The US space agency NASA has resumed its search for life on Mars with the launch of the Mars Reconnaissance Orbiter. Nasa's mantra for seeking life beyond Earth is "follow the water," and Mars seems to have lots of it, albeit mostly frozen into ice or permafrost. On Monday last week a US scientist told the British Association festival of science in Dublin that the sands of Mars could contain up to 50 percent snow and ice.
One fact that is often glossed over, however, is that water may be a necessary condition for life, but it is far from sufficient.
Nobody has a clue about how life may emerge from non-living substances, and what else might be needed.
Because even the simplest living cell is immensely complex, the odds of such a thing forming by chance are virtually zero. If that's the way it happened, then life is a freak phenomenon, and we will almost certainly be alone in the universe. However, the search for life beyond Earth, which underpins the burgeoning field of astrobiology, is based on a belief that chance played only a subordinate role. Instead, some sort of "life principle" is envisaged to be at work in the universe, coaxing matter along the road to life against the raw odds.
The Nobel prize-winning biologist Christian de Duve has dubbed the life principle "a cosmic imperative." Attractive though that may be, there is no direct evidence that the laws of nature are slanted in favour of life. If we found life on Mars or some other planet, and could prove it started from scratch independently of life on Earth, it would bolster the idea of a life principle. Meanwhile it remains an act of faith. De Duve thinks life will emerge more or less automatically on all Earth-like planets. But no planet is more Earth- like than Earth itself. If life is indeed a cosmic imperative, we might expect it to have started many times over on our home planet.
Biologists insist that all known life stems from a single origin. Each species represents a branch on the great tree of life that derives from a universal common ancestor. Support for this view comes from the fact that many specific, complicated features of organisms, such as their genetic code, are the same in all identified species and would be most unlikely to have evolved independently more than once.
But there is a flaw in this reasoning. The vast majority of organisms are microbes, and you can't tell much about their innards simply by looking. Microbiologists have developed ways of sequencing the genes of microbes to position them on the tree of life, but the procedure often doesn't work. Because these techniques are customized to identify life as we know it, they wouldn't work with life as we don't know it.
So how can we be sure that the world about us isn't seething with alien bugs? I began researching this with Charles Lineweaver at the Australian National University. We identified several ways in which multiple genesis episodes might have left traces in Earth's geological or biological record. The real prize would be the identification of a truly alien microbe right under our noses. But how would we spot such a thing amid the welter of familiar life?
A possible answer was provided by my wife, a science journalist. To make proteins, organisms use amino acids, whose molecules resemble left-handed gloves. Look at them in a mirror and they would be right-handed. The right-handed forms are not hard to make, but life does not use them. The best explanation for this preponderance of left-handed amino acids is that it represents a frozen accident: early on in the genesis process, a random choice was made and life got stuck with it. But if there were a second genesis, then the odds are 50-50 that the opposite choice would be made. This "mirror life" might resemble "our" life in most important respects, but not in its handedness. And because left and right-handed life couldn't mix, mirror life would peacefully co-exist with our form of life.