Benner and colleagues identify just two absolute requirements for life to exist: a suitable temperature range to allow chemical bonding, and an energy source (for example, the sun or radioactive decay). This contrasts with the common belief that life absolutely requires liquid water. Indeed, the authors speculate on the possibilities of life emerging in cold, icy environments, just like that of Titan, which meets both requirements and many 'weaker' ones. "Life may even exist in more exotic environments, such as the supercritical dihydrogen-helium mixtures found on gas giants," speculates Prof Benner, referring to the large gaseous planets such as Jupiter and Saturn. He even wonders if we may have missed exotic forms of life here on Earth. "This question is not as absurd as it might seem," says Benner. "Just 50 years ago...life in the deep ocean was not known."

Titan, currently being studied by the Cassini space probe, is perhaps an ideal place to look for life. The stunning pictures and data already sent back from the moon suggest a world of yellow clouds and oily black methane lakes, an environment that is thought to resemble that of the Earth billions of years ago. This puzzling moon is too cold for large quantities of liquid water to exist, however, which for many probably rules out life. Humans and, indeed, simple bacteria are mostly made up of water, so it is difficult to envisage life without it. But Benner believes this focus on water can blinker the search. "Why not use the hydrocarbons that are naturally liquid on Titan as a solvent for life directly?," he muses. "In many senses, hydrocarbon solvents are better than water for managing complex organic chemical reactivity."

We will soon know more. Next month, the European-built Huygens probe will detach from Cassini and touch down, or perhaps splash down, on Titan's surface. "The Huygens mission will be the first real input into this field for some time. Its potential for providing an 'Aha!' experience with respect to weird life is enormous," says Benner.

All life on Earth is widely supposed to have descended from a common ancestor. One consequence of this is that every organism uses the same general biochemistry. For example, all forms of life make use of proteins made from the same set of building blocks. But this may not be the only way to do things. Could creatures exist elsewhere in the Universe with a completely different biochemistry? Experiments in recent years have partly addressed such questions by re-engineering protein and DNA systems. For example, alternative amino acids to those found in living systems are capable of standing in for their natural counterparts. Professor Benner and colleagues now provide a wide-ranging exploration of just how far the chemistry of life can be pushed. "Is water necessary? Is carbon essential? Why not silicon?," asks Benner. One of the leading theories on the origins of life supposes that the earliest organisms used RNA instead of DNA to pass on their genetic information and to catalyse reactions. If this is correct, it demonstrates that alternative biochemistries are indeed possible. Benner suggests that 'RNA organisms' might still exist. Because such life forms would not need the biochemical machinery to produce proteins, they would be much smaller than bacteria, hinting at possible environments we might look for them in. "Many minerals have pores that are smaller than one micron across. These might hold smaller RNA organisms," says Benner.

While more exotic worlds might well harbour life, Mars remains the best bet. "There was water on Mars when there was life on Earth," Benner points out. "This would not be particularly weird life, of course, in that it would be living in water, but it could easily be weird by Earth standards." However, Benner concedes that "a simple 'We don't know' is often the best answer for some questions. "Until life is encountered elsewhere, or aliens contact us, we will not have an independent second dataset. We may not even then, if the alien life itself shares an ancestor with life on Earth."

End

Contact details

Prof. Steven A Benner, Department of Chemistry, University of Florida, Gainesville, FL 32611-7200, USA

Tel: 352 392 7773

e-mail: benner@chem.ufl.edu

Current Opinion in Chemical Biology is a review journal covering all aspects of the interface between chemistry and biology. Each issue contains articles themed around a particular subject of current interest. The article described above is from a section on the Molecular origins of life, edited by Nicholas V Hud and David G Lynn. For more information on the journal or to request the full text of the article, please contact the in-house editor Matt Brown at COChemBiol@elsevier.com. You can view the current issue at http://www.sciencedirect.com/science/journal/13675931