A geological period in the Proterozoic eon that began about 720 million years ago and ended 635 million years ago. It was characterized by a very harsh climate and extensive glaciation, which may have covered the entire planet (the ‘snowball Earth’ theory). Life consisted of single-celled microorganisms for much of this period, but in the later stages is thought to have undergone rapid evolution, with the emergence of multicellular life forms following melting of the glaciers and the consequent upheaval in the climate.