Both petroleum and coal are made up of complex carbon-based molecules, and both originated with living creatures of some kind. Both are vital sources of energy for the modern world and both were formed by geologic processes over millions of years. However, petroleum was mainly formed from the remains of ocean-dwelling microorganisms. Coal, on the other hand, originated from decayed vegetation in ancient swamps and bogs. In any case, it took millions of years for both coal and oil to be produced. This is the case because it took that much time for overlying sediments to produce the unimaginable heat and pressure that would one day allow us to harvest these energy resources. "However" is one clue word or phrase in the passage that indicates a contrast.