Is Mars habitable? In 1907 Alfred Russel Wallace (1823-1913), co-discoverer of the theory of evolution, answered the question in his book of the same title. He concluded that Earth was the only planet in our solar system capable of supporting life and that the probability of the necessary conditions existing elsewhere in the universe were extremely small.
His book followed claims made by a US astronomer, Percival Lowell (1855-1916), that canals on the surface of Mars were evidence of intelligent life. Lowell was convinced Mars could support a temperate climate and was probably home to colonies of intelligent life.
Wallace dismissed the idea. He said Mars was too cold, and its small mass meant that it could not retain an atmosphere.
Almost a hundred years later, the debate about life on other planets is as strong as ever. And unless someone comes up with proof either way, it will never end.
With US space agency NASA's announcement last year that it had found apparent fossil evidence of bacteria-like organisms on Mars, President Bill Clinton was moved to comment on the existence of alien life and its potential impact on society.
But scientists remain at loggerheads over the evidence. Dr Monica Grady, curator of meteorites at London's Natural History Museum, has first-hand experience of meteorite ALH84001, the rock supposed to contain evidence of alien life.
Dr Grady was born in Yorkshire and, despite living in London she describes herself as "a Yorkshire lass". She has a passion for her subject, even though she went to a convent school where, she admits, "science was not a strong point and geology was not even taught".
Her love of geology came from field study days and being so close to the Yorkshire moors. "I wanted to do weather forecasting originally, but I couldn't do geography and physics at A-level because they clashed," she says. "Eventually I did a joint geologychemistry degree at Durham. I wanted to do research, and went to Cambridge to do my PhD, where they wanted someone to work on meteorites."
Just what is it about meteorites that attracted her in the first place? "They are so beautiful. If you go deep within the matrix you can even find small diamonds."
The thought that alien life may be discovered deep within a meteorite was not an immediate consideration when, in 1984, another female geologist, Robbie Score, found meteorite ALH84001 in the Alice Hills region of Antarctica. The entry in her field notebook simply stated that the rock "might be interesting". It was another 10 years before it was identified as a Martian meteorite.
Scientists at first believed the rock originated in the asteroid belt. An analysis of tiny gas pockets trapped in its matrix showed that at the time of their formation the composition of the gas was identical to that of the Martian atmosphere.
The news that the meteorite might contain evidence of fossilised life hit the headlines more than a year ago. But did NASA jump the gun? Dr Grady believes not. "A group of reputable scientists worked very hard and did a lot of soul-searching before they published," she says. "They hedged their bets at all stages. The scientific community has benefited enormously."
Her views on the probability of alien life have not been changed by meteorite ALH84001. She says: "It is possible that life may have arisen on other planets in our solar system. And the most likely of those is Mars. But I don't believe there is life like us."
But what about elsewhere in the universe? "We are one star in a galaxy of 100,000 million and our galaxy is one in 100,000 million. Each of those stars has the potential of a planetary system and each planetary system has the potential for life. But, when you think about the special circumstances under which life has arisen, I just don't know."
James Williams lectures in science education at Brunel University