Proterozoic the Earth was an anaerobic environment Proterozoic Paleoproterozoic Paleoproterozoic microfossils Proterozoic Era Proterozoic Proterozoico "The Proterozoic Eon" Proterozoic Mesoproterozoic Proterozoico the Earth was an anaerobic environment http://www.ucmp.berkeley.edu/precambrian/proterozoic.php http://www.ucmp.berkeley.edu/precambrian/proterozoic.php Precambrian, late proterozoic Proterozoic "The Proterozoic Eon" Proterozoic eon 1.6 billion years ago 1.6 billion years ago the Earth was an anaerobic environment