Glaring white ice 300 feet deep covered all the oceans. Temperatures dropped to minus-20 degrees. The land was barren, dry, frigid, lifeless.

That was the Earth 750 million years ago in what may have been the planet's coldest and longest ice age. But it may also have been a vital period in the evolution of plants, animals and eventually even people, a researcher says."Without these ice events, it is possible there wouldn't be any animals or higher plants," said Paul F. Hoffman, a Harvard University researcher and co-author of a study to be published Friday in the journal Science.

The research supports a six-year-old theory that the Earth was a "snowball planet" during a Precambrian era more than 570 million years ago. That era ended with the sudden appearance of complex new life forms.

This bloom of new species is considered by many to have been a key event in the evolution that helped create a temperate planet and led, millions of years later, to the appearance of humans.

Chemical and isotopic analysis of rocks laid down along the coast of an island that later became part of Africa shows that, between 750 million and 570 million years ago, the Earth went through at least four deep ice ages, each lasting millions of years, Hoffman said.

During those periods of ice - perhaps the coldest the Earth has ever experienced - the oceans froze, creating a planetwide surface of white. This icy glare reflected the sun's heat, causing the planet to get even colder. Continents, said Hoffman, probably were in a dry, cold soak.

"Once the seas froze over, there was no more evaporation," he said. "There was no more snow or rain." The ancient ice ages ended when carbon dioxide, belched from volcanoes, became concentrated enough in the atmosphere - about 350 times the present concentration - to create a super greenhouse effect. The carbon dioxide trapped enough solar heat to melt the frozen oceans.