See a gallery of possible forms of shadow life
While some researchers are attempting to create brand new life in the lab, others are searching for alien life on Mars and, eventually, elsewhere in the solar system. This burgeoning field of astrobiology has a less well-known offshoot right here on Earth: the search for a 鈥渟hadow biosphere鈥�- a second, independent form of life unrelated to sort we know ().
After all, many astrobiologists now think that given the right conditions any sufficiently complex molecular soup has a good chance of generating life if it simmers long enough. If that鈥檚 so, it seems plausible that life may have arisen on Earth not once, but several times. New origins of life are unlikely today, because existing life would gobble up any aggregations of prebiotic molecules before they could edge over the threshold. However, opportunities for the origin of life may well have existed for long periods on the early Earth. Some of these origins may have been dead ends, out-competed by other life forms 鈥� but others could still be living among us, unnoticed.
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As big as Darwin
鈥淚 think if we found a second sample of life on Earth, it would be as big as Darwin鈥檚 theory of evolution,鈥� says cosmologist and astrobiologist Paul Davies at Arizona State University in Tempe. 鈥淚t would answer the most fundamental question we can imagine, which is: are we alone in the universe?鈥�
Sceptics might scoff that shadow life could pass unrecognised for so long, but Davies and his collaborators have a simple rejoinder: we鈥檝e never looked properly. Such life would probably take the form of single-celled microbes, so we would not expect naturalists to spot it casually like some rare parrot. And the techniques microbiologists use to detect life 鈥� staining for DNA, sequencing DNA, and culturing microbes in the lab 鈥� assume that the target microbes have the normal biochemistry.
鈥淭hey couldn鈥檛 detect an alternative form of microbial life,鈥� says Carol Cleland, a philosopher of science and astrobiologist at the University of Colorado in Boulder. Given that fewer than 1 per cent of microbes have been cultured and described, there is plenty of room for shadow life to be living right under our noses.
However, the task of searching for shadow life on Earth is much tougher than looking for life on other planets. 鈥淭his planet is heavily contaminated with life as we know it,鈥� says Shelley Copley, a biochemist at the University of Colorado. That means researchers can鈥檛 just look for evidence of metabolism or the presence of large biopolymers, because ordinary life would swamp any signal from shadow life. Instead, shadow-stalkers have to get more creative.
One promising avenue is to explore extreme environments that are beyond the reach of conventional life, such as ultra-dry deserts, ice sheets, the upper atmosphere or the hottest hydrothermal vents (see The most extreme life-forms in the universe)
Another is to devise ways of detecting alternative biochemistries. In the first and so-far only experiment of this kind, Richard Hoover, a microbiologist at NASA鈥檚 National Space Science and Technology Center in Huntsville, Alabama, went looking for 鈥渕irror life鈥�. Normal organisms use right-handed sugars and left-handed amino acids almost exclusively, and eschew their mirror-image equivalents. But what if shadow life developed the opposite preference? Hoover and his colleague Elena Pikuta created nutrient broths containing only left-handed sugars and right-handed amino acids, inoculated them with unusual extremophile microbes and waited to see if anything grew.
鈥淢uch to our great astonishment, we found that we did have some microorganisms that were capable of growing,鈥� Hoover recalls. But on closer examination, the shadow microbes turned out to be ordinary bacteria with unusual metabolisms.
Cleland thinks there are other places to look. 鈥淲hat I think we should do is go out looking for anomalies,鈥� she says. For example, some researchers have reported nanobacteria which show some of the characteristics of life but are too small to be ordinary cells.
An even more promising anomaly, Cleland says, is 鈥� desert varnish鈥� 鈥� a thin, manganese-rich layer that forms on the surface of rocks, especially in hot, dry places. 鈥淓veryone thinks they know what desert varnish is, but everyone disagrees,鈥� says Cleland. 鈥淭here is no agreement on whether it is produced by living or nonliving processes.鈥� The layered varnish looks a lot like the primitive microbial mats called stromatolites, but for the most part, microbes are absent. Bacteria-like objects are occasionally present, but have never been fully characterised. 鈥淲hat they really are, I have no idea,鈥� says Ronald Dorn, a who studies rock varnish at Arizona State University.
In September 2008, Cleland and her colleagues took samples of desert varnish. They hope to find unusual ratios of elements that might point to some sort of metabolic process, but with a signature that differs from that of familiar life. They hope to have some results later this year.
Not likely, says Norman Pace, a microbiologist at the University of Colorado who is one of the researchers Cleland has recruited to examine the desert varnish. 鈥淭he only reason to invoke [shadow life] is that we don鈥檛 know what causes desert varnish,鈥� he says. Still, he鈥檚 willing to have a look.
Davies hopes that more researchers will start looking for shadow life. Even if they don鈥檛 find it, the search could turn up previously unknown branches on the familiar tree of life. 鈥淪o it鈥檚 worth doing anyway,鈥� he concludes, 鈥渆ven if you鈥檝e convinced yourself that we鈥檙e alone in the universe.鈥�
See a gallery of possible forms of shadow life
