The Proterozoic was an eon early in the Earth's history, visitable in the Doctor Who Expansion. It occurred between
2,500 and 542 million years ago.
The great oxygenation of Earth's atmosphere occurred during this period. However, levels were approximately a hundredth of those in the current time. Multicellular life developed here.
The atmosphere means that unprotected characters entering this time period will suffer 40 atmospheric damage per turn.