Its just one of the trace elements (among cobalt, copper, zinc, chromium, flouride etc for various other specialised mechanisms) that the life forms on earth have evolved to utilise for specific purposes. Its very hard to 'exactly' trace back the evolutionary mechanisms for specific metabolic pathways in highly evolved creatures and be sure that a particular narrative is the only one.
I know it kinda doesn't answer you question exactly. Life just used and incorporated whatever chemicals (in roughly the same proportion) were available in the environment around them.