Assumptions are necessary but also tricky. Soundly based, they provide essential structure. Without them, we could take nothing for granted; all matters at every time and place would be up for grabs.

But when wrongly rooted, assumptions can create utter havoc. Consider some mistaken beliefs involving Africa, this past week’s poster continent because of a U.S. presidential visit.

When Bush stopped in Senegal on Tuesday, he touched the recent roots of many Americans. The main stop: Goree Island from whose infamous “Doorway of No Return” hundreds of thousands of Africans were shipped to the Americas in chains. Assumption at the time: that Africans represented an inferior human strain, even that slavery would somehow improve their condition. Result: chaos in West Africa then, with lingering consequences for failed states like Liberia now.

More than 10 percent of modern Americans have near-term ancestral roots in Africa. Go farther back, and the figure is 100 percent. Paleoanthropology, the science of human origins, now demonstrates – without a doubt – that humanity itself first emerged in Africa. The assumption used to be that we stemmed from somewhere more “advanced”: Asia or perhaps even Europe. Not until 1924 did a brilliant but erratic anatomist named Raymond Dart provide concrete evidence for our common African heritage. It took decades – and the debunking of England’s Piltdown hoax – but finally scholars accepted Dart’s Australopithecus Africanus as proto-human. Now each year brings news of earlier hominid discoveries – all in formerly disparaged Africa.

Sadly, Dart didn’t stop there. Having usefully disproved one mistaken assumption, he embarked on the unfortunate promotion of another – perhaps the most tragic and dangerous assumption operative today. Some of us mellow with age; not Raymond Dart. In 1953 the increasingly cantankerous fossil hunter claimed that we exist today because our remote ancestors were, in effect, “killer apes.” Our impulse towards lethal violence, Dart said, is both what set us apart and why we survived.

Arcane and isolated science? Exactly the reverse. Secretary of Defense Donald Rumsfeld may not know it – and his nominal boss George W. Bush (whom New York Times columnist Nicholas Kristoff describes as less interested in ideas than anyone he’s ever interviewed) surely does not – but this administration’s neo-con take on human nature is partly rooted in the extremely questionable “killer ape” notions of Raymond Dart.

You can see the elderly Dart in action on a masterful 1980s television series entitled The Makings of Mankind. The old fellow emerges from a South African cave armed with “the knuckle-bone of a giraffe” which, he says, “could be used as a remarkable dagger and even as a more formidable club … [with an arthritic swing] like that!” Next he brandishes a hyena jaw which “could r-r-rip up a belly.” Whether these old bones were used as weapons seems immaterial; clearly Grandpa’s having a great time. (The same, one can’t help but notice, is true for septuagenarian weapon-wielder Rumsfeld.)

Elsewhere Dart made his case with still more lurid enthusiasm: “Man’s predecessors … seized living quarries by violence, battered them to death, tore apart their broken bodies, dismembered them limb from limb, slaking their ravenous thirst with the hot blood of victims and greedily devouring living writhing flesh.” Dart concocted the term osteodontokeratic – bones, teeth, and horns – to summarize the scope of our supposedly all-important prehistoric arsenal.

Dart’s “killer ape” quickly made its way into popular discourse and then pop-culture. Dramatist-turned-author Robert Ardrey popularized the concept in his 1960 best seller African Genesis whose first paragraph has our species evolving “on a sky-swept savanna swept with menace.” Ardrey proceeds to mock “The Romantic Fallacy” and “The Illusion of Original Goodness.” His final chapter, called “Cain’s Children,” concludes, “man is a predator whose natural instinct is to kill with a weapon.” Even civilization is explained as “a compensatory consequence of our killing imperative.”

Hollywood loved the idea. Stanley Kubrick’s 2001 Space Odyssey portrays it graphically. Accompanied by the opening chords of Richard Strauss’s “Thus Spake Zarathustra,” a previously preyed-upon ape discovers bone weaponry. In the words of one (laudatory) review, “As he orgiastically smashes and pulverizes parts of the skeleton on the ground, the soundtrack bursts forth in an ecstatic, jubilant climax.” Next in line for smashing and pulverizing is the tribe next door.

Then there’s the 1968 version of One Million B.C., of interest mostly because Raquel Welsh’s animal skins cover so little of her own. The film begins with this grim narration: “A hard, unfriendly world [volcanic rocks] … creatures who sit and wait [perching vultures] … creatures who must kill to live [feeding vultures] … and Man [even by ’60s counter-culture standards, badly in need of a barber] … superior to the creatures only in their cunning [wild boar is lured into pit trap] … their laws are simple: the strong take everything [men with spears push and shove at pit’s edge for the privilege of pegging the pig].” My own review of B.C.: irredeemably a B movie despite Raquel’s C cups.

Paleoanthropology, on the other hand, has had little truck with “killer apes.” Dart’s 1953 paper, “The Predatory Transition from Ape to Man,” barely found a journal publisher. Another South African cave – Swartkrans – displayed the same jumble of hominid and other animal bones, but more careful analysis revealed that the proto-humans in question were more hunted than hunters. And the inclusion of meat in our diet almost certainly originated in scavenging rather than hunting.

What made us special, it now seems, was a mix of erect posture, large brains, and – most relevant for the issue of human nature – an ever-growing capacity for language. Verbal communication has been well on its way for hundreds of thousands of years. Indeed a wide variety of evidence now suggests that our success as a species is rooted not in competition but in language-facilitated cooperation.

So why are we in such a mess today? An obituary – also from Southern Africa – anticipates Part Two next week. “Africa’s Movie Star Bushman Dies” appeared early this month. His name was N!xau (exclamation point = click), and he starred in “The Gods Must Be Crazy,” a different sort of film with a very different theme. It portrays hunter-gatherer life, more or less as we all lived it until 10,000 years ago.

What was human nature like back then? Neo-cons echo Raymond Dart and assume that, then as now, we’re all Cain’s Children bred to the murderous bone.

N!xau’s life and death second guess the assumption of humans as essentially self-aggrandizing. Said the movie’s producer, “He went to America, to Paris, to Japan. He was a world star, but he came back and he went back to his old roots.”

And then these words to be explored next week: “Nothing that was important to us was important to him.”

Dr. Whitney Azoy, a cultural anthropologist and former U.S. diplomat in Kabul, has worked for 30 years with Afghanistan and the Muslim world. The second edition of Dr. Azoy’s study, “Buzkashi: Game and Power in Afghanistan,” is available from Waveland Press.