Richard Dawkins - The sheer magnitude of geological time
If you hold out your hand to represent the length of geological time and you say the middle of my tie is the origin of life and the tip of my finger is the present then the dinosaurs, which went extinct about 65 million years ago, lived about there (palm). Most of this (tie to wrist) is bacteria. You have multicellular life evolving about here (start of wrist), dinosaurs about here (palm), humans at my fingernail, and the whole of recorded human history, everything from the Egyptians, biblical times, the Romans, the Assyrians, the Greeks, all of human history disappears in the dust from one stroke of a nail file. That's the scale of human history, it's the dust from one stroke of a nail file on the same scale as the time that has been available for evolution.
http://youtube.com/watch?v=qR_z85O0P2M