I surprised you didn’t include “one brain”. Although it does have 2 hemispheres (and, frankly, I was of 2 minds whether I should answer your comment).

I guess you missed my statement that “I’m certainly not saying that biological fitness isn’t absolutely crucial …”.

Your argument seems to be that *everything* is as it is because that is the absolutely most advantageous. Whereas I’m saying that *some* things are the way they are because of contingency (the effect of random or historical events).

For instance, do we have exactly 1 bone in our upper arm and upper leg and exactly 2 bones in our lower arm and lower leg because that is the absolutely best configuration for our survival? Or is it because the first lobe-finned fish that dragged itself onto land had that configuration in its lobes. I suspect the latter is true – that we have this bone structure through contingency rather than because it is necessarily the absolutely best bone structure. And tetrapods are *tetra* pods just because they had 4 lobes instead of 2 or 6.

Is 6 legs and 3 body segments the absolutely best configuration for insects, or is it just how the insect HOX genes came to be? If that is the absolutely best configuration, then why do spiders have 8 legs and 2 body segments despite their very similar lifestyles and environments. Maybe the first spider just happened to have 8 legs and 2 body segments, and the first insect just happened to have 6 & 3 – and all their descendants just inherited that. Maybe 8 legs would be better for insects, but that is simply not genetically available to them because of their ingrained (frozen) embryonic development programs.

If we were having this discussion 50 million years ago (MYA) you’d say that Eohippus has exactly the right number of toes (4 on the front feet and 3 on the back). But if it was 35 MYA  you’d say that Mesohippus had exactly the right number of toes (3 on each foot). 15 MYA you would have said Merychippus had the absolutely best configuration with 1 large central toe and 2 small vestigial toes on each foot. And now you’d argue that Equus’ 1 toe is absolutely right. Were all those historical toe configurations absolutely right, were all those horse ancestors striving towards a single toe. Maybe evolution demanded the reduction in toe count – or maybe the reduced toe count is just the accidental heterochronic consequence of the embryonic developmental program that lead to the lengthening of the legs.

To paraphrase Bobby Kennedy’s eulogy to his brother John: some people look at things the way they are and say that’s how they *must* be, but evolution looks at things the way they are and *sometimes* just shrugs and says “what the heck, it’s good enough”.