Sentence:  That the habitability of our planet was severely endangered by the significant rise in oxygen levels and reduction in methane due to oxygen-producing bacteria combined with the low luminosity of the Sun in the Proterozoic era (2500– 542 million years ago) is suggested by geological evidence for severe global <WSD>glaciations</WSD> in this epoch. 
Meaning of the word marked with <WSD> tags:  The process of covering with a glacier, or the state of being glaciated; the production of glacial phenomena; an ice age