system
Given the following post, respond as if you're commenting on Reddit:
human
Why do some species use copper to transport oxygen through their blood while most other species use iron?

I'm under the impression that copper is less efficient at transporting oxygen, so why don't all animals use iron instead? Did some just never evolve to be able to use iron even though it would actually be better for them?
gpt
So, this is a very abbreviated explanation, but you have to think of it in terms of evolutionary problem solving. Life finds new answers to new problems all the time in evolution, and this is a great example. Consider that the main disadvantage to hemocyanin is that it does not bind the oxygen as effectively or plentifully, so it's just not as effective as hemoglobin. Hemocyanin also performs better in colder, less oxygen-rich environments, like the oceans. The groupings of creatures that use molecules other than hemoglobin to transport oxygen through tissues are much more primitive, and their bodies are much smaller (which is a big part of it). Hemocyanin is less efficient, but in a small body that doesn't have far to transport stuff, it can work just as well as hemoglobin. They don't need to evolve using hemoglobin, hemocyanin works fine for their use case. Hemocyanin users are also cold-blooded, so they tend to just need less oxygen in general because they're not thermoregulating. They are not lacking anything by using hemocyanin because it works perfectly for their life history.

Hemoglobin is something that evolved to solve two issues unique to vertebrates: thermoregulation and being big. Thermoregulation is a problem particularly in mammals; they expend energy and oxygen warming their bodies, therefore they need more oxygen to reliably reach their tissues. Hemoglobin is the answer to that problem. The other thing to consider is that a blue crab only needs the oxygen to make it a matter of inches, so the binding between the oxygen molecule and the metalloprotein does not need to be that strong. In a blue whale, the oxygen may need to reliably be transported hundreds of feet, so we need hemoglobin, to which oxygen bonds much more strongly, to make sure that happens. So yeah, animals using hemocyanin are not at any disadvantage because they don't NEED hemoglobin to meet their oxygen needs. For a smaller, cold-blooded creature hemocyanin is more than enough. Hemoglobin is necessary for animals that have a much higher oxygen demand and potentially a much farther oxygen transport distance. 

And just for fun, hemoglobin (red blood) and hemocyanin (blue blood) are not the only options out there. There's also Chlorocruorins, used in many annelids which makes the deoxygenated blood green, vanabins in sea squirts that use vanadium as the metal binder (it doesn't work very well), and coboglobins which appear yellow (there's a few more, but those are the fun-colored ones). There is also a group of fish called cod icefish which live in environments so cold and so-oxygen rich that they don't use any oxygen transport proteins at all, so their "blood" is clear. But yeah, everything evolves to solve a new problem, and hemocyanin users have no need for hemoglobin, and hemoglobin users could never survive using hemocyanin. One is not "better" than another, they're just different use cases.