What seems a bit unlikely is that such a faculty for emitting vibrations in air would evolve naturally, complete with a 'reader' for the involved frequences that translates particular frequency-domain time series directly into abstract data in the host organism's etc etc etc.
If you start with a trivial olfactory communication mechanism, and then evolution continually selects for higher bandwidth...
Sigh.
No, look, there's actually a difference. Seriously, there is.
The difference is that evolution doesn't work by sitting down with a drawing board and devising clever and exotic engineering solutions to problems. It works by exerting selection pressure on a variety of mutant forms of what's already there, preferentially selecting for the forms that give the most pro-survival, pro-reproduction capabilities. And given a choice between evolving an entirely new and unprecedented structure or mechanism for something, versus adapting an old structure or mechanism, evolution will
always pick the latter.
This is why even though wings have evolved in parallel at least twice times in vertebrates, in both cases the wings evolved as adaptations of the arms,
not as completely new limbs sprouting from the existing skeleton. This is why whales and dolphins have tail skeletal structures adapted from the legs of land-based mammals, not redesigned-from-first-principle skeletons more suited for sea life. This is why all land vertebrates have two eyes and not four or six, and so on.
Sign language, spoken language that works by vibrations of air or clacking carapace pieces like castanets or whatever, what they all share is
voluntary control of a muscle. For any organism that is by nature mobile and not sessile, voluntary muscle control is a basic physical faculty. All organisms have it, and mutations to the physiological structure of an organism don't stop the organism from having functional voluntary motor control.
Therefore, repurposing voluntary motor control for new purposes is fairly simple.
...
To "start with a trivial olfactory communication mechanism,"
as distinct from Earthly pheromone communication, you need a creature with voluntary control over its own chemical production, in the same way that we have voluntary control over our muscles. Earthly creatures do not, on the whole, have this; their secretion of pheromones appears involuntary and automatic, insofar as I can tell. Certainly there's no Earthly organism that can deliberately decide in reaction to its environment whether to put an A, C, G, or U on the end of an RNA molecule, as opposed to having autonomous mechanisms that work on RNA without the creature's awareness.
The underlying problem here isn't that evolution, working on a simple example of the mechanism you propose, would fail to create a complex and fully effective version of the mechanism.
The problem is that there is unlikely to
be a simple example for evolution to work on in the first place, unless we posit an extremely strange biochemical system in which the organism's brain is capable of consciously directing, on a highly detailed level, exactly what chemicals the body synthesizes and in what order.
...
It's like saying "imagine humanoids with angel wings." For angel wings to evolve- that is, wings that sprout out of the back and leave the arms unimpeded- you would need a pre-existing organism with six limbs. There's no reason that can't exist, but it would be reflected elsewhere in the evolutionary tree of that planet on a fairly large scale, because basic details of physiology like "how many limbs does this organism have" and "how to basic biochemical pathways work" tend to take
tens of millions of years to evolve.
By contrast, gross physiology evolves a lot faster because minor mutations are less crippling and can therefore accumulate.
For example, chihuahuas, pugs, dachshunds, and Great Danes share a common ancestor no more than twenty to forty thousand years ago, and each species was differentiated from the others by no more than a few thousand years of intense selective breeding. Physiologically the difference between them are
huge, but they all involve reshaping of existing biological structures. Nobody's ever managed to breed a six-legged dog, or a dog with new sensory organs that don't exist in other animals, or a dog that can deliberately alter its own scent gland performance as a way of communicating complicated instructions to other dogs.
Why? Maybe that's the case for you, but there are things in the animal kingdom that make much more "conscious" use of biochemistry. I don't see why it'd be so hard to evolve a mechanism for this, certainly not compared to things like the voicebox or ears.
Because all it takes to create a 'simple ear' is a flap of skin that vibrates in response to vibrations in the air, and there are enough reasons to be sensitive to changes in ambient pressure and vibration for evolution to start acting on this fairly efficiently. All it takes to create 'simple vocal chords' is a set of muscles somewhere along an organism's windpipe that can selectively interfere with the airflow to create sounds. Both of these are the kind of things a random mutation can create given basic physical structures (like the organism having skin at all, like the organism having a windpipe at all combined with muscles for breathing and regulating the passage of air).
Creating a 'simple chemical-speaker' requires the organism to have some mechanism for deciding "Okay, now I'm going to add an extra T group to the RNA molecules I'm emitting, now I'm not, now I'm going to, now I'm not." This is a whole different order of problem than "now I'm going to flex this muscle, now I'm not, now I am, now I'm not." ALL mobile animal life in all plausible ecosystems shares the ability to decide to flex muscles, so an extra flap or blob of muscle tissue in the right place doesn't require the evolution of new biochemical pathways, new control structures, or new exotic failure modes for the organism's development.
The "chemical-speaker" requires all these things, because you need a special RNA-rewriting mechanism that (and this is important)
postdates the evolution of brains. This is very different from the way things work in Earthly life, where most of the basic mechanisms for how our RNA/DNA encoding system and read/write machinery work developed in pre-Cambrian times when all life was single-celled organisms. There aren't a lot of radical new features in RNA/DNA reading and writing, at the basic level, that emerged
after the development of multi-celled life. Some, but not a lot.
The problem is that most possible ways to alter RNA/DNA reading and writing result in rapid increases in mutation rate and death for the organism's descendants. It therefore takes evolution a long,
long time to create workable mechanisms for this sort of thing. Much longer than it takes to evolve minor adaptations to existing physiological structures that already work and just need to be reshaped a bit.
[1] Hey, that's exactly a mechanism! Length of a polymer chain is each discrete signal, you have different length emitters and different length receptors each with their own lengths, you get small to no leakage between similar lengths because protein binding sites are nifty that way, then you transmit and receive a intensity distributions in the polymer-length domain exactly the same way we transmit and receive intensity distributions in the frequency domain.
Yes, Q could design this capability into an organism and it would
totally work.
