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THE UNLUCKIEST dog in history died on 28 June, 1911 in the town of Nakhla, Egypt. According to onlookers, it was struck by a rock from outer space, part of a meteor shower that peppered the region. Despite the dog's death, the Nakhla meteorite was a cause for celebration, because it was to play a major role in the story of extra-terrestrial science.

The Nakhla meteorite belonged to an extremely rare class of meteorites which contain igneous rock. This suggests that they originate from volcanoes, which in turn means that they originate from a planet. In comparison, the vast majority of meteorites are leftovers from the creation of the solar system and, prior to hitting Earth, have nothing to do with planets.

Some scientists argued that the Nakhla meteorite had come from Venus or Mars, while others believed that it was a piece of Earth rock that had been jettisoned into space during a devastating asteroid impact, returning to the planet millions of years later. The mystery of the meteorite's origin was eventually solved in 1982, when the Nasa scientist Donald Bogard analysed tiny bubbles of gas trapped within another igneous meteorite. The ratio of elements matched perfectly the ratio measured by the Viking spacecrafts that landed on Mars in 1976 and tested the Martian atmosphere. This strongly implied that the Nakhla meteorite was from Mars.

Martian meteorites hit the headlines in 1996 when Nasa scientists claimed to have found evidence for life on Mars. After analysing a meteorite found in Antarctica, known as ALH84001, they believed that they had discovered bacteria fossils. There was no consensus, though, as to whether or not the sausage-shaped nodules really represented Martian life, and so scientists began to reexamine other Martian meteorites. The Natural History Museum in London has a sample of the Nakhla meteorite, and in 1998 it distributed minute fragments to research groups around the world, so that they could look for more signs of Martian life.

Earlier this year, David McKay, who had been involved in the original 1996 Nasa announcement, claimed to have found bacteria fossils in the Nakhla meteorite. His discovery remains controversial, but if he is right then the implications are enormous. Although the ALH84001 meteorite is 4 billion years old, the Nakhla meteorite is only 1.3 billion years old. If there was life on Mars just a billion years ago, then there is a much greater likelihood that there is still life on Mars. Furthermore, if there are signs of life in the Nakhla meteorite, then the tragic event that took place in 1911 would be the first time that a Martian killed an Earthling.

'The Code Book - the Science of Secrecy from Ancient Egypt to Quantum Cryptography' by Simon Singh is published this week by Fourth Estate