I learned something new today. There is a good deal of evidence that 650 million years ago this planet underwent the worst ice age it has ever seen. An ice age that covered the entire planet, from pole to pole in a solid sheet of ice up to 1,000 feet thick.
We enter an ice age roughly every 100,000 years. Generally they follow the same story. The earth's orbit slowly takes it further and further away from the sun, gradually dropping temperatures globally. The polar ice caps begin to spread toward the equator, though they never get there. They always stop quite a distance away as the planet slowly returns to its position nearer to the sun. But this ice age was different. Not only was the planet further from the sun, it also, through various reasons that would take too long to explain here, was losing vast quantities of CO2 out of the atmosphere, locking it in limestone deposits beneath the ocean. CO2, a greenhouse gas, normally helps the atmosphere warm and retain heat. So much was being stripped from the atmosphere temperatures plummeted. The ice spread from the poles much faster than it ever had before, or has since.
And here's the wild part. Snow covered glacial ice is the best reflector of the suns rays on the planet, reflecting nearly all of the sun's energy back into space. Ocean water is the poorest reflector, absorbing far more heat than it reflects back. So as the glaciers spread across the planet, glacial ice replacing ocean water, the planet reflected more and more heat back into space. Eventually it reached a critical mass, creating a runaway, self-perpetuating cooling event. The less liquid water there was the less heat the planet retained cooling the planet further, creating more ice, which replaced even more of the liquid water. In no time (cosmically speaking) earth was completely locked in ice, a giant snowball hurtling around a sun that was powerless to help it.
The planet stayed like that for millions of years. Eventually volcanic activity below earth's surface, which ebbs and flows entered a very active period. Over a span of about a million years volcanoes, which emit large quantities of CO2, gave off enough of the greenhouse gas to get the planet warming again. And as more ice melted and more water was released the planet retained more heat and thawed ever faster. Eventually the ice age to end all ice ages was over.
Now here's the really cool part. Before this global freeze life had been evolving on the planet for nearly three billion years. And in that time it hadn't gotten beyond single-celled organisms. These organisms were trapped below (and in) the ice. They either had to adapt or die. Those that did survive were truly the heartiest of the hearty. The cream of the genetic coffee. When the ice finally melted the earth saw the biggest boom in evolution that had ever occurred. It helped that the chemistry involved in a global, prolonged freeze / thaw created more oxygen than had ever existed in the atmosphere before (oxygen levels in the air went from 1% to %20). The hearty survivors of the global ice holocaust, in the newly-created, oxygen rich world went wild. For the first time in three billion years multi-cellular creatures appeared. And in what is called the Cambrian Explosion, multi-cellular life...exploded. After three billion years of stagnancy, huge numbers of complex life forms begin appearing in the fossil record over a period of only 80 million years.
The global freeze had the potential to wipe out all life, such as it was, on earth. Instead it ended up kick-starting evolution, creating a massive diversity of life forms that eventually led to us. It's very possible that had this massive ice age never occurred earth would still be a planet dominated by single cell "slime" life. So give thanks to the universe's unforgiving nature. Had it been kinder to our planet we'd likely not be here.
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