Were not in the Garden of Eden anymore. Darwin speculated that life began in a warm pond on the primordial Earth. Lately other scientists have suggested that the magic joining of molecules that could go on replicating might have happened in an undersea hot spring,on another planet or inside an asteroid. Some astronomers wonder if it could be happening right now underneath the ice of Europa or in the methane seas of Titan.
Two dozen chemists,geologists,biologists,planetary scientists and physicists gathered in Tempe,Arizona,recently to ponder where and what Eden might have been. Over a long weekend they plastered the screen in their conference room with intricate chemical diagrams through which electrons bounced in a series of interactions like marbles rattling up and down and over bridges through one of those childs toys,transferring energy,taking care of the business of nascent life. They charted the fall of meteorites and the rise of oxygen on the early Earth and evidence in old rocks that life was here as long as 3.5 billion years ago. The planet is only a billion years older,but estimates vary on when it became habitable.
In front of a 2,400-member audience one night they debated the definition of lifeanything highly statistically improbable,but in a particular direction, in the words of Richard Dawkins,the evolutionary biologist at Oxford. Or,they wondered if it could be defined at all in the absence of a second example to the Earths biospherea web of interdependence all based on DNA.
Hence the quest for extraterrestrial examples is more than a sentimental use of NASAs dollars. Lets go look for it, said Chris McKay,a planetary scientist at NASAs Ames Research Laboratory in Mountain View,California,who is involved with the Mars Science Laboratory,which will be launched in November.
The rapid appearance of complex life in some accounts has rekindled interest recently in a theory fancied by Francis Crick,one of the discoverers of the double helix,that life originated elsewhere and floated here through space. These days the favourite candidate for such an extraterrestrial cradle is Mars,which was once a water world. Perhaps,some think,its microbes hitched a ride to Earth on asteroidsunless,of course,the microbes went the other way and whats to be found on Mars are the dead remains of long-lost cousins of Earth.
The conference was sponsored by the Origins Project at Arizona State University in an effort to get people together who dont normally talk to each other,said Lawrence Krauss,a physicist who helped organise the meeting. Talk is indeed hard across disciplines and geological ages. John Sutherland,a biochemist at Cambridge University in England,said geologists and astronomers were more interested in talking and speculating about the origin of life than chemists were,even though it is basically a problem of nitty-gritty chemistry. The reason,he explained,is that chemists know how hard it is.
The modern version of the Garden of Eden goes by the name of RNA world,after the molecule ribonucleic acid,which plays Robin to DNAs Batman today,but is now thought to have preceded it on the biological scene. RNA is more versatile,being able not only to store information,like DNA,but also to use that information to catalyse reactions,a job now performed by proteins. That solved a sort of chicken-and-egg problem about which ability came first into the world. The answer is that RNA could be both.
If you want to think of it that way,life is a very simple process, said Sidney Altman,who shared a Nobel Prize in 1989 for showing that RNA had these dual abilities. It uses energy,it sustains itself and it replicates.
One lesson of the meeting was how finicky the chemical reactions needed for carrying out these simple-sounding functions are. There might be a reason why amino acids and nucleotides are the way they are, Krauss said.
What looks complicated to us might not look so complicated to a piece of a carbon molecule awaiting integration into lifes dance. Complexity is in the eye of the beholder, said Dr Sutherland,who after 10 years of trying different recipes succeeded in synthesising one of the four nucleotides that make up RNA in a jar in his lab. With the right mixture and conditions,complicated-looking molecules can assemble themselves without help. When everything is in the pot, he said,the chemistry to make RNA is easier.
Sutherlands results were hailed as a triumph for the RNA world idea,but there is much work to be done,said Steve Benner,who constructs artificial DNA at the Foundation for Applied Molecular Evolution,in Florida. Nobody knows whether Sutherlands recipe would work on the early Earth,he said. Moreover,even if RNA did appear naturally,the odds that it would happen in the right sequence to drive Darwinian evolution seem small.
Some others,including astronomers and geologists,have another view of biological inevitability. Life is a natural consequence of geology,said Everett Shock,a geophysicist at Arizona State. Most of what life is doing is using chemical energy, he said,and that energy is available in places like undersea volcanic vents where life,he calculated,acts as a catalyst to dissipate heat from the Earth. In what he called a sweet deal,life releases energy rather than consuming it,making it easy from a thermodynamic standpoint. Biosynthesis is profitableit has to be; they live there, said Shock,referring to microbes in undersea vents.
Some scientists say we wont really understand life until we can make it ourselves. On the last day of the conference,J. Craig Venter,the genome decoding entrepreneur and president of the J. Craig Venter Institute,described his adventures trying to create an organism with a computer for a parent. Using mail-order snippets of DNA,Dr Venter and his colleagues stitched together the million-letter genetic code of a bacterium of a goat parasite last year and inserted it into another bacteriums cell,where it took over,churning out blue-stained copies of itself. Venter advertised his genome as the wave of future migration to the stars. Send a kit of chemicals and a digitised genome across space.DENNIS OVERBYE