Until about 75,000 years ago—that is, for the majority of our existence on Earth—humankind was restricted to Africa, though we sent out occasional forays into the rest of the world, almost all unsuccessful, all limited in scope. Around 70,000 years ago, everything changed. People raced across the continents like so many imported fire ants. Human footprints appeared in Australia within as few as ten thousand years, perhaps within four or five thousand. Stay-at-home Homo sapiens 1.0, a wallflower that would never have interested Lynn Margulis, had been replaced by aggressively expansive Homo
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