Follow geologists as they hunt for, pickaxe, and test rock samples from the 2.5 billion year old Huronian Supergroup, a sedimentary formation in Ontario, Canada. The scientists are in search of an exact record of how much oxygen gas Earth's developing atmosphere contained at key moments in geologic time. These crustal relics, which have interacted directly with ancient atmospheres, have the power to tell scientists when and how the Earth built up its incredible life-support system to foster more and more complex organisms.