Hard is relative. For example, the filling of the atmosphere with a powerful oxidizer was a major extinction event (the "Oxygen Catastrophe"), yet we do not fin

Hard is relative. For example, the filling of the atmosphere with a powerful oxidizer was a major extinction event (the "Oxygen Catastrophe"), yet we do not find it extreme because we are adapted to it. Even environments that are energy-poor, or cold enough as to inhibit chemical reactions, may support rich forms of life that operate on larger time scales.