NOVA reports on the different ways scientists explain how life emerged on Earth. The program relates the discovery of extremophiles—bacteria that thrive in harsh subterranean and deep ocean environments similar to those believed to have existed on primitive Earth and describes an attempt to determine when life began on Earth by searching rock formations in Greenland for higher-than-expected ratios of carbon 12 to carbon 13 (in ratios currently only known to be created by life processes).