per cent of all land species vanished within a short period of time----a key event int he history of life on earth.