Biologist Ajit Varki, coauthor (with the late Danny Brower) of Denial: Self-Deception, False Beliefs and the Origins of the Human Mind, argues that the defiance of mortality is what makes us human.

You write that the evolution of human intelligence required the crossing of a “death anxiety barrier.” What do you mean by that?

The key to human intelligence is our “theory of mind,” our genetic ability to be aware of ourselves as individuals and also of the self-awareness of others. Imagine that you’re the first human with this ability, and then one of your friends dies. Because you can identify with the dying friend, you would be the first person who truly understands death and that everyone dies, and you would retreat into a fearful, anxious state.

What would the consequences be?

Terror would have made you dysfunctional in passing your genes on. You would be scared to fight another caveman for a mate. If you were a woman who had just watched another woman die in childbirth, you would say, “I don’t want that!” and you would avoid procreation. So at the same time in our evolution we had to come up with a compensating psychological trick: the ability to deny the reality that we will all die.

You contend that the primordial phenomenon of men lying about themselves to impress women was one of the key processes in that evolving denial of reality. How so?

With a theory of mind, a male will present himself as having all sorts of abilities and knowledge and resources he may not have. Conversely, the female has to use her theory of mind to be a good lie detector. To convince women of the lies they’re telling, it helps if men believe their own lies—if they evolve an ability to deny reality to themselves. So you’ve got pathological lying going on; meanwhile, you’ve got depressed people who are committing suicide.

It sounds like a Stone Age existentialist novel!

We’re not yet fully adjusted to all this. One of the theories about major depression is that depressed people are the true realists—if you really want to know the facts, talk to them. The rest of us, fortunately, are in a state of denial and optimism. What is optimism? Denial of reality. What is extreme optimism? Extreme denial of reality.

Yet you feel that the denial of reality that enabled our species to survive and flourish could now be threatening our collective future.

And at an individual level, too. We know what we are supposed to do in terms of exercise and diet, yet most of us ignore it; we have this magical way of thinking, of denying the reality we face. Look at our national debt: we just ignore it and somehow imagine it’s going to go away. Our failure to do anything about climate change is the ultimate form of denial—now, of course, that reality is staring us in the face.