If the anthropocene were to become an official epoch, it would be the first time a geological boundary could be witnessed by scientifically literate human beings. But first, scientists must come up with a start date. Some argue it began in the Industrial Revolution, when factories caused an increase in carbon emissions. Others have said it began earlier than that, when agriculture caused widespread deforestation. But William Ruddiman, a paleoclimatologist at the University of Virginia, thinks it goes back much farther than that. “[The atom bomb] is an important marker in human history, b...