This was a time considered critical in the evolutionary development of multi-celled animals and therefore the most important interval for biological evolution in general.

In a paper to be published in the May 25 edition of Nature, U of T physics professor Richard Peltier and Texas A&M oceanographers William Hyde, Thomas Crowley and Steven Baum note that the late Proterozoic era (600-800 million years ago) was the most important period of evolution for multi-cellular creatures.

However, this period was also a time in Earth's history that has come to be referred to as the Snowball Earth. At that time, the planet was thought to be completely ice-covered.

Geological and paleomagnetic evidence indicates that for alternating periods, the Earth was completely covered by ice sheets over the continents and sea ice over the oceans, followed by sudden warming trends that melted the ice.

"If the suface of the planet was covered by ice, the question arises as to how early life managed to survive under such environmental stress," says Peltier.

To find an answer, the scientists employed several different models of the climate systems and ran detailed computer simulations of the climate thought to have been characteristic of that time.

To simulate the Snowball Earth, they reduced the amount of sunlight reaching the Earth -- to account for the fact that the sun was about six per cent less luminous than it is now -- and varied the concentration of atmospheric carbon dioxide within the range expected for that time.

In most of the simulations, their analysis revealed the presence of a belt of open water near the equator when the general circulation of the ocean was taken into account.

"It is this open water that may have provided a refuge for multi-celled animals when the rest of the Earth was covered by ice and snow," Peltier explains.

The findings of this research are critical to understanding how early life evolved, he states. "This could help clarify how multi-celled animals managed not only stay alive, but to thrive given the Earth's harsh conditions. "The extreme climates may even have exerted pressure on these 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. "It was during the warm Cambrian era -- immediately following the late Proterozoic -- in which life proliferated."

The late Proterozoic period was also a time when the supercontinents Rodinia and Pannotia formed and subsequently rifted and disassembled.

Located over the south rotational pole in the position of present-day Antarctica, these supercontinents were made up of the current land masses of Africa, South America, Antarctica, Australia, Greenland, Laurentia and parts of Asia.

According to Peltier, the entry of the Earth into the snowball state required not only the weak sun and atmospheric carbon dioxide levels not significantly higher than present-day, but also this high degree of polar continentality.

Funding for this research came from the Natural Sciences and Engineering Research Council of Canada and the National Science Foundation in the U.S.

[Janet Wong is a news services officer with the Department of Public Affairs.]