Natural selection is cleverer than you are

April 1, 2022 • 1:07 pm

The title of this very short post is widely known in our trade as “Orgel’s Second Rule” after evolutionist Leslie Orgel. Of course the Rule doesn’t mean that natural selection is conscious or has a pre-planned goal or outcome: simply that sometimes natural selection can achieve a result so wild and unexpected that it looks as if there was a clever mind behind it. (I say this so that the ID types won’t attribute to me a “mind” behind evolution.)

I lectured about this during this trip when explaining how, in Antarctic “icefish”, the gene making the enzyme trypsinogen—produced by the pancreas to help digest food in the intestine—became duplicated over evolutionary time, and thereafter one duplicate became rearranged by natural selection so it produced a bizarre glycoprotein having a small amino-acid sequence repeated many times over. That sequence allows the new protein (the old one’s still there) to glom onto the small particles that would enable ice to form and grow in the body. It is an anti-freeze protein that, produced in large quantities, reduces the freezing point of the fish’s blood below the -1.9° C of Antarctic waters. Therefore the fish’s cells don’t freeze and it can survive in water that would kill most other fish.

We know the evolutionary source of this antifreeze protein because it bears, at the beginning and end, the “start” and “stop” bits of the DNA that makes trypsinogen. Those “vestigial sequences” are evidence of evolution—of the ancestry of the antifreeze protein.

But I’ve digressed. I wanted to talk about something I just realized. I took my daily half-hour constitutional on the top deck (daily when the weather’s good, that is), and came back thirsty. It was gloriously warm and sunny outside, and I grabbed my aluminum water bottle from the cabin fridge. (We all get these bottles to save plastic, as Hurtigruten is green.)

I don’t like “hydrating”, but this time I needed to. And this time, because I had a thought, I timed how long it took me to take the several deep swallows needed to slake my thirst. It was about five seconds.

Now over those five seconds, my body didn’t have time to absorb and use the water that it needed. The thirst, of course, is a signal that your body needs water. But what struck me is this: I drank sufficient water for my body’s needs before those needs had even begun to be satisfied!

In other words, not only is thirst a way of telling you need water (and hunger for food and so on), but the slaking of thirst is a way of telling you when you’ve had enough water. It’s as if by simply ingesting a medicine you need, you’re cured before the medicine even does its thing. Or it’s as if you have an infection, and the infection starts to go away five seconds after you swallow your antibiotic. Or it’s as if your headache went away the minute you swallowed your aspirin.

Of course, we didn’t evolve to take antibiotics, but we did evolve to drink water when needed. And natural selection has been clever enough to find a way to tell us that we’ve drunk enough before that water has entered our system.

Presumably the “that’s enough” reflex evolved because you don’t want to drink too much water. You could get bloated, or perhaps a predator is lurking nearby to snatch you as you guzzle from the water hole. I don’t know how it happened, but it did happen.

Maybe this seems tedious and obvious to you, but it still amazes me. What mechanism operates in your throat and stomach to let you know that you can stop drinking?

UPDATE: Well of course biologists have thought about this problem before; I certainly didn’t think I was the first. And, sure enough, in the first comment below Cyrus Martin, senior editor of Current Biology, steered me to a paper in his journal that dealth with the issue, “Thirst,” by David Leib et al.

Here’s the most relevant part, but the article has lots of information about the generators of thirst and the physiological distress it signals:

 Drinking quenches thirst in anticipation of water absorption

There is a delay of tens of minutes between the ingestion of water and its full absorption into the bloodstream. However, drinking can quench thirst within seconds, long before the ingested water has had time to alter the blood volume or osmolality. Thus, thirst is not quenched by the reverse of the process that generates it; instead, the brain terminates thirst by using sensory cues from the oropharynx to track ongoing water consumption and then estimate how this water intake will alter fluid balance in the future, after the water has been absorbed. These anticipatory signals are transmitted from the oral cavity to the SFO [JAC: the “subfornical organ” i the brain that detects changes in blood osmolarity and partly generates the “thirst signal”] where they inhibit the same thirst neurons that respond to change in the blood volume and osmolality. This circuit organization allows SFO thirst neurons to make a comparison between the physiological need for water, which they measure directly by monitoring the blood, and the amount of water that has recently been consumed, which they measure by tracking oropharyngeal signals of fluid intake. SFO thirst neurons compare these two values to decide when drinking should be terminated. It is likely that a similar integration occurs within other structures of the lamina terminalis that control drinking behavior and hormone release.

