You have just invented a time machine and you want to investigate the past (the dream of every paleontologist!). When is the earliest time in Earth history that you could go back and not have to have an oxygen tank so that you could breathe normally?

Question

Quizplus · Accepted Answer

The Cambrian Period is the earliest time in Earth history that you could go back and not have to have an oxygen tank so that you could breathe normally. The atmosphere during the Cambrian Period had about 12% oxygen, which is enough for humans to breathe.

You Have Just Invented a Time Machine and You Want