Far fetched? Ever seen the human brain down to it's structure, nerves, cells, proteins, hormones that flow through it, etc, or that of one that is abnormal? Or aged? The Bible cover any of that? Oh, that's right, it does - demonic possession, or a curse from God visted unto the children of sinners...
And you are correct about that - easier to think a petty vindictive bogeyman in the sky causes cerebral palsy rather than nerves not sheethed in sufficient myelin, or demented rather than clumping nerves misfiring in the that of someone with Althzeimer's
You attempt to use bits of science configured to to a religious scaffolding - YOUR religion, conveniantly.
Despite Noah supposedly taking 2 of EVERY animal (or at least not the unclean ones, or, but he did anyways, and...), we have the remains of creatures which are extinct, most of those not mentioned in the Bible, despite the fact that Adam named every one of them - or was that only those in Eden, or outside in the world where the Gods also created them...?
Anyways, not every creature to have lived has been discovered, as new ones are found all the time. Of course there's gaps, but those are narrowing over time. Take the illustrious "Where's the missing link?" question STILL asked by creationists. In terms of humans, turns out there isn't one, there's several. Some are not even links, but branches, dead ends that didn't lead to modern humans. So much for this single linear threads of transition theory you refer to as Darwinism which is not.
Speaking of.... It has been known for decades that the first identified Neanderthal was riddled by athritis. The stoop shouldered image that lingered was incorrect. The rest found have not been malformed by such. Further, as they began to disappear, hybridized fossils of H Neanderthalis and H Sapiens have been found. Intermediary cranial features, limb lengths... Now we can openly admit that facial characteriscs of Caucasions (higher bridged nose, narrower elongated skull, sloping forehead) which resemble Neanderthals do because they came from them.
It is curious that chimps, as you mentioned, so closely resemble humans, reckon that intelligent designer was running short on ideas. The farther back, the closer they become. Bear in mind that the transitions are not merely variants on the same plane, as the greatest differences occur further down the strata. Creatures evolve, lineages separate, and links widen over time. That's what evolution does.