I noticed that peeing is rewarding? What the hell?! How did enough of my (human) non-ancestors die because peeing wasn’t rewarding enough? The answer is they weren’t homo sapiens or hominids at all.
I would split it into two questions:
(1) what’s the evolutionary benefit of peeing promptly?
(2) In general, if it’s evolutionarily beneficial to do X, why does the brain implement desire-to-X in the form of both “carrots” and “sticks”, as opposed to just one or just the other? Needing to pee is unpleasant (stick) AND peeing is then pleasant (carrot). Being hungry is unpleasant (stick) AND eating is then pleasant (carrot). Etc.
I do think there’s a generic answer to (2) in terms of learning algorithms etc., but no need to get into the details here.
As for (1), you’re wasting energy by carrying around extra weight of urine. Maybe there are other factors too. (Eventually of course you risk incontinence or injury or even death.) Yes I think it’s totally possible that our hominin ancestors had extra counterfactual children by wasting 0.1% less energy or whatever. Energy is important, and every little bit helps.
There are about ~100-200 different neurotransmitters our brains use. I was surprised to find out that I could not find a single neurotransmitter that is not shared between humans and mice (let me know if you can find one, though).
Like you said, truly new neurotransmitters are rare. For example, oxytocin and vasopressin split off from a common ancestor in a gene duplication event 500Mya, and the ancestral form has homologues in octopuses and insects etc. OTOH, even if mice and humans have homologous neurotransmitters, they presumably differ by at least a few mutations; they’re not exactly the same. (Separately, their functional effects are sometimes quite different! For example, eating induces oxytocin release in rodents but vasopressin release in humans.)
Anyway, looking into recent evolutionary changes to neurotransmitters (and especially neuropeptides) is an interesting idea (thanks!). I found this paper comparing endocrine systems of humans and chimps. It claims (among other things) that GNRH2 and UCN2 are protein-coding genes in humans but inactive (“pseudogenes”) in chimps. If true, what does that imply? Beats me. It does not seem to have any straightforward interpretation that I can see. Oh well.
Another factor for the evolutionary benefit of peeing promptly is it decreases the risk of Urinary Tract Infections. It also lets you drink more water.
I wonder if the carrots were implemented as a means of encouraging doing more of the thing than the organism would do otherwise. If there were no carrot aspect to hunger, the organism would only eat just enough to stop the pain of hunger but no more, missing the opportunity to build up fat stores for the future. If there were no carrot aspect to peeing, the organism would expel just enough to make the pain go away but wouldn’t empty the tank, so to speak, making them carry around some tiny amount of extra weight than otherwise. This might be a just-so story, but it’s the answer that comes to mind for me.
I would split it into two questions:
(1) what’s the evolutionary benefit of peeing promptly?
(2) In general, if it’s evolutionarily beneficial to do X, why does the brain implement desire-to-X in the form of both “carrots” and “sticks”, as opposed to just one or just the other? Needing to pee is unpleasant (stick) AND peeing is then pleasant (carrot). Being hungry is unpleasant (stick) AND eating is then pleasant (carrot). Etc.
I do think there’s a generic answer to (2) in terms of learning algorithms etc., but no need to get into the details here.
As for (1), you’re wasting energy by carrying around extra weight of urine. Maybe there are other factors too. (Eventually of course you risk incontinence or injury or even death.) Yes I think it’s totally possible that our hominin ancestors had extra counterfactual children by wasting 0.1% less energy or whatever. Energy is important, and every little bit helps.
Like you said, truly new neurotransmitters are rare. For example, oxytocin and vasopressin split off from a common ancestor in a gene duplication event 500Mya, and the ancestral form has homologues in octopuses and insects etc. OTOH, even if mice and humans have homologous neurotransmitters, they presumably differ by at least a few mutations; they’re not exactly the same. (Separately, their functional effects are sometimes quite different! For example, eating induces oxytocin release in rodents but vasopressin release in humans.)
Anyway, looking into recent evolutionary changes to neurotransmitters (and especially neuropeptides) is an interesting idea (thanks!). I found this paper comparing endocrine systems of humans and chimps. It claims (among other things) that GNRH2 and UCN2 are protein-coding genes in humans but inactive (“pseudogenes”) in chimps. If true, what does that imply? Beats me. It does not seem to have any straightforward interpretation that I can see. Oh well.
Another factor for the evolutionary benefit of peeing promptly is it decreases the risk of Urinary Tract Infections. It also lets you drink more water.
I wonder if the carrots were implemented as a means of encouraging doing more of the thing than the organism would do otherwise. If there were no carrot aspect to hunger, the organism would only eat just enough to stop the pain of hunger but no more, missing the opportunity to build up fat stores for the future. If there were no carrot aspect to peeing, the organism would expel just enough to make the pain go away but wouldn’t empty the tank, so to speak, making them carry around some tiny amount of extra weight than otherwise. This might be a just-so story, but it’s the answer that comes to mind for me.