Snowball Earth Part 2: The Evidence For
with a membrane bounce nucleus). But the first eukaryote organisms have been identified in rocks dated to more than a billion years old, so why did it take so long for multi-cellular life to emerge?
A series of global freeze-thaw cycles suggested by the "Snowball Earth" theory would help to answer this problem. With each "Snowball Earth" the majority of organisms on Earth would be killed in massive extinction events. A few forms of bacteria and algae would survive in the hydrothermal vents of the deep sea. Volcanic activity at the oceanic ridges, hot spots, and island arcs would continue and provide havens from some organisms. Other organisms, called psychrophilic (cold-loving) organisms, such as ones found in the Antarctic today, would have also survived. From these surviving organisms the a new group of organisms would emerge and then subsequently die off in the next "Snowball Earth" episode. Finally, in the last episode life emerged and formed the body plans of organisms living today.
The Trigger
The evidence for a "Snowball Earth" is quite compelling, but what would have been the trigger event to cause such an extreme climatic event in the first place? First, we must look at the sun. In the Late Proterozoic the sun was about 6% weaker than it is today (over time the sun has gotten stronger, and warmer). This makes the Earth more susceptible to glaciation, but there is no geological evidence of massive glaciation before the Late Proterozoic when the sun was even cooler.
A better explanation may be the position of the continents. During the Late Proterozoic the continents were all located near the equator. When the continents are located in the lower latitudes the Earth loses an important mechanism for keeping the amount of CO2 in the atmosphere above a critical level. The presence of ice sheets on high latitude continents prevent chemical weathering from taking place, so CO2 is not converted to carbonate rock, and allows the levels of CO2 in the atmosphere to stabilize. When the continents are at the equator this mechanism doesn't work, CO2 is continually removed from the atmosphere and turned into carbonate rock even as the Earth cools and ice sheets form. Eventually the sea ice reaches the 30 degree latitude limit and the "Snowball Earth" is unavoidable.
This trigger mechanism is only a theory as there is no direct evidence, but it appears plausible based on the carbon cycle.
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