The bones were already in the sediments by the time the lake dried up. The lake contained fish that the human ancestors could have used for food. The lake was deep enough that a person could drown in it. There were no other lakes in the immediate area before the lava dammed up the river.
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 that lie on top of the layer of lava. Ttherefore, ancestors of modern humans lived in western Asia between two million and one-and-a-half million years ago.
Which one of the following is an assumption required by the argument?