The Singular supplement that every human should take is B12.
B12 is produced by soil bacteria. We're essentially washing our vegetables in disinfectant (chlorinated water) + Cooking. We've killed off most soil bacteria.
Soils are also Cobalt depleted. Further lowering the B12 content of all foods.
B12 is produced by lower gut bacteria (in humans), but not absorb-able in high quantities. Scientists posit that, humans with dirty underwear and who'd invariably scratch their butts, might have, in the past, transferred the necessary quantity of B12 bacteria onto their hands, then rest of the body, and through handling food.
Disclaimer, there is also some science to suggest we don't need B12 supplementation either, and that measured deficiencies had other causes.