It's wild to me that that molecule and it's derivatives are so localized to certain species. For folks information, Tyrian purple is indigo that has been substituted on both ends with bromine groups (6,6'-dibromoindigo). Indigo itself is only found in a couple species (indigo and woad are the most prevalent). Also Tyrian purple was harvested by "milking" a certain snail for minute amounts of the dye.

I went to a farm near me to buy some madder (the roots of which contains the anthraquinoidal dye alizarin) and one of the farmers, an older man, told me all of that above and I asked him if he was a chemist? Non chemists don't use words like "substituents."

"In a past life," he said.

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The origin is freezing point depression. Broadly, salty water - and the water can come from skin moisture alone - has a lower water concentration (<100% water) than pure ice (100% water), so water tends to move to the area of lower concentration. (This is equivalent to the equilibrium melting point going down.) But this movement requires melting, which absorbs substantial energy corresponding to the latent heat and makes the system and the surroundings much colder. This translates into much more severe freezing of tissue than with ice alone. Make sense?

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Why do you say that? Are they always symmetrical? What about phantom limbs or the cannibalisation of certain areas?

How does limb dominance factor into that?

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I can't read the citation now, but the thyroid hormones affect mitochondrion function, so iodine was key to the metabolism of mitochondria.

It's really no different from other hormones: they are only synthesized in one organ and then released into the body. Sex hormones are mainly (not exclusively) synthesized in the gonads, etc.

It is fascinating that the body regulates thyroid hormones the same way a chemical plant would regulate a key chemical today. The pituitary gland monitors the level of thyroxine in the blood and emits thyroid stimulating hormone in greater or lesser amounts if the thyroxine level is too low or too high. The thyroid, physically located separate from the pituitary, then produces thyroxine depending on the TSH level it senses. If the sensing function and the synthesis function were physically located at the same place, the body couldn't control what was happening.

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Plus it is commonly found in seawater. So if life evolved in seawater it would use elements available to it.

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for your 2nd para, how do we know all this is correct? isnt cmb the only information we have from early universe and it comes after 100k years of big bang? (if so, all this 10^-36, 10^-6 secs after the big bang sounds like a bunch of hokum, when there is zero evidence to verify it)

Others have made good points about halogen availability in marine life, but I think it's also good to look at it from a biochem perspective. T3 and T4 use phenol rings with iodine substituents sticking off them. The size of these groups will be pretty fuggen big compared to organic groups with similar functions. This makes sense, because thyroid hormones have a direct effect on the overall pace of metabolism across the whole organism. You want that lock to turn with a big key, so that it won't easily get accidentally triggered. Put another way: the harder it is to accidentally trigger thyroid hormone receptors, the better an organism will be at autoregulating its metabolism.

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I use Dulse flakes ( dried seaweed ) daily to get my dose of iodine. It's loaded with the stuff.

You know how salt has historically been used as a de-icer? That's because salt water has a lower freezing point than normal water, meaning it has to get colder to freeze.

When it gets cold enough anyway, salt water forms something with the cool name of a "eutectic frigorific mixture," which establishes crystals of high-heat-transfer ice crystals on skin at much, much lower temperatures than normal ice. That means really bad frostburn with no sensitivity to know you're even being burned.

This has apparently been idiotically embraced as an "internet challenge" enough to merit a Wikipedia page: https://en.m.wikipedia.org/wiki/Salt_and_ice_challenge

>The mixture of ice and salt create eutectic frigorific mixture which can get as cold as −18 °C (0 °F).[3]

>The salt and ice challenge can quickly cause second- and third-degree injuries similar to frostbite or being burnt with the metal end of a lighter, as well as causing painful open sores to form on the skin. Due to the numbing sensation of the cold and possible nerve damage during the stunt, participants are often unaware of the extent of any injuries sustained during the challenge, only feeling pain once the salt on their skin enters lesions created during the challenge. Skin discoloration from the challenge may remain after the challenge has been attempted.[4][5][6]

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To add to this, it’s also why you’re mostly likely to find halogen-containing biomolecules in aquatic species, especially marine mollusks. The dye Tyrian purple would be a classic example.

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