Wall Street Journal: At the end of the last ice age, about 11,000 years ago, northern Africa became a grassland, home to fish, elephants and thousands of people. Then, 5,000 years ago, scientists say, it abruptly turned into an enormous wasteland--the vast desert that we now call the Sahara. The shift from savanna to sand was the result of natural climate change, triggered by a cyclical alteration in the sun's orbit. The decline in rainfall pushed residents south and east and perhaps contributed to the rise of the Egyptian...