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 fossil record.
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.
White landscape
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.
Testing questions
The scientists report 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 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.
"Identification of such sites would be a critical test of our open-water result," they say.