Speculations on the origin of life have, until recently, been purely theoretical. …

• (Introduction)
• It's a small world
• Test tube evolution
• Links in a chain
• Looking for the right path
• The world that time forgot
• Merger or takeover
• Box 1
• Box 2

Biology Articles » Evolutionary Biology » Origin of Life » It's alive – isn't it? » Looking for the right path

Looking for the right path

- It's alive – isn't it?

But would these kinds of reactions have been plausible on the early Earth? One scientist who thinks not is Robert Shapiro at New York University, US. He argues that many of the prebiotic routes suggested for the synthesis of nucleotide bases are so artificial that it is unlikely that they ever took place on the early Earth, and that, even if they did, any yields from the reactions would have been so small and the products would have decayed so rapidly that there would have been little chance of them getting together to form nucleotides.

For instance, the adenine nucleotide, adenosine, can theoretically be produced entirely abiotically. However, according to Shapiro, ribose can only be derived from formaldehyde in relatively small yields, and a whole bunch of closely related products are produced as well; this is also the case for the synthesis of adenine from ammonia and hydrogen cyanide. To produce adenosine abiotically in the laboratory, however, prebiotic chemists extract ribose and adenine from the other compounds, and react them together under optimum conditions.

As Shapiro says in his recent book Planetary dreams: 'It would be much more realistic to heat together the entire formaldehyde and cyanide products, which would furnish the mother of all messes. Better yet, the chemist should simply mix the cyanide and formaldehyde starting materials. But we know what happens in that case; the two substances have a great affinity for each other and their reaction takes off in a direction that bears no relation to life as we know it'. At least a theoretical prebiotic synthesis path has been developed for adenosine: no such path has yet been defined for the pyrimidine nucleotides.

	The bases of DNA and RNA

Even proponents of the RNA world hypothesis admit that there are major problems with the prebiotic synthesis of RNA nucleotides. Writing in The RNA world, Gerald Joyce, a professor in the departments of chemistry and molecular biology at the Scripps Research Institute, California, US, and Leslie Orgel of the Salk Institute for Biological Studies, San Diego, US, state: 'Scientists interested in the origins of life seem to divide neatly into two classes. The first, usually but not always molecular biologists, believe that RNA must have been the first replicating molecule and that chemists are exaggerating the difficulties of nucleotide synthesis. The second group of scientists are much more pessimistic. They believe that the de novo appearance of oligonucleotides on the primitive Earth would have been a near miracle. Time will tell which is correct'.