“That Mars is inhabited by beings of one sort or other is as certain as it is uncertain what those beings may be.” With these dramatic words, the American astronomer Percival Lowell informed the world about a network of canals he thought existed on the Red Planet. Lowell conjectured that Mars was a dying, drying planet, whose inhabitants built straight channels to bring melt water from the polar caps to the arid equatorial regions. He produced elaborate maps of a canal network to support his theory.

That was in 1906, when the idea of life on Mars seemed entirely plausible. Then in the 1960s space probes sent to Mars failed to reveal any sign of the much-discussed canals. In 1977 two spacecraft called Viking landed on Mars and found a freeze-dried desert bathed in deadly ultra-violet rays. The craft were equipped with robot arms that scooped up dirt and then analysed it for microbes. Nothing was found. Mars looked completely dead.

Recently, however, opinion has begun to shift. Photographs of the surface show what look like dried-up river channels and lake beds. There are hints of an ancient ocean. Evidently Mars was once warm and wet and maybe not unlike our own planet. Could life have flourished there in the remote past? Might it still be clinging on today in some obscure niche?

There is a good chance we shall learn the answers to these questions in the coming decades. Mars is the one planet in our solar system that is just about accessible to human exploration, and the motivation to go there is strong. It could be our only chance of finding a second genesis – another location in the universe where life has emerged from nonlife. The consequences for science and philosophy are incalculable. The discovery of a second tree of life would transform our view, not only of the nature of life, but of our own place in the universe.