Although the crux of your claim that diffusion is the rate-limiting step of many biological processes may be sound, the question you actually ask, “Why hasn’t evolution stumbled across a better method of doing things than passive diffusion?”, is misguided. Evolution has stumbled across such methods. Your post itself contains several examples of evolved systems which move energy and information faster than diffusion. These include the respiratory system, which moves air into and out of the lungs much faster than diffusion would allow; the circulatory system, which moves oxygen and glucose (energy), hormones (information), and many other things, all much faster than diffusion; and neurons, which move impulses down the axon much faster than diffusion. In none of these cases has the role of diffusion been completely removed, which is what you are looking for I suppose, but life has increased the total speed from point A to point B by several orders of magnitude by finding an alternate mechanism for most of the distance.
Beyond that type of optimization, I agree with you that this is a local maximum problem. Life’s fundamental processes are based on chemical reactions in aqueous solution, which necessarily involve diffusion, and any move beyond that is a big, low-probability step.
That said, particularly in the case of the brain, I don’t think evolution has thoroughly explored its possibility space, and there is room for further optimization within the current “paradigm”. As one example, the evolution of human intelligence proceeded at least in large part by scaling brain size, but we are currently limited by the constraint that the head needs to be able to fit through the mother’s pelvis during birth. Birds, based on totally different evolutionary pressures involving weight reduction for flight, seem to be able to pack processing power into a much smaller volume than mammals. If humans could “only” use bird brain architecture, then we could presumably increase our intelligence substantially without increasing the size of our heads. But despite the presumed evolutionary pressure (we suffer much higher maternal mortality than other species due to our big heads) there has not been enough time for us to convergently evolve brain miniaturization to the extent of birds.
Although the crux of your claim that diffusion is the rate-limiting step of many biological processes may be sound, the question you actually ask, “Why hasn’t evolution stumbled across a better method of doing things than passive diffusion?”, is misguided. Evolution has stumbled across such methods. Your post itself contains several examples of evolved systems which move energy and information faster than diffusion. These include the respiratory system, which moves air into and out of the lungs much faster than diffusion would allow; the circulatory system, which moves oxygen and glucose (energy), hormones (information), and many other things, all much faster than diffusion; and neurons, which move impulses down the axon much faster than diffusion. In none of these cases has the role of diffusion been completely removed, which is what you are looking for I suppose, but life has increased the total speed from point A to point B by several orders of magnitude by finding an alternate mechanism for most of the distance.
Beyond that type of optimization, I agree with you that this is a local maximum problem. Life’s fundamental processes are based on chemical reactions in aqueous solution, which necessarily involve diffusion, and any move beyond that is a big, low-probability step.
That said, particularly in the case of the brain, I don’t think evolution has thoroughly explored its possibility space, and there is room for further optimization within the current “paradigm”. As one example, the evolution of human intelligence proceeded at least in large part by scaling brain size, but we are currently limited by the constraint that the head needs to be able to fit through the mother’s pelvis during birth. Birds, based on totally different evolutionary pressures involving weight reduction for flight, seem to be able to pack processing power into a much smaller volume than mammals. If humans could “only” use bird brain architecture, then we could presumably increase our intelligence substantially without increasing the size of our heads. But despite the presumed evolutionary pressure (we suffer much higher maternal mortality than other species due to our big heads) there has not been enough time for us to convergently evolve brain miniaturization to the extent of birds.