LONDON (Reuters) - Canadian and American scientists think they may have found the answer to how life survived on Earth about 600 million years ago when the planet resembled a giant snowball.

They used calculations and climate models to show there may have been areas of open water that allowed early creatures to live while the rest of the planet was in a deep freeze.

``This could help clarify how multi-celled animals managed not only to stay alive but to thrive given the Earth's harsh conditions,'' Richard Peltier, a physics professor at the University of Toronto, said in a statement.

Scientists believe that about 600-800 million years ago, the Earth was covered by ice and then a sudden warming period thawed the deep freeze and led to an important period of evolution.

But they were puzzled by how life could have survived and then thrived under such harsh conditions.

Peltier and colleagues William Hyde, Thomas Crowley and Steven Baum of Texas A&M University said their theory, reported in the science journal Nature, could explain it.

They believe the harsh climates could have given the creatures an evolutionary push.

``The extreme climates may even have exerted pressure on the multi-celled animals to evolve and adapt, possibly leading to the rapid development of new forms of animals and their movement into new, unpopulated habitats when the Earth exited the snowball state,'' said Peltier.

The researchers used computer simulations of what are thought to have been the climate characteristics at the time and took into account less sunlight and the varied concentrations of atmospheric carbon dioxide.