Oil is one of the oldest fuel sources known to man. Its impact on the world is not simple; while it has powered the vehicles that have made human mobility possible, it has also propped up some of the most repressive regimes in recent history. In the last installment of our four-part history of energy series, we speak to journalist and author Paul Roberts to discuss the complex role that oil has played in shaping the industrialized world, and the costs/benefits that oil has as an energy source in the 21st ce