1 - Beginnings
I recently read an article (Ch. 11 in Charles Darwin, A Celebration of His Life and Legacy, J. T. Bradley & J. Lamar, eds., 2013) that reviewed some research and ideas about how life might have got started on this planet. The “metabolism first” idea is that first there was some kind of confined system that harnessed chemical (or heat- or electrical) energy to build up, from simple molecules, more complicated organic molecules, probably proteins, that could catalyze all the chemical interchanges that constitute “metabolism”; and then later it included something like DNA or RNA “informational” molecules that would allow the system to copy itself.
The “replication first” idea is that first there was something like DNA or RNA molecules, whose chemical structure favors duplication; and then, later, protein molecules were assembled around the DNA or RNA to give them a metabolic home from which they could take in materials and energy from the outside, and make more copies of themselves.
Living cells, now, of course, do both: they take in molecules and molecular energy for their growth and biochemical functions, and they replicate their informational DNA as the first step in cell division, the basis for all embryological development and for life’s continuing onwards.
I think the article does not pay sufficient attention to the “coding question,” which is central to the operation of life as we know it. It is difficult to understand how coding got started. The problem is this. DNA by itself is a relatively inert molecule: put it in a test tube, and nothing happens. In a cell, however, it is used for the information it contains. Along its length, the order of its bases (abbreviated A, G, C, T) constitute a chemical code containing information (…AACAGTACAGGGT…, and so on) for making many different kinds of protein, including proteins that unlock the information in DNA to make it available, and including proteins necessary for its own duplication.
It’s as if you have a locked box containing instructions for doing all kinds of useful things, but you don’t have a key for the box. There are instructions for how to make the proper key, but those instructions are in the box! In short, the molecular instructions for making proteins are encoded in DNA sequences, but the DNA can’t be used without the proteins being already in place. How could such an interdependent DNA-protein system ever get set up in the first place?
There are some recent ideas about this question, which I hope to address in some future note(s). There will have to be some chemistry involved, naturally.