Dwarfing all other mountain ranges, the Himalayas are the single most impressive feature on the Earth's surface. To get that way has taken 40 million years of mountain building. In the process, the Himalayas have also dragged world temperatures to the lowest since complex life evolved. And that may have triggered a new bout of evolution. Gabrielle Walker investigates the connection between climate, the rise of the Himalayas, and the ascent of man.