Blam posted a story a couple weeks ago, in which it was theorized that pieces of comets hitting the earth may have been responsible for the onset of such historical eras as the dark ages, the end of the great Egyptian civilazations, etc.
The scene I was referring to occurs after the aliens have conquered most of Africa. They're puzzling over a spear, trying to throw it with their trunks, convinced that it could never be thrown straight despite what the native humans say. They finally ask one of the Africans there to prove it; the old man picks up the spear, turns, and promptly throws it into the side of one of the aliens, (eventually) killing him, thus proving that not only could humans throw the spear straight, they could throw it with deadly force as well.
My favorite dumb alien/smart human book. :)