Humans appear to have done a thorough job of contaminating the Earth’s rivers, oceans and atmosphere, says Rachel Nuwer. Is there anywhere pristine left on the planet? Somewhere between 1.8 million and 12,000 years ago, our ancestors mastered the craft of fire building. Anthropologists often cite this event as the spark that truly allowed us to become human, giving us the means to cook, keep warm and forge tools. […]