Scientists have put forward a theory that could explain how life
survived a time when the Earth nearly became a snowball.
There is growing geological evidence that our 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 has puzzled researchers is how such a cold and desolate
environment could have prepared lifeforms for the evolutionary
explosion that can be seen about 50 million years later in the
But a computer model now suggests 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 is thought to have been a
combination of a dimmer Sun - by about 6% - and lower levels of the
greenhouse gas carbon dioxide 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 co-workers,
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 believe, could have provided a hold-up for lifeforms
until volcanic activity had pumped sufficient CO2 into the
atmosphere to raise temperatures and defrost the Earth.
The scientists reported their findings in the journal Nature,
where they note some questions that still need to be answered.
One is that a liquid ocean may have soaked up carbon dioxide,
preventing the big build-up necessary for the warming phase to
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.
"Identification of such sites would be a critical test of our
open-water result," they said.