<Li> Cryogenian: The middle period in the Neoproterozoic Era: 635 - 720 Ma . </Li> <Li> Tonian: the earliest period of the Neoproterozoic Era: 720 - 1000 Ma . </Li> <Li> Mesoproterozoic: the middle era of the Proterozoic Eon, 1000 - 1600 Ma . Corresponds to "Precambrian Y" rocks of older North American geology . </Li> <Li> Paleoproterozoic: oldest era of the Proterozoic Eon, 1600 - 2500 Ma . Corresponds to "Precambrian X" rocks of older North American geology . </Li>

How did the composition of the atmosphere change during the middle precambrian