including animals. Organism size and complexity increased considerably after the end of the snowball glaciations. This development of multicellular organisms may have been the result of increased evolutionary pressures resulting from multiple icehouse-hothouse cycles; in this sense, snowball Earth episodes may have "pumped" evolution. Alternatively, fluctuating nutrient levels and rising oxygen may have played a part. Another major glacial episode may have ended just a few million years before the Cambrian explosion. One hypothesis which has been gaining currency in recent years: that early snowball Earths did not so much affect the evolution of life on Earth as result from it. In fact the two hypotheses are not mutually exclusive. The idea is that Earth's life forms affect the global carbon cycle and so major evolutionary events alter the carbon cycle, redistributing carbon within various reservoirs within the biosphere system and in the process temporarily lowering the atmospheric (greenhouse) carbon reservoir until the revised biosphere system settled into a new state. The cool period of the Huronian glaciation is speculated to be linked to the decline of greenhouse gases during the Great Oxidation Event. Similarly, the possible snowball Earth of the Precambrian's Cryogenian between 580 and 850 million years ago (and which itself had a number of distinct episodes) could be related to the rise of more advanced multicellular animal life and life's colonization of the land. However, a very recent study, based on findings of previous studies, suggested land plant evolution was driven by the Cryogenian glaciations, which they also theorized to be the reason why the Zygnematophyceae (sister group of land plants) became unicellular and cryophilic, lost their flagella and evolved sexual conjugation. Effects on ocean circulation Global ice cover, if it existed, may—in concert with geothermal heating—have led to a lively, well mixed ocean with great vertical convective circulation. Occurrence and timing Neoproterozoic There were three or four significant ice ages during the late Neoproterozoic. Of these, the Marinoan was the most significant, and the Sturtian glaciations were also truly widespread. Even the leading snowball proponent Hoffman agrees that the 350 thousand-year-long Gaskiers glaciation did not lead to global glaciation, although it was probably as intense as the late Ordovician glaciation. The status of the Kaigas "glaciation" or "cooling event" is currently unclear; some scientists do not recognise it as a glacial, others suspect that it may reflect poorly dated strata of Sturtian association, and others believe