HAPPY BIRTHDAY?
Soon the United Nations estimates, the world’s population will reach seven billion. Because censuses are infrequent and incomplete, no one knows the precise date — the Census Bureau puts it somewhere next March — but there can be no doubt that humanity is approaching a milestone.
The first billion people accumulated over a leisurely interval, from the origins of humans hundreds of thousands of years ago to the early 1800s.
Adding the second took another 120 or so years.
