Is it just a coincidence that the key energy molecule in cells, ATP, has as its basic unit the nucleotide, which also happens to be the basic unit of DNA/RNA (and, for that matter, with the nucleotide also being the basic unit of critical intracellular “second messengers” such as cAMP and cGMP, as well as ubiquitous and important co-enzymes like NADP and coenzyme A)?
Is this simply an example of the evolution of a molecule where the most fundamental, and presumably oldest, structure underwent various changes, each one splitting off to perform a particular, often related, function (e.g. the homologies among receptor tyrosine kinases and among protein growth factors)? Or is there a more fundamental reason for the common structural unit of DNA/RNA, ATP, co-enzymes, and cyclic nucleotides (perhaps an energy consideration)?
I am way over my head here, so please, if possible, make your answer such that an ignoramus will have a chance of understanding it.
I don’t know a lot about this in particular, but the explanation in your second paragraph is quite a compelling one, as, after all, that’s how evolution works. Look at the prevalence of the immunoglobulin fold in all kinds of proteins with wildly divergent functions, for instance. It’s much easier to adapt something you already have lying around than it is to come up with something completely new to fulfill some new function.
Barring evidence that there is some fundamental underlying reason of which I’m unaware, I’d tend to lean toward that.
I’m no expert on molecular cell biology, but I can say that it doesn’t have to be exclusively one or the other of your explanations. In general, biological molecules are almost always originated from a slight modification of an existing one, but if they don’t work well at what they do, they’ll keep getting modified (or replaced) until they do work well enough (given the various engineering trade-offs that have to be made in any system). So, my guess is that they did have a common ancestor, but there is probably also something about the purine ring that makes it particularly useful as a base for long-lived molecules in the cell (my WAG would be that the purine ring is extra stable inherently and is not attacked by any common cell enzyme, but that’s a pretty wild guess).
Finally, I’ll add that there is a third factor that could come into play: it’s often cheaper (in various ways) to build different things from common components. So, even if the purine base isn’t particularly better than another base in isolation, it might be cheaper for the cell to build it on a purine base rather than synthesizing a whole different base.
And again, it’s not like these three factors can’t all be in play at once.
It’s not a coincidence to the extent that it wouldn’t make much sense to invent a wholly new molecule to transport energy. Much easier to modify something that’s lying around (and that idea is true whether you’re talking about an initial adoption, or the long-term viability of using that molecule).
It is a coincidence to the extent that there are other potential choices that could do the job approximately as well.
There are a lot of evolutionary biologists who believe that the first life may have been self-replicating RNA chains, existing without even a proper cell wall in the earliest phases. Then, later, DNA became a permanent storage/template because it is more a more stable molecule, but RNA continued being a primary carrier of information. If that idea is the case, you’d expect such life to exist in a soup of loose RNA nucleotides and thus the list of “other potential choices” might be pretty short.
IIRC, when DNA is replicated, you need the triphosphates. The energy from the phosphate bond is used in adding the nucleotide to the growing DNA molecule. So one could speculate that the original energy-carrying function was needed for DNA assembly, and was later multi-purposed to other cellular functions.