Would copper-based blood be viable for a human-like land-based species in an earth-like atmosphere or would it be worse than iron-based blood?

  • $\begingroup$ yes i know i ask dumb questions my current "universe" is all over the place, as is my brain $\endgroup$ Commented Oct 17, 2022 at 23:09
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    $\begingroup$ I don't think this is a dumb question at all. +1 $\endgroup$ Commented Oct 18, 2022 at 13:00
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    $\begingroup$ Spock, is that you? $\endgroup$
    – Michael
    Commented Oct 18, 2022 at 14:57

2 Answers 2


No, it would be less efficient.

Hemocyanin is a protein that transports oxygen in some invertebrates the “blood” of animals that contain this protein, such as horseshoe crabs is blue when oxygenated because of the copper present in the protein. Hemocyanin is substantially less efficient at carrying oxygen compared to hemoglobin (what you have in your red blood, assuming you’re not a sentient horseshoe crab or octopus). Therefore a copper based blood would be less efficient if we look at the real world examples of copper based blood as a reference.

Hemocyanin is more efficient at lower temperatures and lower oxygen concentrations than hemoglobin, which is why it is present in animals like cephalopods, crabs, lobsters and other denizens of the deep.

Hemocyanin would not be sufficient as an oxygen carrier for any vertebrate breathing earth’s atmosphere.

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    $\begingroup$ Ah fair point @AlexP. I’ll edit my question to reflect that it’s inefficient for vertebrates in earth’s atmosphere as OP wanted $\endgroup$
    – user71781
    Commented Oct 17, 2022 at 23:26
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    $\begingroup$ Does the fact that Hemocyanin is less efficient than Hemoglobin necessarily imply that any molecule using copper for oxygen coordination will be less efficient than iron? $\endgroup$ Commented Oct 18, 2022 at 9:34
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    $\begingroup$ Good answer. This is probably as far as is reasonable to speculate concerning the tiny fragment of chemistry that is known to humanity. What might emerge from the vast ocean of the unknown is anyone's guess. $\endgroup$
    – Slarty
    Commented Oct 18, 2022 at 11:38
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    $\begingroup$ There are various variants of hemoglobin that have different saturation curves, optimized for different environments & use-cases (pulling oxygen from the blood into muscles, or pulling oxygen from the mother's blood into the fetus). It's not unlikely that known forms of hemocyanin are optimized for their current environment, but that variants are possible that would work better for air-breathing vertebrates. Whether any of those would actually be an improvement over hemoglobin, and whether evolution could find the time to evolve it with competition present, is a different question. $\endgroup$
    – AI0867
    Commented Oct 18, 2022 at 12:26
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    $\begingroup$ I agree with @H.Rappeport's implication: just because the complexes that we are aware of favour haem doesn't mean that something copper-based of which we are not aware (because the complex molecule has not evolved on Earth) couldn't be better $\endgroup$ Commented Oct 19, 2022 at 19:56

would it be worse than iron-based blood

Yes, as NixonCranium's answer states. Haemoglobin is a more efficient carrier of oxygen, so for equal concentrations of oxygenating compounds in the blood, haemoglobin is objectively better.

Would a copper based blood be viable for a human-like land-based species in an earth-like atmosphere

This is a very different question to checking what works best. Evolution moves incrementally into the "adjacent possible" solutions, so once a body plan is substantially committed to a solution, it's not often that you see evolutionary pressures to reverse that. More typically, what you end up with is further layers of adaptation to push a non-optimal solution into something which works for a new ecological niche; and evolution is a surprisingly good way to make things "viable".

With aquatic mammals, we actually have interesting examples of evolution going in the other direction. Hemocyanin is objectively better for deep-sea dwellers, but whales have evolved a blood supply with twice as much haemoglobin which allows them to dive to similar depths. It's easy to see how successive small increases in haemoglobin concentration would give animals an advantage in feeding range.

There's no reason to think that the same would not be true for animals with copper-based blood. If there are no haemoglobin-based animals already occupying that ecological niche, it would actually be surprising if something else didn't move in. Animals who can expand their feeding range further towards the surface would have a clear advantage over those who can't, so it's easy to see how they could evolve more concentrated blood to deal with the environment. And from there, animals who can make more use of the feeding or nesting opportunities on dry land would again have clear advantages. This doesn't mean that it would be the objectively best solution, any more than a whale's haemoglobin concentration and air-breathing are objectively the best solution for life in the depths of the ocean, but it's perfectly well within the bounds of what could be evolutionarily viable.


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