If there's life on Mars, it would be one of the most important discoveries of all time. It would mean life on Earth was not some special unique event - it would mean there's likely to be life throughout the Universe. In the first of two special programs on Mars, Horizon explores how the search for Martians is now hotting up, and why many scientists are becoming more and more convinced that life may have arisen on Mars, and that there may even be something living there now.

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July 1998, downtown Miami, Florida. Six apartment blocks have just been demolished, to allow the construction of two brand new 40-story skyscrapers. As construction workers prepare the site, they notice a strange phenomenon in the ground - a perfectly preserved circle of large holes, almost 13 meters across. What they had stumbled upon would generate huge excitement and controversy: either they had unearthed a rare and mysterious 2,000 year old Indian site - or a 1950s septic tank? ?or an ancient inverted American Stonehenge? or a unique Mayan village in North America. For a while theories ranged far and wide. But finally, after examining the strategically-placed holes, and the range of artifacts found around the circle - stone tools, shark bones, axe heads - archaeologists began to believe that this was a genuinely unique site - the remains of a mysterious forgotten tribe called the Tequesta.

In 2000, the discovery of a tiny fossilized jawbone hit the headlines. The jawbone, only a few centimeters long with seven rows of teeth, was found abandoned in a dusty museum where it had lain unnoticed for decades. Its rediscovery has caused a sensation in the world of paleontology because scientists now believe it may be a tiny remnant of a 'missing link', an ancient extinct animal that could provide a vital clue in our understanding of one of the great mysteries of science - how, 360 million years ago, a slimy fish-like creature grew legs and walked out of the water, onto the land to become our ancestor. This mystery has taken scientists a century to unravel. And this tiny jawbone may be a final clue. According to the theory of evolution all four-limbed animals, everything from human beings to dinosaurs, are descended from one creature, the first of its kind to crawl on the Earth. Long ago, this almost mythical beast must have evolved from a fish with fins, must have developed legs and made the great evolutionary move from water to land. How and why this huge evolutionary change should have happened was the source of fierce speculation and it was always believed that one day scientists would find the fossil evidence that would explain everything.

Thirty years ago a small and rare tropical plant was put into these tanks to grow. What came out was a monster. Today it is a potential threat to seas and oceans across the world.

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There is a controversial theory that for millions of years the Earth was entirely smothered in ice, up to one kilometer thick. The temperature hovers around -40?C everywhere, even in the tropics and the equator. If it did, then virtually nothing could survive this ferocious climate. There are some tantalizing 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.

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In 2001, scientists announced an amazing discovery: the oldest skull of a human ancestor ever found. The 3? million year old fossil was remarkably complete, and unlike any previous fossil find. Its discovery - by a team led by Meave Leakey of the famous Leakey fossil-hunting family - has revolutionized our understanding of how humans evolved. The great mystery of our evolution is how an ape could have evolved into the extraordinary creature that is a human being. There has never been another animal like us on the planet. And yet ten million years ago there was no sign that humans would take over the world. Instead the Earth was dominated by the apes. More than 50 different species of ape roamed the world - ten million years ago Earth really was the planet of the apes. Three million years later, most had vanished. In their place came something clearly related to the apes, but also completely different: human beings!

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Out in deepest space lurks a force of almost unimaginable power. Explosions of extraordinary violence, are blasting through the Universe every day. If one ever struck our Solar System it would destroy our Sun and all the planets. For years no one could work out what was causing these awesome explosions. Now scientists think they have identified the culprit. It's the most extreme object ever found in the Universe; they have christened it a 'hypernova'.

Doctors Panayiotis Zavos and Severino Antinori claim they are ready to embark on the greatest human experiment of our age. They say they will attempt to clone a human being before the year is out. Most people think the objections to this are ethical - human cloning would create many moral dilemmas. There is another question that few ever ask: is the science actually ready yet for cloning healthy humans? Horizon follows the latest research, which has led many scientists to believe that Zavos and Antinori's plans to clone the first human could end in tragedy. The programme also meets couples like Matthew and Desir?e Racquer (above) who think cloning offers them the only way to raise a child who is truly their own. Dr Lorraine Young at the Roslin Institute laboratoriesFor decades, cloning remained within the realms of science fiction. The idea that instead of combining a sperm and an egg, a new human could be made from a single cell taken from an adult, seemed completely absurd. But that all changed in February 1997, when the Roslin Institute (right) introduced the world to Dolly the sheep - the first animal cloned from an adult. Ever since Dolly, scientists have been continuing to experiment with cloning animals. So far, they have succeeded in cloning sheep, cattle, pigs, goats and mice, fueling the belief that humans could be next.