That IS clever, isn’t it? But wait—there’s more:

The specific oropharyngeal mechanisms that are used to track water consumption are not well defined. One signal appears to be temperature, because cold liquids inhibit SFO thirst neurons more efficiently than warm liquids, and oral cooling alone can reduce both thirst and the activity of these SFO cells. One explanation for this temperature-dependence is that water ingestion tends to cool the oropharynx, and as a result animals may learn to associate changes in oral temperature with the post-ingestive effects of water consumption. In addition to temperature, other somatosensory signals that report on the sensation of water in the oral cavity are likely to be important. There is also evidence that signals from further down the gastrointestinal tract, such as stretch receptors and osmosensors in the stomach, may play a role in thirst satiation. In none of these cases, however, is the identity of the relevant sensory neurons and the neural pathway by which they transmit information to the lamina terminalis clear.

I was just wondering last night why cold water is so much more desirable when you’re thirsty, and why people don’t just guzzle lukewarm water when they’re thirsty. This temperature-dependence, as it says above, may be a phenomenon involved with learning the relationship between temperature of water and how well your body is satisfied with the water. But, as usual, we don’t know the full answer.

I was just wondering last night why cold water is so much more desirable when you’re thirsty, and why people don’t just guzzle lukewarm water. This temperature-dependence, as it says above, may be a phenomenon involved with learning the relationship between temperature of water and how well your body is satisfied with the water. But, as usual, we don’t know the full answer.

58 thoughts on “Natural selection is cleverer than you are

  1. I get “Hangry” that is, low blood sugar makes me easy to anger and generally grumpy. But I eat a single mouthful of something and I almost immediately feel better, even though I know there’s no way that my body could have processed that bite of food into usable sugars in the cells.

    I don’t know if it is the placebo effect or my body is actually responding to the brain signals that I’ve eaten something, prepare for nutrients. But it happens to me too.

    1. Even better, your blood sugar doesn’t actually drop when you’re hungry and angry (unless you have diabetes and have taken insulin). The liver keeps producing glucose for many hours after your last meal, and can keep doing so by catabolizing proteins during many weeks of starvation. Blood sugar is meticulously defended irrespective of food intake or else the brain stops working. So it’s something else with a fast response time that gives you that hungry feeling, and something else that tells you to stop eating. Maybe ghrelins and leptins are the signal mediators.

      But whatever, something has evolved to stimulate thirst, hunger, and satiety that has the effect of reducing the perturbations of homeostasis that would result if you had rely on the actual chemical impact of ingested food or water. These effects are hard to study.. Thirsty animals allowed water will stop drinking when a balloon is inflated in the stomach. But this might mean only that it is uncomfortable to drink with your stomach distended by a balloon. So it goes.

    2. You would surely not survive well in a pre-modern society Kevin, if you cannot survive some hunger/thirst pangs?! 😉

  2. “… simply that sometimes natural selection can achieve a result so wild and unexpected that it looks as if there was a clever mind behind it. ”

    It struck me that this notion is in the realm of illusion.

    So bent lines, tables that look different, and all those illusions are one category – then biological illusion would be a different category, with mimicry, camouflage, etc. with how we perceive it.

    1. IIRC Richard Dawkins called that ‘design-oid’ in his ‘Climbing Mount Impossible’, It looks ‘designed’, but it is not.
      I like this ‘designoid’ designation.

      1. Absolutely

        I just find that perception of design to be subsumed by the phenomena of illusion in general – blind spots and limitations in human perception.

        I just don’t think I noticed the simple _illusion_ of it before – being so dazzled by the biology.

        Though I’m sure Dawkins has written that before – “illusion”.

  3. The water is stimulating some sort of “filling” response in the stomach. Filling is a sensation the body “knows”. I know a doctor who said eat nuts to feel full. S’all I know about “filling” phenomena (if it is one).

    I do not know the volume of the stomach.

      1. Yes yes, sorry, I was hastily writing down thoughts … I have to get a moment to … digest … the update, thanks.

  4. Or it’s as if you have an infection, and the infection starts to go away five seconds after you swallow your antibiotic. Or it’s as if your headache went away the minute you swallowed your aspirin.

    Isn’t a headache a symptom of an underlying condition? So it would be as if the headache went away when you had yourself treated for the underlying condition.

    And it is as if you no longer feel the symptoms of the infection right after you have it treated.

    1. The way I understood Jerry’s exploration that you quoted, Chetiya, was that the immediate slaking of thirst by drinking (which is indeed what happens) sounds at first blush to be as absurd as imagining an infection or headache starting to go away as soon as you take antibiotic or aspirin (which is of course not what happens), before the drugs can be absorbed and transported to their site of action. Thirst-slaking is different.

