Past life on Mars could have left traces in surface rocks. In 1996 NASA scientists announced that a rock blasted off Mars by an asteroid impact had been found to contain tiny features reminiscent of fossilised microbes. The meteorite, known as ALH 84001, has been subjected to intense scrutiny. Although the jury is still out, the prevailing view is that ALH 84001 will not provide definitive evidence for life on Mars.

If there was ever life on Mars there is a serious chance it could have spread to Earth by hitching a ride in a rock like this one. An average of one Martian meteorite per month strikes our planet, and over geological time billions of tonnes of Mars rocks have rained down on us. Experiments with artillery shells and centrifuges have shown that microbes could withstand the shock of being kicked off Mars. Once in space, the cold vacuum conditions would act as a preservative.

Radiation presents the greatest hazard to life travelling through interplanetary space. But cocooned inside a rock a couple of metres across, a microbe would be shielded from the worst effects. Calculations suggest that hardy bacteria could endure millions of years in orbit round the sun if ensconced within a suitable rock. The final hazard – high-speed entry into Earth’s atmosphere – need not present a problem because the frictional heat would not have time to penetrate to the interior of the rock.

This analysis carries a startling implication. Being a smaller planet than Earth, Mars cooled quicker, so it would have been suitable for life sooner. The deep subsurface, where early life could have sheltered from the ferocious barrage of impacting asteroids and comets, may have been habitable as early as 4.4 billion years ago. By contrast, Earth was probably a searing hell until 3.9 billion years ago. So perhaps life began on Mars and later spread to Earth inside ejected rocks. If this speculation is correct, then we will all be descended from Martians, and any organisms found there today would be our distant cousins.

Exciting though this prospect may be, the discovery that Mars and Earth life share a common origin would leave tantalisingly unsolved the biggest of the Big Questions of existence: are we alone in the universe? If life arose on Mars independently of life on Earth, then it would be clear that the laws of nature are intrinsically bio-friendly, able to coax life into being on any Earth-like planet. In that case, the universe should be teeming with life. But if Earth life is just a side-branch of Mars life – or vice versa – then it is still possible that life’s origin was a freak event, unique in the universe. Either way, the Red Planet is likely to contain vital clues to the answer.

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