Life doesn't need water. In fact, all kinds of weird liquids could be solvents for life like water is here on Earth. Scientists say the list of alien water-substitutes is long, from frigid nitrogen to supercritical CO2 to methane to formamide. Whatever inhabits these other liquids would have to take on some truly odd forms, right down to DNA like we've never seen before.
At the Astrobiology Science Conference 2008 that wrapped up yesterday, chemist Steve Benner proposed that formamide might make a great solvent for life with some bizarre biochemistry that mimics our DNA, but in a way we can only imagine. Benner's a great speaker and scientist, but has a tendency to lapse into flights of chemical minutia, so I'll take a page from a New Scientist feature in June that aptly sums up the point:
A suitable solvent is only part of the story of life, of course. Apart from a few viruses, all life on Earth uses deoxyribonucleic acids (DNA) to encode the information needed to build and run an organism. But is there an alternative? Could genetic information be stored another way?
DNA consists of a double helix, like a twisted ladder. Every rung of the ladder comprises a pair of molecules called bases. These bases are the part of DNA that actually encode the genes. There are four types, known by the initials G, A, C and T, and they form the alphabet of every genetic code. The struts of the ladder consist of deoxyribose sugars linked by charged phosphate groups.
Biologists have methodically altered different parts of the DNA molecule to explore which aspects of its structure are necessary for it to function properly. They have identified several parts that can be changed without disrupting the molecule. For example, you can replace deoxyribose with another sugar, such as threose. Different and more molecules can be used to represent the bases too.
But that's where the known options end, says Steven Benner, a synthetic biologist at the Foundation for Applied Molecular Evolution in Gainesville, Florida. Benner has found that replacing the phosphate groups with uncharged substitutes brings disaster. The DNA strand becomes unstable, collapses into a ball and sinks to the bottom of his experimental solution like dregs in a beer keg.
Before these experiments, people wondered why the phosphates were there - whether they were simply a redundant evolutionary artefact, rather like a male nipple. It's now clear that they serve a vital function. The charges keep DNA stiff by organising a cradle of water molecules along its chain; without them, DNA easily wads into a ball - another demonstration of how water is integral to life as we know it. An alien's DNA equivalent in ammonia or methane, say, would therefore need some very different structures to avoid rolling up. Those charged phosphates might have to be replaced by something greasier, like hydrocarbon or benzene molecules, says Jack Szostak, a molecular biologist at Harvard University.
Spearheaded by Paul Davies of the BEYOND Institute, several talks at the conference featured ideas about weird life. It appears to be gaining a serious head of steam among scientists, and Davies told me that we might even try looking for these strange creatures on Earth, as part of an alternate-chemistry "shadow ecosystem."
Source: original reporting, New Scientist (sub required)