      Now, it’s undoubtedly true that one may feel better immediately on swallowing a pain medicine or an antibiotic, although women with urinary tract infections will more often, in my experience, agonize for a few hours waiting for antibiotics to “kick in”. Immediate improvement in well-being is the placebo effect. Nonetheless it is mediated in the brain, and the brain is a physical object that obeys the laws of physics. There must be some neurochemical mechanism for the placebo effect, therefore, —dopamine, anyone? — and this could be mediated by the same neural reward pathways that already must exist for thirst-slaking, grafted on to and conditioned for things we invented later in our evolution. Viewed this way, immediate thirst-quenching is a kind of placebo effect that prevents the deleterious consequences of over-drinking that would occur if the animal drank until the blood osmolality returned to normal, because all the water in the stomach was yet to enter the blood. Over-hydration can be fatal and not just because a predator jumps you while you are guzzling too long at the watering hole.

      The Cell article is good. Jerry made an aside once about his undergraduate evolutionary biology courses often being not satisfying to teach because overly focused pre-med students who swelled the classes didn’t find the material relevant. I had a flash of recognition there. But I can say that once into medical school, a grasp of evolutionary principles is essential for understanding anatomy, physiology, biochemistry, and behaviour. Our professors enthusiastically taught us to think that way. I could talk for hours about the evolutionary biology of a little peptide called anti-diuretic hormone, which plays a central role in the excretion (or not) of excess water, and its close relative oxytocin.

  5. It is an anti-freeze protein that, produced in large quantities, reduces the freezing point of the fish’s blood below the -1.9° C of Antarctic waters.

    Wasn’t there an effort a while ago to splice the antifreeze gene from flounder into tomatoes to keep them from freezing on the vine?

    1. On Page 196 of my copy of Michael Pollan’s “The Botany of Desire” he mentions recent (2001) genetic engineering effors to splice genes from the flounder to produce frost tolerance in plants.

      So, basically, Yes.

  6. Any such mechanism is likely to be very imprecise. That is, my guess is that cold receptors + pressure receptors in the mouth, throat, esophagus, and stomach tell you you’ve drunk some water but there is no quantification of how much you ‘need’ or lack nor how much you just took in.

    The other side of the coin–thirst–is well known to be delayed and imprecise. Anyone who works or walks in the hot desert knows that you need to drink more water than you are thirsty for to avoid debilitating dehydration.

  7. From the Thirst paper:

    One of the most remarkable features of animals is their ability to sense their changing internal needs and then generate specific behavioral responses that restore homeostasis.

    My suspicion is that predictive processing is involved. The various neural nets ‘predict’ that we are thirsty from the various bodily sensations and so generate a ‘prediction’ that drinking will slake the thirst – and as long as the prediction doesn’t generate a feedback error we don’t need to process the progress of restoring homeostasis.

    Which might explain why cold water is more desirable when slaking thirst. It gives a much clearer and perhaps wider set of sensory inputs to alert the predictive processing that relief is on the way.

    1. An interesting anecdote: my cardiologist has gone into afib a number of times after chugging very cold water following a good workout. Now he drinks lukewarm water to hydrate after working out; I’m sure it’s not as satisfying, but I’ve never asked.

    2. In Indonesia you may get a small thermos flask with warm water on your bedside table.
      I’m not sure whether that is for you to take in more water in such a hot climate, or just that it is considered disagreeable to drink cold water when you’re still going to sleep.

  8. My question is why are we advised to drink much more water per day than is necessary to assuage thirst? I often hear amounts such as nearly a gallon of water (15.5 cups or 3.7 liters) for an average sized man living in a temperate climate.

    Does anybody really drink that much?

      1. Good article!

        So, 1945 is when the FDA issued guidance.

        I have been wondering if a water drinking fad developed in the late 80’s-early 90’s, but perhaps this has always been the case – or just how old one is, when they become more responsible…. dunno.

      2. I still have to take that article in, but I think sodium and salty foods bear mentioning, as they especially dehydrate and – as other discussion here has brought to light – perturb the potassium/sodium equilibrium, and potentially over the time span of digestion, so 2-3 days (I’m just brainstorming here, I’m not dictating – I need to read up on this myself).

        Then there’s the booze but let’s not go there….

  9. Fascinating.

    Maybe we prefer cold water because in a hot natural environment running water (in a brook, say) must usually be both cooler and cleaner than stagnant water. Or maybe we prefer it because the mechanism that natural selection developed to sense we’ve drunk enough works best with cold water (given that in nature water is usually cold)—the preference would have evolved as a consequence.

