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WHEN Darwin published his theory of evolution in 1859, he gave a convincing account of how life on Earth has evolved over billions of years, from simple microbes to the diversity of the biosphere we see today. But he pointedly left out of his account how life got started in the first place. “One might as well speculate on the origin of matter,” he quipped.

Almost a century-and-a-half later, the origin of life remains one of the great unsolved problems of science. The pathway from non-life to life is so mysterious that scientists can’t even agree on whether life on Earth is a bizarre chemical fluke, unique in the universe, or the expected outcome of intrinsically bio-friendly laws of nature. Much hinges on the answer, including the issue of whether or not we are alone in the universe.

In recent years it has become fashionable to assume that life emerges rather readily under Earth-like conditions and is therefore likely to be widespread in the universe. This belief underpins the ambitious astrobiology programmes of NASA and the European Space Agency, the goal of which is to discover a second sample of life.

Peter Ward is an astrobiologist at the University of Washington in Seattle who is prepared to think outside the box when it comes to alien biology. His book is a comprehensive review of the state of our knowledge, and ignorance, of what it takes for life to get started and where we might look to discover alien organisms. It is also a delight to read.

Understandably, much of Ward’s discussion concerns Mars, which by common consent offers the best hope for finding traces of extraterrestrial life. The recent discovery of methane in the Martian atmosphere has fuelled speculation about subsurface microbes clinging tenaciously to life billions of years after the surface of the Red Planet degenerated into a freeze-dried desert.

One of the complicating factors in seeking a second genesis is that the planets are not completely quarantined from each other. Debris splattered into space by comet and asteroid impacts gets distributed around the solar system. Mars and Earth in particular have been trading rocks throughout their history, and it is clear that microbes could hitch a ride and be transported in relative safety from one planet to the other. While this boosts the chances of finding life on Mars, it implies that Martian organisms might not be alien at all, but merely members of another branch on the terrestrial tree of life.

Indeed, as Ward points out, Mars might well have been a more favourable planet for life to begin on. If that is right, then the chances are that life on Earth came from Mars, cocooned within rocky ejecta, in which case we are all descendants of Martians. Thrilling though such a discovery would be, it would fall short of the second, independent genesis that would signal that life is a universal phenomenon.

The search for alien life need not involve expensive interplanetary missions. If life does form readily then we might expect it to have arisen many times on our home planet. Ward discusses the intriguing idea that alien organisms may lurk all around us, unrecognised for what they are because they fail to respond to standard biochemical analysis. For example, there could be microbes that use RNA instead of DNA, or employ a different genetic code. Such organisms might very well be passed over by microbiologists focused on life as we know it. Ward also makes a case that some viruses could be relics of ancient alternative forms of life.

“Aliens may lurk around us, unrecognised for what they really are”

Mars and Earth are by no means the only planets in the solar system that are, or were, fit for life. The surface of early Venus might have been promising. There are tantalising hints of chemical imbalances in the Venusian clouds that just might be the result of continuing microbial activity in the upper atmosphere.

Europa, one of Jupiter’s moons, is another favourite of astrobiologists on account of a liquid-water ocean beneath its icy crust. Ward, however, is sceptical of Europa’s biological potential, because it lacks a decent energy source. By contrast Titan, a moon of Saturn recently visited by the ESA probe Huygens, is a veritable petrochemical factory with abundant chemical energy, but its frigid conditions would prove lethal for anything resembling terrestrial life.

Even our own moon could play an important role in the story of life, for its largely inert surface will have preserved rocks propelled from Venus, Mars and Earth that may contain vital clues, or even fossils, from the remote past. It may also preserve a record of catastrophic events such as gamma-ray bursts, which are thought to have been responsible for some of the mass extinctions of terrestrial fauna and flora.

While the search for signs of alien life goes on, some scientists are trying to cook it up in a test tube. In fact, viruses have already been made from scratch, and it seems only a matter of time before truly autonomous living cells are created in the laboratory.

It would be a mistake, however, to conclude that because we can synthesise life from non-life, life does indeed form readily in the universe. It is one thing for a primitive cell to be painstakingly designed and manufactured by a team of trained organic chemists in a laboratory full of expensive equipment, quite another for it to happen spontaneously in the rough and tumble of the natural world.

Ward surveys several popular scenarios for how and where Mother Nature may have worked this extraordinary magic, ranging from deep-ocean volcanic vents to desiccated deserts. The range and diversity of the theories serve to confirm that, to date, we really don’t have much of a clue about how or where life began, or how long it took. But Ward for one clearly thinks we are on the verge of obtaining the answers.