A Short History of Nearly Everything by Bill Bryson (excerpt from book) "If your two parents hadn’t bonded when they did, possibly to the second, possibly to the nanosecond, you wouldn’t be here. And if their parents hadn’t bonded in a precisely timely manner, you wouldn’t be here either. And if their parents hadn’t done likewise, their parents before them, and so on, obviously and indefinitely, you wouldn’t be here. Push backwards through time and these ancestral debts begin to add up. Go back eight generations to about the time that Charles Darwin and Abraham Lincoln were born, and already there are 250 people on whose timely coupling your existence depends. Continue further, to the time of … the Mayflower Pilgrims, and you have no fewer than 16,834 ancestors earnestly exchanging genetic material in a way that would, eventually and miraculously, result in you."