Just Add Water (and Volcanoes)
A new study by Martin A. Nowak and Hisashi Ohtsuki at Harvard University offers some interesting new perspectives on how life emerged on Earth, by examining the transition from chemistry to biology… what some might think of as the ‘primal gap’ leading from goopy organic chemistry to true Darwinian selection and evolution.
Emergence of life (EOL) research is actually a pretty big field of research, and has been since the Miller-Urey experiments decades ago. Many people in the field call it ’emergence’ of life instead of ‘origins’ of life, because realistically the onset of life probably wasn’t a one-off event, but an emergent phenomenon that occurred in many spots around the globe in the late Hadean (about 4 billion years ago). It actually very likely that life started in many places on Earth, in different seafloor hydrothermal vent systems, and might have included several different molecular profiles using different amino acids to build different RNA variants.
Today the work is a lot more sophisticated than the old Miller-Urey bottles, and has managed to work out most of the steps needed for abiogenesis. The clincher to the whole EOL problem came when it was discovered that RNA has autocatalytic properties. Basically, some configurations of RNA can do a pretty good job of making copies of themselves, in a solution containing dissolved amino acids and a supply of labile chemical energy in the form of polyphosphate bonds (not necessarily ATP). The beauty of this discovery is that it closed what was once a gap: how to get life started when you need both an assembler and a set of assembly instructions. How do you get both at once? Irreducible complexity? Sorry, wrong answer.
RNA in our cells does two things: as transfer or messenger RNA it acts as a set of instructions, as ribosomal RNA it acts as an assembler. It turns out that even very simple, short lengths of RNA can do both. Some sequences, made with some amino acids, are better at catalyzing their own replication than others. In a natural environment where lots of organic compounds are present, and there’s also a source of chemical energy… a place like a seafloor hydrothermal vent back in the late Hadean, say… you’d have thousands of different polypeptide chains forming and unforming all the time. Including other organic compounds that you also find in carbonaceous chondrite meteorites, such as saccharides, and phosphate from Hadean seawater, the variety of different nucleic acid variants cooking up and decomposing on a regular basis would be enormous.
Now, if some nucleic configurations had greater stability than others, and were better autocatalysts than others, you immediately set up a quasi-Darwinian competition, except instead of life forms competing and replicating, you have different randomly-generated molecules. Over time, the molecules that were most stable and most able to copy would come to dominate the system. This is not very different from what drug researchers sometimes do; cook up a variety of isomers and closely-related chemicals in a reactor, then let intrinsic differences in assembly kinetics and thermodynamic stabilities winnow thousands of products down to just a few. The unstable ones, and the ones slower to form, sacrifice their components to other molecules that are more stable and can self-assemble faster.
The great thing about RNA is that if you start to make larger self-replicating strands, sometimes you get ‘mutations’ that will generate smaller, broken strands… but these can also work as helper enzymes. Some successful RNA configurations will generate lots of fragments, some will generate few. The ones that make fragments that end up promoting the kinetics of the self-replicating ‘parent’ would succeed relative to other configurations. Boom… now you have a replicating molecule with digital memory (its sequence) and a constellation of proto-enzymes (the assemblers). Enclose all this in a semi-permeable phospholipid micelle and you’re off and running.
Life is easy. You just need rocks and hot water plus time.