About two million years ago, lava dammed up a river in western Asia and caused a small lake to form. The lake existed for about half a million years. Bones of an early human ancestor were recently found in the ancient lake-bottom sediments on top of the layer of lava. Therefore, ancestors of modern humans lived in Western Asia between 2 million and 1.5 million years ago.

Which one of the following is an assumption required by the argument?

(A) There were not other lakes in the immediate area before the lava dammed up the river.

(B) The lake contained fish that the human ancestors could have used for food.

(C) The lava under the lake-bottom sediments did not contain any human fossil remains.

(D) The lake was deep enough that a person could drown in it.

(E) The bones were already in the sediments by the time the lake disappeared.