There's a popular claim that the seven billion people alive today outnumber all other humans who have ever lived. It's meant as a stark reminder of humanity's population explosion over the last 200 years...but is it true, or total crap?
This idea has gained a bit of new currency with the recent UN announcement that humanity has crossed the seven billion mark just thirteen years after we reach six billion and 25 years after we reached five billion. Considering humanity only crossed the billion person threshold around 1800 and the two billion mark in 1923, it doesn't seem totally impossible that all the people alive today might really outnumber their deceased counterparts going back to the very beginnings of humanity.
Not totally impossible...but also not correct, either. BBC News spoke to the Population Reference Bureau in Washington DC, and they estimate that about 107 billion people have been born since humanity first emerged, which they set 50,000 years ago. (That sounds like an underestimate, considering the current scientific consensus favors a date more around 200,000 years ago, but the tiny population means that even an extra 150,000 years would only tack on another few million or so.)
The key to this relatively high figure is the amount of births per thousand people per year, which today is about 23. Throughout much of history, lots of people died before reaching reproductive age, which meant many more children had to be born to keep the population growing. The Population Reference Bureau says the figure in the Middle Ages was probably at least 80 births per thousand, and it might have been significantly higher than that.
The BBC also points out that these figures actually vindicate the most famous population estimate in science fiction history - that of Arthur C. Clarke in the beginning of the novel version of 2001: A Space Odyssey, where he observes, "Behind every man now alive stand 30 ghosts, for that is the ratio by which the dead outnumber the living." While the 2012 version of that statement would have to refer to 70 ghosts, Clarke was absolutely correct when he wrote it back in 1968, when the population was 3.5 billion - and the ratio of dead to living was about 29 to 1.
All this probably means that the living will never outnumber the dead, unless humanity's population explodes by several orders of magnitude beyond what we've already experienced. For what it's worth, one path to this might involve this estimate from the UN Population Division, which notes that, should 1995 fertility rates hold constant, the world population in 2150 would be 256 billion - not that that was meant as a serious estimate.
Our one planet almost certainly couldn't actually hold 107 billion people all at once, so the only real way to prove this old chestnut true would probably be to take to the stars and colonizing other worlds...and then start breeding like rabbits, because what the hell else are you going to do once you've colonized an alien planet?