[Earw0rm]

My point is that, in a higher organism, it may be essential to how a lot of their processes function, in that it was infrastructure that already existed at the time those processes were developed, so they were in turn developed to depend upon it.

So "the reason it originally exists" and "what breaks if you take it away" aren't necessarily the same thing.

As with, say, digestion, or an major organ like the liver, it's reasonable to think that it does simple things in simple animals, and more complex things in more complex ones.

Take out an animal's liver, it's not one process that stops working, it's dozens. There's one or two that will kill it quicker, so those are the ones it dies of, but artificial livers are hard to build as they implement so many vital processes.

[fc417fc802]

I don't dispute any of that but it's stating the obvious, it's nothing more than topical conjecture (even if it's almost certainly correct), and (most importantly) it does nothing to answer the question. What essential functions are being performed?

Take your liver example. We can largely answer that same question. I can't off the top of my head but the answer is fairly well established even if incomplete to varying degrees depending on the species.

There is widespread consensus on why a liver is needed for survival whereas there is not for sleep. That's particularly interesting when you consider that sleep is more common across the tree of life than dedicated livers are (at least AFAIK).

[Earw0rm]

Sleep is more common across the tree of life because it's older.

Older than bilateral symmetry even - jellyfish are thought to sleep, sponges however do not. Jellyfish don't have spinal columns, lungs, gills, livers, kidneys, hearts, guts or blood, but they do have nerves and they do seem to sleep.

There is widespread consensus as to which processes failing will kill you first in acute liver failure, but it governs dozens of processes that, medium term, are essential to life; not all are widely understood.

In the case of sleep, it seems to be nervous system dysregulation that kills. It's notable that comatose patients don't seem to suffer the ill-effects of sleep deprivation. But still, "the thing that kills an animal subject to extreme sleep deprivation" is not necessarily "the original process for which sleep was evolved".

Human brains do some fairly complicated vital things during sleep (REM, spindles, slow wave activity), but that can't be the original essential function - the simplest animals that sleep (jellyfish) don't really have brains, although they do have nervous systems.

Whereas those animals which lack nervous systems (sponges) can't be said to sleep, although it's reasonable to ask.. "how would you tell?", or ask whether the question itself makes sense for something that lacks the ability to sense, plan, act.

So another framing is "anything which can be awake, must also be asleep". But one might equally argue, we don't know why animals are awake.

We can go one step further and suppose that, in order for an animal to act, to exercise will, it must do so at above its average metabolic rate, and in doing so it necessarily incurs metabolic debt.