Signs of human impact on the planet are everywhere. Sea levels are rising as ice at both poles melts; plastic waste clogs the ocean; urban sprawl paves over landscapes while industrial agricultural empties aquifers. Between climate change, urban development, and straight-up, old-school pollution, the Earth we inhabit now would be scarcely recognizable to our earliest ancestors 150,000 year ago. But even that doesn't go back nearly far enough, according to the most recent research. William Ruddiman, a paleoclimatologist at the University of Virginia, thinks we should look for the start of t...