Friday, August 28, 1998 Published at 10:30 GMT 11:30 UK
Earth's huge 'snowball event'
Ice covered the earth 750 million years ago
Today global warming is one of the world's biggest environmental problems. But scientists in the United States are now proposing that life as we know it owes its existence to a similar greenhouse effect occurring millions of years ago.
A research team from Harvard University in Massachusetts has proposed that a fall in the level of carbon dioxide in the earth's atmosphere 750 million years ago caused a drop in global temperatures which came close to wiping out all life on earth.
The entire earth became covered with snow, which reflected the sun's heat and further accelerated the process of freezing - a phenomenon which has been dubbed a "snowball event".
Writing in Science magazine, they say that levels of certain carbon isotopes in the rocks suggest almost a complete halt in biological activity during the period when the earth was frozen.
There may have been four periods of global glaciation between 750 and 570 million years ago. These ice ages may be the coldest the earth has ever experienced.
At the time life was very primitive, just single-celled creatures. After the glaciations only a relatively small number of microbes would have survived, the scientists say.
During the glaciations the earth remained covered in ice for 10 million years, until volcanic activity released more carbon dioxide into the atmosphere.
The result was a greenhouse effect which raised global temperatures again.
The ice ages, especially the one 570 million years ago could have given evolution a boost and allowed the start of the evolution of the wide variety of animals known today.
'Evolution of new life forms'
He believes the new theory explains why higher animal and plant life evolved at such a late stage of geological history.
Simple living organisms were present on earth three billion years before animals emerged, but it took the profound environmental changes that signalled the end of a snowball period to provide the conditions for the evolution of new life forms.
He explains that in terms of evolutionary biology, a particular set of circumstances is needed to explain the boost evolution received 570 million years ago.
These may include a crash in the population, followed by repopulation a in different set of circumstances, which allow natural selection to take place governed by a different set of environmental stresses.
"A succession of snowball glaciations each lasting millions of years, and then by rapid recovery under a greenhouse situation, it seems to be just what the biologist ordered," Mr Hoffman added.