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Denial, as the old joke goes, is not just a river in Egypt. It's proof of modern society's deepest sins: delusion, repression, irrationality, cynicism and manipulation. It's the failure to be able to tell fact from fiction, and, worse, the devastating failure to know oneself. Deniers are the ones who can't face the fact that they're alcoholics, wife beaters or anti-Semites, the ones who insist that everyone else is wrong even when the weight of evidence is crushingly against them.

But to Ajit Varki, an oncologist and specialist in human origins, and Danny Brower, a geneticist who died in 2007, denial is something altogether different: a prerequisite for human intelligence.

That startling premise is explored in their book Denial: Self-Deception, False Beliefs, and the Origins of the Human Mind.

The existence of the book was unlikely in the first place: In a phone interview from San Diego, Calif., Dr. Varki explains that he met Prof. Brower only once, by chance, for a single hour in 2005.

Dr. Varki had given a lecture on the molecular differences from chimpanzees that might have made humans unique, when Prof. Brower pigeonholed him and told him that he was asking the wrong question.

Instead of asking what makes humans unique, Prof. Brower said, why not ask why other smart, self-aware animals – elephants, apes, dolphins, whales, even magpies – don't match humans in intelligence, even though many have been on Earth far longer? What held them back?

Dr. Varki went home and thought about the question. Two years later, he sent Prof. Brower a long e-mail, but he got no answer back – Prof. Brower had died unexpectedly of a rare blood-vessel disease, leaving an unfinished manuscript of a book about his idea. Eventually, Prof. Brower's widow encouraged Dr. Varki to complete the book.

Denial's basic idea is that the big breakthrough came when two things happened simultaneously.

First, humans became aware not only of their own minds but also of others' – they were able to put themselves in someone else's shoes and to imagine what even people they had never met might be thinking. We can do it with fictional characters as well, picturing how, say, Homer Simpson might react to this article.

Psychologists call this having a full theory of mind. It manifests itself in uniquely human activities such as teaching, torture, romantic infatuation, organized sports, grandmothering, cuisine and even blushing. That human superpower can lead to a Mother Teresa or to a Hitler, Dr. Varki points out.

Many animals show some self-awareness. "We've got all these very smart animals and birds that are right on the brink of understanding," Dr. Varki says. Dolphins, chimpanzees, orangutans, elephants and maybe some birds can recognize themselves in mirrors.

Some even react to another's death. They may mourn that a relative or companion is no longer there, but they don't appreciate that another mind was there and has gone. For along with the advanced ability to understand others, Dr. Varki and Prof. Brower argue, comes the crippling knowledge that one will die.

They speculate that over the course of evolutionary history, many individual elephants, apes, dolphins or early humans might have made it to that point, and then become so traumatized by the knowledge that they died out before passing along their genes. Perhaps that meant falling into deep depression or hiding away in the woods.

And so comes the second necessary breakthrough – the ability to deny reality, including the reality of death: I know that I'm going to die but carry on as though I'm immortal, meanwhile spreading my DNA around like nobody's business.

The fusion of these two developments spread like wildfire through the several thousand anatomically modern humans who lived, say, 100,000 years ago. This mind-over-reality theory is one explanation for why we are the only surviving human species, Dr. Varki said. (The book describes the process as a kind of "intellectual arms race.")

Even today, Dr. Varki speculates, it might be that one cause of depression is that a person loses the ability to deny death, making life seem futile. After facing the hell of reality straight on, the depressed person cannot cope any more.

Like the radical notion that the Earth revolves around the sun, that species evolve through natural selection or that the Earth's crust is made up of moving tectonic plates, this one seems questionable at this point in its life cycle. Those theories were eventually verified, but there is no way to prove this one yet, Dr. Varki admits.

But while the book explains the value of denial, Dr. Varki's driving concern is for us to temper it with a dose of reality.

His worry is that we are using our powers of denial to ignore what he calls "local climate destabilization," also known as global warming.

Most types of denial, he says – about high national debt loads, or eating too much red meat or smoking cigarettes or refusing to wear seatbelts – aren't fatal to the entire species. Climate destabilization, like a nuclear holocaust, could be.

In fact, if his theory is correct, it might also explain why we haven't found intelligent life on other planets: Perhaps life there evolved much as it did here – depending on creatures that became fully aware of other minds and their own mortality, developed the denial mechanism and then blithely wiped themselves out, taking everything else along for the ride.

Denial, our secret weapon, could turn out to be too much of a good thing.