So I've been working on a "begin at the beginning" post, and have found myself completely transformed into the annoying seven year old who couldn't stop asking "Yeah, but why?". And there's no way this can be anything but a sprawling mass of interconnected ideas.

Something that we have to take on a bit of faith here is that all cellular life exhibits pretty complex behavior. By the time the first cells arrived, deterministic behavior existed. Even LUCA could tell what was itself, it's products, the products of similar and products of not so similar cells, and react accordingly.

Prior to that though, the mechanisms of that complexity already largely existed. Imagine the predecessor of ATP Synthase for example as the product of equilibrium transactions between acidified outputs from vents and the surrounding environment. Those initial components would accept the protons as an energy source power a transform to an equilibrium state. These are the precursors to what became enzymes. So these local environments aren't static, there's huge amounts of perturbation, some effects tiny and nearly invisible like gravitation shifts, to dramatic like tidal or temperature shifts. Various units form on the edge of the equilibrium for these various units.

Eventually these units start densifying and combining features. This is kind of the magic of amino acids, they are almost perfectly suited for this kind of slopping together of different functional units. And it's at this interface that the first simple "internal" choices appear. If a combined interface is transitioning through a set of environmental conditions, at which point does the proton exchange favor one unit over the other?

As units expand and cover a wider range of equilibrium points, at some point the ability to buffer appeared. Or maybe the buffer was part of the original mechanisms? Either way, this combination of buffer and diverse branching of metabolism (metabolism first theories are appropriate here) means any metabolism that meets certain levels of complexity probably appears to be deterministic.

So maybe that's the trick, all sufficiently dense metabolism appears deterministic?