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[i2amroy]

I'd probably call it a rather unstable equilibrium at the beginning though; if we have two similar creatures but one is using hemoglobin and the other hemocyanin than evolution is going to favor the hemoglobin one hard-core. It's one of those things that if it manages to make it into the widespread intelligence than it's more of a hanger-on where the other benefits the species had outweighed any that their close competitors had more than the cost of having hemocyanin as their blood type limited them.

On the note of copper vs. iron, I would like to point out that the chance of you encountering a copper-rich and iron-poor planet is vastly less likely to occur than one that is iron-rich (or has no real metals at all). This is because the Iron peak means that stars can fuse everything up to iron to generate energy, but as soon as they start to fuse iron they begin consuming energy, and suffer core collapse very quickly. Most higher elements than iron are generally only generated in the tiny moments of a supernovae, and in much smaller quantities, meaning that iron-based planets are going to be much more common than copper-rich ones (and both will be much more common than a iron-rare copper-rich one). As a matter of fact, judging by the abundance of the chemical elements in the Milky Way Galaxy, iron is approximately three orders of magnitude (Cu# * 10^3 = Fe#) more common than copper is.

[Grim Portent]

Edit: And why even use the same hemocyanin as is found on Earth? A larger molecule with more oxygen binding sites is entirely within the realm of possibility as well.

A large version of hemocyanin would be unstable and energy intensive to create. Just because it can happen doesn't make it practical or useful.

[lemon10]

Just because hemoglobin is the most efficient terrestrial molecule for blood doesn't mean that its the most efficient out there.

Honestly, the odds that any single one of our evolved mechanisms are "the best" in the entire universe is exceedingly unlikely (indeed, this extends to all life on earth). It might be above average compared to the average species, but there is no reason to think that its particularly special.

Even if the odds are 95% that you are going to end up with functions of around equivalent efficiency, that means that some species are going to end up being lots more (and some much less) effective at some things.