History Files


Sinian World

How Life Survived the Big Freeze

Adapted from a BBC News report, 25 May 2000

Scientists in 2000 put forward a theory that could explain how life survived a period of prehistory in which Earth nearly became a snowball.

Geological evidence at the time was growing to show that the planet went into a big freeze at least twice during the late Proterozoic era, 600-800 million years ago, with the polar icecaps a kilometre deep extending to the equator. What previously puzzled researchers was how such a cold and desolate environment could have prepared lifeforms for the evolutionary explosion that can be seen in the fossil record about fifty million years later.

But a new computer model suggested that there may have been gaps just big enough in the ice coverage to provide a refuge for developing life to flourish.

White landscape

The cause of the big freeze - popularly known as 'Snowball Earth' - is thought to have been a combination of a dimmer sun - by about 6% - and lower levels of the greenhouse gas carbon dioxide (CO2) in the atmosphere.

This would have lowered temperatures and allowed the polar icecaps to grow - their spread even accelerating the cooling process as more solar radiation was reflected back into space off the expanding white landscape. William Hyde from the Texas A&M University, and colleagues, tested such ideas on their coupled climate/ice-sheet computer model - with energy from the sun and carbon dioxide levels adjusted to what they could have been in the late Proterozoic era.

They found that a 'snowball' Earth was quite possible. But by playing with the variables, they also found scenarios that left the planet with an ice-free 'oasis' of uncovered ocean. This, they believed, could have provided a hold-up for lifeforms until volcanic activity had pumped sufficient CO2 into the atmosphere to raise temperatures and defrost the planet.

Testing questions

The scientists reported their findings in the journal Nature, where they noted some doubts that still needed to be addressed. One is that a liquid ocean may have soaked up carbon dioxide, preventing the big build-up necessary for the warming phase to begin.

Another is that early multicellular organisms would have required shallow sea-floor areas. Any ocean refuge would therefore have had to contain ice-free continental shelves while the rest of the world was frozen over. Identifying such sites would be a critical test of the open-water result.



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