    At any rate, this explains why we’d find it quite disagreeable to continue drinking water once our thirst is satiated, yet have no problem drinking a hot cup of coffee or tea right afterward.

    And all this reminds me of the ridiculous advice that we must drink x number of glasses of water per day, as if our thirst were not enough to know when we need to hydrate.

    1. Supposedly, the advice to drink eight glasses of water a day suddenly appeared in some textbook without reference early in the 20th century. No research-based source was ever located. Since it was a popular textbook, the advice went viral.

      As far as knowing when to hydrate, I think our senses for this are pretty inaccurate. Another commenter mentioned that people need to be advised to drink more water than they think they need when in the desert. I recall studies that show that people who give up coffee to reduce caffeine intake, often experience headaches because they don’t replace it with sufficient water.

      1. Why would one want to reduce one’s caffeine intake? 😁
        I always thought that these weekend headaches were a caffeine withdrawal symptom. However, i think the excess intake of ethanol the Friday evening, resulting in high aldehyde levels, may more often have something to do with it.

    2. This cold water preference must be cultural. I have lived long in the US and have puzzled many a waiter (gender generic) by asking for “Iced tea no ice”. I drink lukewarm water as I prefer it for appreciating the food I eat at the same time. Hopefully some cross-cultural studies have taken place, with and without ice-box.

  10. Not bang-on topic but close enough I hope, though maybe not new to most here:

    It is possible to dangerously over-indulge in water in conditions where it does not make you uncomfortable right at the time. A mathematical friend from some decades ago, who was certainly somewhat experienced, did end up in hospital some hours after an ultra-marathon (running, not nordic skiing). For some reason which I don’t remember he had drunk much more straight water, not energy drink or whatever, than usual and was unaware of this danger. Again I don’t quite remember what more specifically was happening, but I think it related to a dilution of blood sugar over time. Also the race would have been at least 50 miles, maybe more, so stretched over close to 7 hours at least for him–but soon after, a night or two in the hospital.

    I can’t remember exactly, but at least once, on the Vasaloppet course in Sweden, I took that amount of time, likely more. But the many long ones here and in Norway were in the 3.5 to 5 hours range for the mediocre like me; except for the Canadian Ski Marathon, which despite its name is not a race. We used to have a TV exercise program on City TV called the 20 minute workout. So my joke was that this CSM was the 20 hour workout for slowpokes—but over a 2-day weekend, about 85 km/day. I seemed to mostly avoid drink problems, though do vividly remember about 15 minutes of indigestion in that only Sweden event above I ever did, from stuffing down a bun too fast.

    Drifting here re nordic racing, indulge an old fart!—It’s hard to believe how the best can race 50 km. with over 5000 feet of net elevation gain in barely over 2 hours. These days it’s mass start so maybe a bit of ‘game-playing’ early on, which increases possible times. I think at Thunder Bay back in 1995 in probably the only World Championship races ever in Canada, and with individual timed starts, there were some men under 2 hours. And record about 2:15 in the Birkebeiner (the real one in Norway) for 54 km and climbing two small mountain ranges with the finish in Lillehammer noticeably higher than the start near Rena.

      1. And that story reminds us of the Medieval torture method where the victim was forced to drink water unto water overdose.

  11. Good that you got some molecular evolutionary aspects into one of your lectures. How did that seem to be received, or any of your others? Any interesting passengers encountered – maybe for a future post?

    1. Yes. Now that you are speaking with live audiences, what does their interest level and technical background seem to be? I do not recall you commenting on any differences between your zoom talks on the first voyage and the live ones on this expedition…other than your somewhat worried anticipation before the first zoom lectures. How are things working out now that processes are returning to pre-pandemic conditions? (Sorry for the late comment but just got to yesterday’s weit this morning)

  12. If you try infinite things and let the results compete and allow only the fittest to survive then how could the outcome be anything different?

    1. You can’t try infinite things. Even with bacteria and other microbes you can only get to moderately large numbers (a dozen or two digits – you hardly need paper to keep track of them). For large animals (humans, for example) you can’t get more than a few billion (10 digits) without needing a spare planet or ten.
      The potential coding “space” of DNA is a lot larger. The human population today couldn’t “explore” the space of a 17-codon long string, and a credible bacterial population would start running out of elbow room by about 30-35 codons long. Human insulation has 51 amino acids, so 153 base pairs (plus a few codons of administrivia, which is extremely non-trivial).
      Very seriously, you can’t explore the whole of the potential “space” of DNA coding and protein sequencing. And that’s not even considering the other large, complex, variable biological molecules.
      Dawkin’s analogy of “exploring a landscape” developed in “Climbing Mount Improbable” doesn’t do a good job of expressing just how few and far between are the trajectories (“paths”) through the landscape which have been followed by one protein (codon sequence) morphing into another as evolution has gone on. Looking at insulin (only!), and taking a walking analogy for the path, the amount of “protein” space “explored” by human insulin compared to the available landscape would be akin to walking a few yards on the surface of the Earth and declaring that you’ve explored the planet.

