[The following text is from Reason for Hope: A Spiritual Journey by Jane Goodall with Phillip Berman, chapter 9: "Precursors to War". Reprinted by permisson of the publisher. New York: Warner Books, 1999]
It was both fascinating and appalling to learn that chimpanzees were capable of hostile and territorial behavior that was not unlike certain forms of primitive human warfare. War had always seemed to me to be a purely human behavior. Accounts of warlike behavior date back to the very first written records of human history; it seemed to be an almost universal characteristic of human groups. Wars have been fought over a wide range of issues, including culturally and intellectually determined ideological ones. They have functioned, at least ecologically, to secure living space and adequate resources for the victors. To some extent too, they have served to reduce population levels, thus conserving natural resources.
Moreover, as Darwin pointed out, warfare in prehistoric times, since it involved conflict between groups, rather than between individuals, must have put considerable selective pressure on the development of increasingly sophisticated cooperation among group members. Communication skills would also have been crucial: the emergence of a complex spoken language would have given the greatest advantage of all. Qualities of intelligence, courage, and altruism would have been highly valued and the best warriors may well have more women and fathered more offspring than cowardly and less skilled group members. This process would escalate, because the greater the intelligence, cooperation, and courage of one group, the greater the demands placed on its enemies. It has actually been suggested that warfare may have been the principle evolutionary pressure that created the huge gap between the human brain and that of our closest living relatives, the anthropoid apes. Whole groups of hominids with inferior brains could not win wars and were therefore exterminated.
Certainly the first true humans were unique by virtue of their large brains. It was because the human brain is so large when compared with that of a chimpanzee that paleontologists for years hunted for a half-ape, half-human skeleton that would provide a fossil link between the human and the ape. In fact, this so-called "missing link" is surely comprised of a series of vanished brains, each more complex than the one that came before it. These brains, alas, have forever been lost to science, save for a few faint imprints on fossil craniums.