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.
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.