This has lead to another explanation for the origins of life. Perhaps thinking of life as a bunch of replicators has blinded use to it. Living organisms can also be thought of as chemical factories. Genes and their products are simply there to control these reactions. Some believe it was chemical reactions that came before the proteins or RNA that catalyse the reaction. This is called the metabolism first theory. People who follow this theory have described what is needed for a chemical system to be the beginnings of life. 1) A boundary or form of membrane is needed to keep life and non-life away from each other. The 2nd law of thermodynamics states the universe my decrease in order but life increases in order so inside the boundary entropy decreases but this generates heat that causes an increase in entropy outside it. 2) An energy source must have existed. Perhaps some sort of redox reaction to power the chemical reactions in the metabolism first model. Radiation may have been used. 3) The energy source must be linked to the other chemical reaction. For me this is difficult to see how this could happen without proteins there to help things along but perhaps if I knew more chemistry it would be clearer. We use ATP as our energy currency but how could redox reactions or radiation be linked to these ancient chemical reactions? 4) The chemical reactions must be able to change and evolve. If a cycle of reactions was created where A became B and B became C and C became D and D became A again we have something to expand upon. If we had a carbon input such as E we could take compounds off the cycle and expand it (see diagram). These reactions could be powered by a redox reaction of X to Y. Eventually complex molecules could be created 5) One final requirement for these reactions to have been the origins of life is need and that is to be able to replicate. It is hard to imagine how this is possible before a lipid membrane existed for it to divide into two. If possible this would have allowed for Darwinian evolution through the competition for recourses.
The RNA-first approach has some support for it. Minerals have been found that contain boron in ‘containers’ or ‘bowls’ in
I believe these two theories could work together. Perhaps it is only because I find the rna world aesthetically pleasing i want to save it but these reactions could eventually become so complex they make rna. Over time these built up and started to take control and then made proteins to do much of there job when dna then took over as the info store and rna was simply the messenger and helps out in only a few reactions today. This nicely explains why the formation of the peptide bond in the ribosome is still done by rna. We could go even further into theory and suggest there was a polymer before RNA that could act as catalyse and self replicator. PNA has been suggested. Instead of having the sugar-phosphate backbone like RNA and DNA it has peptides attached to bases forming a backbone. Sadly such a molecule does not exist in our cells today or leave fossils in the ground for us to examine so we cannot test if PNA was really the first molecules that lead to life. If PNA did exist it all became RNA and then DNA.
At the moment we have many ideas about what may have been involved with the start of life on earth but it is difficult to prove anything. What we can do is explore the potentials of these molecules or chemical systems. After all theories about the origins of life cannot be tested directly but the do make predictions and by testing these predictions we can hopefully learn a lot.
Albert et al. (2002) Molecular biology of the cell. 4th edition.
Shapiro (2007) A simpler origin for life. Scientific American 296: 24-31.