WILL we ever discover firm evidence of extraterrestrial life? This month NASA reported the strongest signs yet that water once flowed on the now dry surface of Mars. And while flowing water does not of course guarantee that life has evolved there, it certainly boosts the chances. So what next?
One way forward is for future missions to continue with the intended approach of the lost Beagle 2 and other Mars landers. This has been to look for the general footprints that all life is expected to leave behind: information-carrying polymers, molecules such as lipids and peptides, or chemical or isotopic signatures produced by enzymatic processes. Because these signatures are so general, they are likely to be left behind even by life forms fundamentally different from anything found on Earth. The problem with this strategy is that it is not particularly sensitive and there are non-biological ways to create such signatures.
The prevailing wisdom among astrobiologists is that life arose spontaneously on our planet and might have done so many, many times across the universe. But a competing (and in my opinion more likely) view says that within each galaxy and perhaps even between galaxies life is likely to have spread from a single ancestral source. Certainly, when it comes to our very next planet, a planet that has exchanged a lot of material with Earth, I find it hard as a molecular biologist to believe that life on Mars would be totally distinct from life on Earth.
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If this is so, we should be able to use a far more powerful and specific approach. The 1976 Viking mission to Mars revealed a freeze-dried desert irradiated with deadly ultraviolet rays. Today the outlook for discovering possible remnants or descendants of Martian life is brighter. Experiments on Earth have revealed bacteria and archaebacteria thriving in habitats with high radiation and in frozen conditions which, though not as extreme as those on Mars, demonstrate the incredible adaptability of microbes.
Some years ago, I heard Norman Pace, a microbiologist from the University of California at Berkeley, talk about how to prospect for these so-called extremophiles by detecting their genetic material rather than trying to grow them in the lab. Pace described probing the most extreme habitats for life by using the now well-established polymerase chain reaction (PCR) to spot DNA in crude soil samples. The technique, which works by creating millions of copies of a specific stretch of DNA, is the most robust and sensitive detector we have of terrestrial life.
This sparked an idea: why not use the same method to look for their Martian relatives? PCR machines have become as standard in molecular biology labs as toasters are in kitchens 鈥� and smaller. Working with engineers at MJ Research, a manufacturer of automated lab instruments, and a team at Massachusetts Institute of Technology, we have started to design and build a low-power, lightweight instrument and propose to send it off to other bodies in our solar system, most immediately Mars.
But what feature could betray the common ancestry of Earth鈥檚 microbes and their presumed Martian relatives? In fact, about 500 鈥渦niversal genes鈥� are carried in the DNA of every known living thing on Earth. And the one that has changed the least over the past 3 to 4 billion years is a gene that encodes a molecule known as 16S ribosomal RNA. It is the perfect candidate for our quest.
Ribosomal RNA molecules are the main ingredients of ribosomes, the cellular machines that manufacture proteins. The gene for 16S is a DNA chain made up of roughly 1500 nucleotides. Although the DNA sequence varies from species to species, it contains within it a number of short segments that are exactly the same in organisms as disparate as bacteria and yeast at one extreme, and humans or corn at the other. These conserved segments are what we should now search for on the surface of Mars using the sort of lightweight PCR machine we are developing; there would be no need to isolate, culture or grow the organism. Even if microbes only flourish in particular oases on Mars that are unreachable with current technology, PCR should be able to detect any that are dispersed by wind.
But landing an instrument package on another planet is a risky business, and NASA prefers to hedge its bets by using incremental instruments 鈥� those that will generate new information whether life exists or not. Our PCR machine is not like that: if life exists today we may hit the proverbial jackpot and find it; if it does not exist today, we would generate no data. That makes our approach a controversial one.
Still, astrobiologists might regret NASA鈥檚 cautious, incremental approach if, 30 years from now, we finally find life on Mars that we could have discovered earlier by looking for 16S ribosomal genes. Happily, just a few weeks ago NASA finally decided to fund the development of our PCR machine. If all goes well, it may be on board when another mission is rocketed towards Mars in 2009 or 2011 to search the planet for life.