Reinventing ourselves

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Reinventing evolution is a peculiar task. Evolution is, if anything, an endlessly inventive process, constantly spawning novelties and innovation, and imagination is not adequate to explain the variety we find now. Evolution led to mushrooms and oak trees, mosquitoes and jellyfish, paramecia and people...each one representing a reinvention in itself. How can we improve on that?
We can't. Evolution does have the constraint that new features should have local, short term utility, though, and organisms inherit the constraints imposed on them by ancestral adaptations. So let's play the game of imagining we could go back in time and change a few features of life that function well, but with a mind to improving them for far future generations.
In the pre-Cambrian period, about 600 million years ago, our worm-like ancestors had a central task to perform: reproduction. This involved shedding sperm and eggs into the water, nothing more, and they co-opted the openings and apparatus that they already had handy for excretion to do the job. Shedding gametes, shedding feces…similar tasks, right? That expedient sharing of those two functions had long-term consequences we live with right now: Our reproductive organs are coupled to our organs of excretion, which later also got tangled up with our hind limbs. Now we have to deal with yeast infections and a pelvis that doubles as both a birth canal and a base for posture and walking.
What if that worm-like ancestor instead used its other body opening, the mouth, to expel gametes? Reproductive function would be moved forward, away from other messy, but necessary functions. Of course, it would mean that a kiss would be fraught with major new connotations and consequences, but I think we could cope.
Those same pre-Cambrian ancestors are thought to have been literally brainless, and only later would they evolve a central nervous system. Initially, they had only a simple strip of nervous tissue, and later still, part of that strip grew larger and more complicated to form a proper brain. There's no particular necessity that the brain would form in the head--that's again a product of convenience, since more sensory organs were located in the front of the animal, and induced an enlargement of the local part of the nervous system to cope with their input.
So let's meddle again, and instead put the brain somewhere near the middle of the animal. In that position, it can be better protected by the mass of bone and muscle in the chest, and also be more conveniently located relative to the heart and circulatory system. It changes our head from a bulbous housing for a crucial, delicate organ, all poised on a fragile stalk of a neck, to a flexible sensory and feeding apparatus.
Approximately 450 million years ago, our ancestors evolved paired fins. The pectoral (or front) fins evolved first, and later another pair formed in the pelvic region, and then…no more. There was no absolute proscription against additional limbs--some ancient fish, such as the Acanthodians, had multiple pairs between the pectoral and pelvic fins--but our lineage stopped with two pairs.
Let's add more. We don't need to be greedy. Just a single additional pair, for a total of six limbs, would be enough. Six would be very stable, and further, when our particular ancestors needed a pair for tool use and carrying things, they wouldn't need to teeter precariously on just two legs--they'd have four for locomotion. The centaur form would be stable and versatile.
As long as we're playing with the number of appendages, why settle for just five fingers? The first vertebrates to crawl onto the land in the late Devonian didn't. Our early terrestrial ancestors tinkered quite a bit with digit numbers 360 million years ago, with six, seven, and eight-fingered forms, before settling on five as enough. Eight-fingered hands would be interesting--imagine what a piano player could do!
We estimate that about 260 million years ago, in the Permian era, our ancestors evolved a new feature of their metabolism: They were warm-blooded. This was a great innovation that I don't recommend meddling with, since it allows for higher and more constant rates of activity, but there was one peculiarity in the way we handled one little problem. Sperm need to develop at a cooler temperature than the rest of the body. The solution: suspend the testicles in a bag outside the body, where the breezes would keep them cool. I contend that this is an awkward and inelegant way to deal with it (My fellow males know all about some of the other problems dangling delicate bits of our anatomy in exposed places can cause.) Some of our fellow mammals have fixed the problem in cleverer ways--elephants, for instance, keep their testicles tucked deep inside their bodies--and I propose that we adopt similar internal solutions.
The line that led to the mammals evolved in parallel with the dinosaurs, but with a difference in lifestyle. Two hundred million years ago, our ancestors lived a mostly nocturnal life, and our eyes lost features that only work in the bright light of the day, like color vision. We re-evolved the additional color pigments only about 100 million years ago. Thus, we're 100 million years behind on the visual refinements found in animals that didn't have to live in the dark for so long: the birds. 'Eagle-eyed' is a good thing: Let's give mammals the acuity and color capabilities of avian eyes.
Approximately 20 million years ago, the apes branched off of the primate tree and did something very unusual: They lost their tails. A feature that had characterized our lineage for a half billion years was reduced to a tiny stub, for reasons no one knows. Let's bring tails back. Furthermore, let's add to it the versatility that evolved in the New World Monkeys: Let's make it fully prehensile. Having an extra manipulatory appendage would be handy, and I think a nice long tail would be rather stylish, too. (As an aesthetic recommendation, I suggest we keep it furry: A rat-like tail just wouldn't be as attractive.)
We have a long history of many small changes, each one imposing new constraints on subsequent possibilities and shaping our current form. Reinventing the whole process could lead to something that looks as different from our current shape as we do from an insect. In this entirely hypothetical exercise I've imagined a result that is grossly different. Why not have a six-limbed organism, the two forelimbs used for manipulating objects with an assist from a prehensile tail, with a small head used for feeding, reproduction, and sensing the world around it with a pair of highly sensitive eyes, and a large brain safely ensconced in the chest? It would work at least as well as what we've got now, although it may be a less probable outcome.