edit: or it may be that iodine deficiency is historically recent and sufficiently rare to result in any specific adaptations. I was basing it on Lithuania which has iodine deficient soil even fairly close to the coast, but that state of affairs may be geologically recent.
What would cause iodine deficiencies to be recent? As far as I know, remedies using seaweed go back thousands of years; that was enough time for milk and altitude adaptations in some populations, and it seems to me that fixing iodine deficiency would be much more valuable if possible: being completely lactose-intolerant is not nearly as bad as being a shriveled retard who’s infertile.
By recent I meant last ~20 000 years. The lactose tolerance is an exceptionally simple adaptation: full adaptation in 1 step. Whereas I’d imagine available mutations for seaweed craving could only produce a slight preference for a wide class of foods with the first mutations.
Also it may be that there is such an adaptation, it’s just that it’s in a sub-population where it gone unnoticed. In either case we have an apparent fact that there’s no such adaptation, even though it would seem to be advantageous. Which perhaps can’t tell us much about why but can tell us not to expect more complex adaptations to that specific problem.
edit: also, isn’t iodine only necessary for the synthesis of thyroid hormones? In principle it ought to be possible to evolve not to need iodine in the first place, but that obviously won’t happen if iodine is common enough.
Also it may be that there is such an adaptation, it’s just that it’s in a sub-population where it gone unnoticed. In either case we have an apparent fact that there’s no such adaptation, even though it would seem to be advantageous. Which perhaps can’t tell us much about why but can tell us not to expect more complex adaptations to that specific problem.
I suspect such an adaptation would have been noticed. There’s a stupid number of studies in which the researcher takes urine/blood samples from women, measures iodine levels or a proxy for iodine level like TSH, and do a followup on the infants, and, mirabile dictu, the infants tend to have smaller heads or other problems in a fairly linear correlation with the mothers’ deficiencies. (Seriously, there’s like 1 a week of these in my Pubmed alerts for iodine, it’s quite a nuisance. Who funds this crap?) It’s common to record race or ethnicity as part of the covariates; if someone had found an interaction where, I dunno, Europeans were immune to iodine deficiency, I figure I would have heard about it because it would be so much more interesting and sexy a result than the usual one.
also, isn’t iodine only necessary for the synthesis of thyroid hormones? In principle it ought to be possible to evolve not to need iodine in the first place, but that obviously won’t happen if iodine is common enough.
I’m not sure. It may be that iodine is the only way to do the things it does in mammals—either because it’s a local optima or global. For example, you can theoretically have blood which doesn’t require iron for hemoglobin (which requires iron), as demonstrated by octopus blood using hemocyanin which relies on copper rather than iron, but no matter how little iron is available, it’s hard to see mammals ever evolving hemocyanin and abandoning hemoglobin. And more generally, now that the arsenic-bacteria seem to have been debunked, there doesn’t seem to be any replacement at all for phosphate in key roles like ATP; if Earth-life has no access to phosphate, it’s doomed.
(Of course, some other examples suggest the other way: humans lost the ability to synthesize vitamin C a while ago, and could probably recover it; but there’s never been enough selection pressure.)
But they wouldn’t be immune to deficiency, merely eating a diet that prevents the deficiency, through some sort of craving for seafood which is virtually impossible to discern from cultural. We do seem to have a preference for variety in food.
I’m not sure. It may be that iodine is the only way to do the things it does in mammals
My understanding is that it is used in signalling molecules used to control growth, but is not used in any key steps in main metabolism itself. Animals can survive on very little iodine, it seems—stunted and not growing right, but i’d imagine given enough time evolution would find a way to re-do that growth control using another molecules for hormones.
What would cause iodine deficiencies to be recent? As far as I know, remedies using seaweed go back thousands of years; that was enough time for milk and altitude adaptations in some populations, and it seems to me that fixing iodine deficiency would be much more valuable if possible: being completely lactose-intolerant is not nearly as bad as being a shriveled retard who’s infertile.
By recent I meant last ~20 000 years. The lactose tolerance is an exceptionally simple adaptation: full adaptation in 1 step. Whereas I’d imagine available mutations for seaweed craving could only produce a slight preference for a wide class of foods with the first mutations.
Also it may be that there is such an adaptation, it’s just that it’s in a sub-population where it gone unnoticed. In either case we have an apparent fact that there’s no such adaptation, even though it would seem to be advantageous. Which perhaps can’t tell us much about why but can tell us not to expect more complex adaptations to that specific problem.
edit: also, isn’t iodine only necessary for the synthesis of thyroid hormones? In principle it ought to be possible to evolve not to need iodine in the first place, but that obviously won’t happen if iodine is common enough.
I suspect such an adaptation would have been noticed. There’s a stupid number of studies in which the researcher takes urine/blood samples from women, measures iodine levels or a proxy for iodine level like TSH, and do a followup on the infants, and, mirabile dictu, the infants tend to have smaller heads or other problems in a fairly linear correlation with the mothers’ deficiencies. (Seriously, there’s like 1 a week of these in my Pubmed alerts for iodine, it’s quite a nuisance. Who funds this crap?) It’s common to record race or ethnicity as part of the covariates; if someone had found an interaction where, I dunno, Europeans were immune to iodine deficiency, I figure I would have heard about it because it would be so much more interesting and sexy a result than the usual one.
I’m not sure. It may be that iodine is the only way to do the things it does in mammals—either because it’s a local optima or global. For example, you can theoretically have blood which doesn’t require iron for hemoglobin (which requires iron), as demonstrated by octopus blood using hemocyanin which relies on copper rather than iron, but no matter how little iron is available, it’s hard to see mammals ever evolving hemocyanin and abandoning hemoglobin. And more generally, now that the arsenic-bacteria seem to have been debunked, there doesn’t seem to be any replacement at all for phosphate in key roles like ATP; if Earth-life has no access to phosphate, it’s doomed.
(Of course, some other examples suggest the other way: humans lost the ability to synthesize vitamin C a while ago, and could probably recover it; but there’s never been enough selection pressure.)
But they wouldn’t be immune to deficiency, merely eating a diet that prevents the deficiency, through some sort of craving for seafood which is virtually impossible to discern from cultural. We do seem to have a preference for variety in food.
My understanding is that it is used in signalling molecules used to control growth, but is not used in any key steps in main metabolism itself. Animals can survive on very little iodine, it seems—stunted and not growing right, but i’d imagine given enough time evolution would find a way to re-do that growth control using another molecules for hormones.