Fluid beginningsIn ocean of thought on origin of life, scientist proposes theory that may hold water
By Diana Steele
How did life begin on Earth? Geophysicist Joseph Smith provides a theory that holds water, he says -- but not too much.
"The problem with most theories on the origin of life is that there was too much water around for the kind of organic chemistry that needed to take place," said Smith, the Louis Block Professor in the Geophysical Sciences. "Synthesis of biomolecules from organic compounds dispersed in aqueous 'soups' require a mechanism for concentrating the organic species next to each other, and biochemically significant polymers -- like polypeptides and ribonucleic acids -- must be protected from photochemical destruction by solar radiation."
In the March 31 Proceedings of the National Academy of Sciences, Smith postulates that this chemistry could have been facilitated by silica-rich minerals resembling zeolites, porous crystals with channels running through them. Most zeolites are hydrophilic -- water-loving -- and tend to absorb water from their surroundings. But certain silica-rich synthetic zeolites are organophilic, preferentially absorbing organic materials out of water.
A naturally occurring zeolite -- called mutinaite -- was recently discovered in Antarctica, and Smith thinks that this mineral could provide the key to the chemical evolution that led to the origin of life. It's possible that mutinaite, which has aluminum in place of silica, loses aluminum at its surface to become silica-rich through weathering, Smith said. A small amount of remaining aluminum would provide the catalytic centers for assembling organic molecules into polymers.
A famous experiment performed at the University in 1954 by then-graduate student Stanley Miller and his adviser, Nobel laureate chemist Harold Urey, showed that amino acids, which make up the proteins found in all living organisms, could form from chemicals in the atmosphere combined with water and lightning.
No experiment has yet demonstrated how the amino acids assembled into protein and ribonucleic acids (RNA) chains, but Smith is planning such experiments using a synthetic, silica-rich organophilic zeolite.
Amino acids occur naturally in right-handed and left-handed forms, but only the left-handed forms are found in the proteins of living organisms. Smith said, "It's probably an accident that only the left-handed form is used, but it may have started in a zeolite with a left-handed channel." Zeolites with one-dimensional channels could have provided the template for assembly of only one version of the amino acids into the first primitive proteins.
Smith plans a trip to Australia, where some of the oldest and least-metamorphosed rocks and minerals are found, to look for more naturally occurring organophilic zeolites like the mutinaite found in Antarctica. He's hoping these minerals still contain evidence of primary biocatalysis. Further research will include chemical experiments to determine if the zeolites actually carry out the chemistry he proposes, and the use of computer models to study the structure of the channels.