When starting an explanation it is usually best to begin at the beginning to avoid confusion. For evolution this means going back to the first appearance of life on this planet. As is often said, no-one was around at this time so we don’t know exactly what happened but we can make a few hypotheses and test them to see how they stand up.
There are several ways of looking at how life started depending on how you define it. If you define life as an entity that is:
- separated from it’s environment by a membrane or skin,
- intakes food to drive metabolic processes that allow for…
- the replication of genetic information and
- building of supporting molecules and structures for the purposes of creating offspring.
Then you might conclude that the origin of life was a vastly improbable event that could not possibly have taken place without help, then you’d congratulate yourself on discovering the answer to one of sciences greatest mysteries and grab some lunch. Or, you might decide to consider the origin of life to depend on earlier molecules that we would not today consider to be alive but were the necessary precursors that allowed life to arise from non-life, a process known as abiogenesis. Then you go about looking for candidates for such molecules and actually evaluate the evidence.
Scientists consider RNA to be a good candidate for these precursor molecules and refer to the time at which they were the stepping stone to true biology as the RNA world. The attractive qualities of RNA that lead to this hypothesis is that RNA is used in the biological world and so we don’t need to imagine exotic molecules, RNA can catalyze reactions in the same way as proteins so it doesn’t need those supporting structures and it is possible for it to be the template for it’s own replication in the same way as DNA so it is it’s own genetic material.
In this way life could have started out as a single or small number of similar RNA strands that had the ability to replicate themselves and so become more numerous in the environment and “out compete” the existing molecular species that could not replicate. Today ribosomes are in every cell in the world, working to translate the DNA code into the protiens that make up our bodies and perform the chemical reactions that keep us alive. The active components of these ribosomes are made out of RNA and may be a relic that owes it’s existence to the proposed RNA world that could have come before complex life.
Recent research by scientists at the Scripps Research Institute has lent weight to this line of reasoning. They managed to create pairs of RNA enzymes that each catalyze the replication of the other in solution. Variants of these molecules were allowed to compete for the resources available and the more efficient pairs became dominant in the population.
It is tempting to think of this experiment as accurately reflecting how life really arose but it is important to remember that this is more of a “proof of concept” to show that it could happen, not that it did happen just like this. But creating simple chemical evolution in a test tube is exciting. We should also remember the the theory of evolution by natural selection proper concerns only what happens once life is on the scene and has little to say on how it actually got started but it is a question that is very much related and one that is being worked on.