There is a controversial theory that for millions of years the Earth was entirely smothered in ice, up to one kilometre thick. The temperature hovers around -40ºC everywhere, even in the tropics and the equator. If it did, then virtually nothing could survive this ferocious climate. There are some tantalising geological clues that show this theory may be true but the problem is, the clues and the Snowball Earth theory defy the laws of nature.

For over fifty years a group of scientists has been trying to prove this incredible period of Earth history. Struggling against scepticism and disbelief, now finally the 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. Scientists across the world are starting to believe that in the past the Earth froze over completely for ten million years... then warmed up rapidly about 600 million years ago. Almost all life was wiped out. But out of the freeze emerged the first complex creatures on Earth. Scientists now believe that the so-called Snowball Earth theory could hold the key to the evolution of complex life on this planet.

The discovery 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. Telltale signs of past glaciation have been found in places that should have been much too hot - very near the equator. 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?

Back in the 1960s one of the first climate modellers, Mikhail Budyko, stumbled on an ingenious answer. Through some simple mathematical formulae, he calculated that 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. The idea fascinated scientists, but no one thought his runaway glaciation was anything more than a theoretical result. Surely it had never actually happened on planet Earth?

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 came a series of insights and inspirations from a geologist in California, Joe Kirschvink, who came up with a brilliant solution - that 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.

From the baking landscape of Africa to ice-covered Antarctica, Horizon follows the tale of a theory which, if true, would have huge implications. Because scientists now believe this cycle of freezing and frying may have created the unique conditions needed for the evolution of complex life, including our own.