Life originated more than 3.4 billion years ago, when the earth's environment was much different than that of today. Especially important was the lack of significant amounts of free oxygen in the atmosphere. Experiments have shown that rather complicated organic molecules, including amino acids, can arise spontaneously under conditions that are believed to simulate the earth's primitive environment. Concentration of such molecules evidently led to the synthesis of active chemical groupings of molecules, such as proteins, and eventually to interactions among chemical compounds. A rudimentary genetic system eventually arose and was elaborated by natural selection into the complicated mechanisms of inheritance known today. The earliest organisms must have fed on nonliving organic compounds, but chemical and solar energy sources were soon tapped. Photosynthesis freed organisms from their dependence on organic compounds and also released oxygen so the atmosphere and oceans gradually became more hospitable to advanced life forms.