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Imagine a world frozen solid. Vast sheets of ice hundreds of feet thick encase the entire planet, a giant snowball floating through space. This is not some exotic new world deep in space. It’s our world, the Earth, 650 million years ago.

Scientists are starting to believe that in the past the Earth froze over completely for millions of years... then warmed up rapidly over 600 million years ago. Struggling against scepticism and disbelief, now many mysteries have been solved and the scientific community is slowly coming around to the extraordinary idea not just of the dramatic freeze, but of an equally dramatic thaw. Naked Science explores the most extreme period of Earth’s climatic history, meeting all the major players of the controversial theory that is Snowball Earth.

The development of this theory is a classic scientific detective story. For decades there had been a growing 'X-File' of geological anomalies haunting the scientific community. Alien rocks known as ‘dropstones’, telltale signs of past glaciations, have been found in places that should have been far too hot for ice. Tim Raub, a geologist at Caltech University, is in the arid scrub land of Flinders Range National Park, South Australia. Not only is this a strange place to be looking for evidence of glaciation, but Raub’s work suggests that this part of South Australia was even closer to the equator 650 million years ago. Even during the most severe ice age, scientists believed that the ice only reached as far down as Northern Europe and the middle of the USA. So what could these tropical deposits mean?

For Paul Hoffman of Harvard University, the evidence could only mean one thing. If ice was at the equator, then the entire planet must have been covered in ice and snow. He points back to research done by one of the first climate modelers, Mikhail Budyko, which showed that theoretically, if the polar ice caps had spread past a crucial point, a runaway freezing process would have followed, eventually freezing over the whole of the planet. Hoffman believes the dropstones are proof this catastrophic scenario is not just theoretical, but scientific fact it happened deep in Earth’s history.

But there still remained a major problem for the Snowball Earth theory. The idea foundered because according to the model, once the Earth was frozen there was no way out - the Earth would remain frozen forever. The big freeze would wipe out all life; we would not exist today. It seemed patently absurd. But then a series of insights and inspirations from a geologist in California, Joe Kirschvink, who came up with a brilliant solution - volcanoes, protruding above the frozen landscape, would have carried on pumping out carbon dioxide, the greenhouse gas, even though the world had entered the deep freeze. On Snowball Earth there was no rain to wash this carbon dioxide out of the atmosphere. Instead it would have built up to higher and higher concentrations - until eventually it sparked off not just global warming but global meltdown.

It was this period of catastrophic global warming that some scientists believe could hold the key to the evolution of complex life on this planet. Shuhai Xiao of Virginia Tech found animal embryos in South China dating just 3 million years after the end of the Snowball event and in South Australia, Jim Gehling shows the many fossils of wonderful, complex creatures that appeared shortly after and have never been seen before or since. Could it be that this cycle of freezing and frying created the unique conditions needed for the evolution of complex life, including our own?