Why do almost all animals have red blood?

Question

It can’t be a coincidence that almost all of them have red blood. Why can’t it be blue? Green? Purple?

In: Biology

Answer 1

While others have mentioned hemoglobin as the oxigen transporting molecule that gives blood its color there’s a point missing: all of these animals with “red blood” evolved from a common hemoglobin-using ancestor – hence today you see its use spread across the animal kingdom.

P.S. I’m not excluding any sort of converging evolution, where another species not having a common ancestor may have evolved to use hemoglobin as its oxigen carrying molecule.