      1. Given billions of years for life to develop, the number of mutations during that process is close to infinite, even though the vast majority may fail instantly.

          1. I am a simple peasant, so any extremely high unimaginable number is close to infinity to me. And, it does not appear to undermine my argument, so I leave the splitting of hairs up to the experts.

            1. Is YouTube for peasants?

              If so, check out Graham’s number on Numberphile.

              Or Hilbert’s infinity hotel on Veritasium.

              1. “useless”

                memorizing a “phone book” (remember those?) is useless.

                But pi is not a phone book.

                I don’t know what “use” it would have, but I think it really depends on what it is used for and how.

                There’s a competition in England for memorizing pi – there’s a video on YouTube.

              2. I made an argument about why natural selection might be cleverer than we are, but the ensuing discussion is not about the argument itself, but about the nature of infinity, and why you cannot get close to it. And then it derails even further towards numbers in general. And now you expect me to look at videos about numbers. The point of my remark is entirely lost, which is:

                The outcome of endless trying and failing in a process of natural selection may produce a better result than design, simply because some unimaginably high number of options have been tried and only the best survived.

  13. Regarding the secondary question about why cold water is more desirable when you’re thirsty than warm water, could this perhaps be linked to maintaining temperature balance? Thirst often (but not always) coincides with situations in which the body is working hard to shed heat – either as a result of vigorous exercise or of high ambient temperatures – and ingesting cold water can help to achieve this heat loss whilst simultaneously addressing the thirst you are experiencing.

    1. True, but in both cases, the signal (fluid or cold) has to be processed in the brain and interpreted as a prediction that the actual chemical or thermal effect willhappen, sending an order to stop the ingestion of cold fluids long before tonicity and temperature fall to normal. If the brain was deaf to these signals, you would overshoot and be over-hydrated and shivering instead of comfortable. Reducing latency in the feedback loops prevents maladaptive, energy-wasting oscillations around the target value. Even the thermostat in your house does some anticipatory signal processing to maintain the set point efficiently. The advice to drink before you are thirsty when exercising in hot weather is another form of learned anticipatory signal processing which gets around some of the evolutionarily constrained compromises that our natural regulators have to live with.

      1. Yes I understand that Leslie. I was just musing on why we tend to prefer cold water when luke-warm is presumably as good for re-hydration. The mechanisms by which the brain determines we have drunk enough and prevents us overshooting are indeed fascinating. We tend to take homeostasis for granted except when it goes wrong but it is one of the many amazing aspects of our biology.

      2. By the way

        Cold water means more calories burned.

        Sounds like a bad diet plan but its a reason why Michael Phelps could pack the fast food in but win eight (?) gold medals – the pool is a thermal sink. I think its held at a cool T – not really comfy. Ray Cronise (former NASA engineer) made this argument.

        Of course it didn’t hurt that Phelps practiced like a maniac.

    2. Consider an 80kg human (the normal weight for testing ropes and safety gear) who is 1°C overheated throughout (a fairly serious situation). They drink a kilogram of water at 4°C (maximum density). Neglecting the differing specific latent heat capacities of flesh versus bone, the water would cool the user by … I estimate about 0.4°C.
      Actually, that calculation is a bit more optimistic than I’d expected – I’d only been expecting about 0.1°C. But it’s still a fairly small effect in a quite serious situation. That is why the standard survival advice is that water doesn’t do you any good in your bottle. If you feel thirsty, put it down your neck. The benefit you’ll get from sweat cooling will be a lot more than the chill from drinking cold water.
      Besides, after several hours in your sweaty rucksack, the bottle of water is going to be pretty close to ambient temperature. We used to get bottled water delivered in “ton bags” of 1L bottles (because it is easier to shift from the container to stores) and they didn’t go into the cold store because we didn’t have room in there for that much “just” water. Most people just carried a kilo bottle in their coverall pocket and replaced it when empty and passing one of the ton bags.

  14. Have you considered the fact that water that is cold is likely from a running source, not a stagnant one, which would be warm and full of other forms of life. Cold water signals that it is pure. No doubt our ancestors figured this out in a very short time….most of them, that is.

  15. A few gulps of water is sufficient to slake thirst. Why is this not the case with coffee, wine, beer, and spirits?

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