Do we have any theories as to why complex life eventually started requiring various metal elements as micronutrients?
Do we have any theories as to why complex life eventually started requiring various metal elements as micronutrients?
For example, why did zinc, of all things, start getting utilized by brain and prostate tissue in humans?
Metal ions can perform interesting chemical reactions that organic molecules cannot. A positively charge metal ion can also naturally bind to negatively charged proteins. So the organisms that more successfully took advantage of these chemical reactions reproduced more effectively than the organisms that didn't.
The answer is almost certainly "because it was available" but that doesn't exactly explain much.
Animals whose biology that used the metals reproduced more successfully is the explanation. It could longer lives, better reproductive outcomes, or a ton of other reasons but it all comes down to reproduction.
But that's all that can be said. Life is random. There is no "why". The body uses zinc because it can.
Depends how you're using "why". In Russian, they actually have two words for why, one of which implies teleology, and one which doesn't, and merely requests some explanation for a phenomenon. I wish we had that in English.
In this case, it's such a general question you can't do much better, but you could, for example, talk about why oxygen-carrying proteins pretty much always incorporate an ion of something, in a merely cause-and-effect way. (And I actually don't know the answer to that one)
Also, metals can easily accommodate varying number of electrons in the electron shells of their atoms and still be stable. That makes them very good to quickly store and release electrons which means they can help say transfer molecules around (iron for transport of gasses), scavenge free radicals (e.g. manganese) etc.
I'm guessing the main reason is "why not". It's in the environment, and one day some critter mutated a new process that involved it.
It's not just complex life, either. Bacteria use all kinds of strange elements in various enzymes and complexes.