There's a cute statistical argument for predicting when the End of Days will occur, it goes something like this. Suppose you reach into my pocket and pull out some balls. Oi, no giggling, this is serious maths. I've got a number of balls in my pocket, and the balls are numbered 1, 2, 3, etc. up to the number of balls. If I have ten balls they're numbered one to ten, if I have a thousand balls they're numbered one to a thousand, if I have a billion balls then I have big pockets.
I don't tell you how many balls I have, but I will let you take three of my balls. So you dive in and pluck out three balls, and they have on them three numbers, for argument's sake let's say they're numbered 6, 7, and 46. Your task is to estimate how many balls I had in my pocket to start with. Clearly there must have been at least 46 since you found the 46th ball. But presumably not lots more than 46. If there were, say, a billion balls then it'd be incredibly strange that all the ones you found were so low in number. A statistical argument suggests that it's most likely there were about 60 balls given these three draws. If you're interested I used Wolfram Alpha to simulate the experiment and get those numbers, and I simulated it with 56 balls. Not bad, eh?
What's this got to do with the End of Days? Well replace balls with humans. Every human that's ever lived and that will ever live was/will be born in some order, so let's dispense with these illogical “names” we all have and just give everyone a number based on when we were born, a kind of human serial number. The first ever mutant homo sapiens sapiens to pop out of a startled homo sapiens idaltu will be number 1. You and I are numbered somewhere around 100 billion. And so on. Now suppose humanity, as a species, somehow manages to survive everything that nature, the Universe, and itself can throw at it. Suppose that humanity survives for billions more years, colonises the galaxy, and eventually numbers in the trillions. By the time we reach that stage the total number of humans that have ever lived will be in the quadrillions, maybe the quintillions. That's a lot of illions. If that's the case then the chances of me, number 100 billion, being alive right now is vanishingly small. Like me having a thousand numbered balls in my pocket and you picking out three balls numbered less than 1.
One explanation for this is that you and I are statistical anomalies, freaks living on the very edge of the bell curve. We're a hundred fair coin tosses coming up as a hundred heads: utterly implausible but not absolutely impossible. Another explanation is that we're nothing special. We were born at a statistically typical time. If that's the case then we can use the same maths as with the balls-in-my-pocket situation to determine that there won't be quadrillions of humans in the future, in fact the human race will die out in about 10 000 years sometime around the birth of human number 1.2 trillion.
It's an interesting argument because it's obviously totally wrong, but it's not difficult to learn enough maths to corroborate the argument, while spotting the problem with the statistics is rather more tricky. Still, if you believe the argument then you can accept that, roughly speaking, there are currently ten billion living humans, a hundred billion dead humans, and a thousand billion humans yet to be born. This QI Book of the Dead is a work of non-fiction so looks at a few dozen of those hundred billion dead humans. (Books about the humans that haven't been born yet are called “fiction”. Books about still living humans straddle the line between the two genres.)
The brief biographies get a little samey (kid overcomes adversity to become amazing adult – if they're male then they're probably gay) but it's not the authors' fault if we humans tend to fall into the same patterns over and over. Besides, the book is best dipped into sporadically rather than devoured in a single sitting. And (quite) interesting though it is, it's really a gateway book. You know the kind of thing. You're at a party one night and suddenly someone starts passing it around. “Go on,” they whisper, “just give it a little try.” Peer pressure conquers all so you take in a few pages, trying not to inhale. Then suddenly: BANG. You're reading a thousand page treatise on Ignácz Trebitsch Lincoln and a Richard Feynmann book on quantum electrodynamics. You have been warned.