All that is assuming that you do know for certain the evolution did NOT exploit it.

There is some extrapolative argument that hints at the contrary. Adrian Thompson's evolved FPGA circuits exploited a single chip's underlying physics in a manner no digital circuit designer would. By that thread, it would seem possible that evolution has already exploited quantum computation ... just that maybe we haven't had the tech eye to see it yet.

After all, all systems are quantum mechanical.

I don't think 2 is so easy to justify. Quantum state transitions occur all the time in a discordant mess of activity at room temperatures. The only way we can control for that and have quantum transitions occur in useful controlled circumstances is by operating at close to absolute zero, which isn't very conducive to exploitation by terrestrial life.

It may turn out to be a similar issue to jet engines, or semiconductors. The materials and conditions required for them to operate just aren't very easy for terrestrial life to evolve into.

Nature works on the premise of emergence. It's entirely possible that the energy gradient required for a biological system to make successful, permanent changes beneath its fundamental layer of operation is just too much.

Suppose this all started with a few self-replicating proteins. From that we got organelles, and then cells. Then multi-cellular organisms, then tissues, organs, etc.

But working backwards, from protein -> molecular chemistry -> quantum phenomenon, may simply not be the path of least resistance and thus for the overwhelming majority of life in the universe, was not an evolutionary path.

The answer is a combination of 1 and 2. Not every computing device is actually useful. For example, you can find plenty of brain parts that look vaguely like GPUs or FPGAs. None that look anything like a standard CPU. This would be basically impossible to build out of cells, and not useful anyway.

The same thing applies to quantum computers. They’re much much harder to build because they’re more delicate. We’re talking about effects that usually are completely destroyed by a single unwanted atom coming in and hitting something. And there are a lot of atoms flying around in cells. Propagating any quantum signal from even one cell to an adjacent one is impossible. Finally they’re less useful. I can’t think of problems a biological brain needs to solve that require even a moderately fast CPU, let alone a quantum computer, which provides speedups over the CPU for only certain specific problems.

But none of this really matters, because your comment is one long isolated demand for rigor. You wave away my long list of examples because you think something very distantly related exists (in which case, with those low standards, quantum computers already exist), or because the examples are clearly impossible or not useful (without equally seriously considering the same for quantum computers). This is what I mean by skepticism of QC being driven mostly by intellectual fashion.

It's 2. There is no way for cell/proteins to maintain a useful small scale quantum state.