The appearance of life on Earth seems to face so many obstacles — sourcing the right ingredients, for example, and arranging them into living things (while being bombarded by meteorites) — that scientists often feel forced to regard it as almost miraculous. Now two US researchers suggest that, on the contrary, it may have been inevitable.They argue that life was the necessary consequence of available energy built up by geological processes on the early Earth. Life sprang from this environment, they say, in the same way that lightning relieves the accumulation of electrical charge in thunderclouds.In other words, say biologist Harold Morowitz of George Mason University in Fairfax, Virginia, and physicist Eric Smith of New Mexico's Santa Fe Institute, the geological environment "forced life into existence".This view implies not only that life had to emerge on the Earth, but that the same would happen on any similar planet. Smith and Morowitz hope ultimately to predict the first steps in the origin of life based on the laws of physics and chemistry alone.Morowitz and Smith admit that they don't yet have the theoretical tools to clinch their arguments, or to show what form this "inevitable life" must take. But, they argue, it probably used the same chemical processes that now drive our own metabolism — but in reverse.One source of geological energy would have been compounds called polyphosphates, made in volcanic processes. These are 'battery molecules', similar to those that now supply living cells with energy.