Humans are born time travelers. We may not be able to send our bodies into the past or the future, at least not yet, but we can send our minds. We can relive events that happened long ago or envision ourselves in the future. New studies suggest that the two directions of temporal travel are intimately entwined in the human brain. A number of psychologists argue that re-experiencing the past evolved in our ancestors as a way to plan for the future and that the rise of mental time travel was crucial to our species’ success. But some experts on animal behavior do not think we are unique in this respect. They point to several recent experiments suggesting that animals can visit the past and future as well.