From a New York Times guest essay by William MacAskill headlined “The Case for Longtermism”:

 William MacAskill is a professor of philosophy at Oxford University and the author of “What We Owe the Future,” from which this essay has been adapted.

Imagine living the life of every human being who has ever existed — in order of birth.

Your first life begins about 300,000 years ago in Africa. After living that life and dying, you travel back in time to be reincarnated as the second-ever person, born slightly later than the first, then the third-ever person, and so on.