Countdown to 7 billion people on Earth  This year marks the
seventh "billion-person" milestone in the planet's history. On
or around Oct. 31, 2011, the world's 7 billionth person will be
born, the United Nation estimates.  Even more staggering is that
of the 7 billion people on Earth, about 1.4 billion of them will
be old enough to have observed the arrivals of the 6 billionth,
5 billionth, 4 billionth and 3 billionth people in the world.
About 42.5 million people could have blown the party horn for
the birth of the 2 billionth baby.  Yes, population has risen
very quickly over the last century. Demographers do expect a
decline in the population growth rate, but absolute numbers will
continue to rise, likely hitting 9 billion by 2050. Meanwhile,
we look back at history's past population milestones, asking:
"How has the world changed?"  The 1 billionth baby  The world's
first billion-person milestone was a long time coming. Estimates
of historical populations can be rough, but the U.S. Census
Bureau pegs the g