Sunrise on the Serengeti, and life on the savanna is in full swing. Zebras and wildebeests graze the dewy grass; elephants and giraffes munch on acacia leaves; and lions and hyenas survey the scene, looking for their next meal. To visit this place is, in some ways, to see the world as it looked to our ancestors millions of years ago, long before humans began to wreak havoc on the planet—or so the conventional wisdom goes. Indeed, much of eastern Africa is often thought of as a pristine ecosystem, largely unchanged by our kind in the more than two million years since our genus, Homo, arose.