20 with global warming? Would we attempt to live healthily? Would we have children? While mental time travel has clear survival advantages, conscious foresight came to humans at an enormous price — the understanding that somewhere in the future, death awaits. Ajit Varki, a biologist at the University of California, San Diego, argues that the awareness of mortality on its own would have led evolution to a dead end. The despair would have interfered with our daily function, bringing the activities needed for survival to a stop. The only way conscious mental time travel could have arisen over the course of evolution is if it emerged together with irrational optimism. Knowledge of death had to emerge side by side with the persistent ability to picture a bright future. The capacity to envision the future relies partly on the hippocampus, a brain structure that is crucial to memory. Patients with damage to their hippocampus are unable to recollect the past, but they are also unable to construct detailed images of future scenarios. They appear to be stuck in time. The rest of us constantly move back and forth in time; we might think of a conversation we had with our spouse yesterday and then immediately of our dinner plans for later tonight. But the brain doesn't travel in time in a random fashion. It tends to engage in specific types of thoughts. We consider how well our kids will do in life, how we will obtain that sought-after job, afford that house on the hill and find perfect love. We imagine our team winning the crucial game, look forward to an enjoyable night on the town or picture a winning streak at the blackjack table. We also worry about losing loved ones, failing at our job or dying in a terrible plane crash — but research shows that most of us spend less time mulling over negative outcomes than we do over positive ones. When we do contemplate defeat and heartache, we tend to focus on how these can be avoided. Findings from a study I conducted a few years ago w