Humans have been moving food around the world for thousands of years. Toward the end of the second century BC, merchants traveled along the Silk Road, transporting noodles from Xi'an, grapes from Dayuan and nutmeg from the Moluccas Islands to eager buyers along its 4,000-mile network. While it's possible to trace the evolution of food through that matrix of ancient caravan routes that linked China to the West, it's hard to measure its environmental impact. It's likely that, as with any road, wildlife corridors were disrupted. But greenhouse gas emissions were fairly low, consisting of the methane from the belches and farts of the horses, yaks and Bactrian camels, and the fires that humans burned along the way.