A note on human population, from The Economist

"Demographers like to dramatize this recent population growth by asking a spooky question. Of all the people who have ever lived, how many are alive today? The answer requires a lot of guesswork, except for the very recent past; but a fair estimate for the number of people born throughout human history is 80 billion - 100 billion. With mankind now numbering 6 billion, the astonishing number must be: 6-7%. The figures are even more spectacular if you count man-years lived rather than people, because life for early man was usually short: at birth, he could expect 20 years of it in 10000BC, only 27 as late as 1750AD, and 58 today. On that reckoning, those alive today account for one-sixth of the time that humans have collectively spent on earth."
--From The Economist, 31 December 1999