And perhaps this was just as well, for this change to the very fabric of reality would cost Humanity, or rather what would become of them, very dearly. There was more in store for the world's self proclaimed Masters than a rudimentary change of species, or few alterations to their society that nobody would ever remember, or care about if they did.
No, indeed, the dominant race of the Earth, impetuous and wrathful Masters, and lost in the throes of their own hubris, were to be brought down indeed. For not only were they to lose their humanity, but their independence, and even their very personhood.
As Humanity's form transitions to follow the new course of reality, history itself seemingly being rewritten by some unseen hand, their bodies will undergo a most novel metamorphosis. If one were to observe with the omniscience of an outside observer, they might not catch on to the peculiar nature of the transformation at first, just like those whose minds will be unaffected by the shift's effects. Eventually, however, the dire circumstances will be unmistakable. During their alteration, the form of the Human race will reverse. Limbs becoming their polar opposite, arms and hands into sturdy legs and clumsy paws, hooves, or claws, as their own legs become new forelimbs. Their body morphing, mass shifting and organs migrating, and their entire skeletal structure adapting to frame their new body. Their reproductive organs or their posterior starting to bulge out, starting to take on new features, becoming something more... Forming into a head.
Most alarming, however, would be to the fate of the victim themselves. What becomes of their head might depend on many things. It could be something mirrored in the rest of their kind, or it could be unique to them. But perhaps their neck will begin to pull back into what was once their shoulders, even as their lips start to pucker, forming into some creature's anus, and their nose stretching into a tail, the rest of their unneeded features molding closed. Perhaps their mammaries might start to swell, forming into round, sensitive orbs if they weren't already, skin sagging loosely around them, as their neck starts to stiffen and elongate, skull dissolving into hot, throbbing, spongy flesh as their hair falls away, head molding into a sensual form as rich seed drools from their pursed lips. Or perhaps they may still find themselves being pulled back by the neck into the malleable flesh of their changing body, their face becoming soft and smooth, before filling out with feathers, scales, or fur, whilst their mouth fills with pleasure, stretching into a sensitive, delicate slit, leaking sweet juices as it aches to feel release...
When all is said and done, the resulting creature will look (And, for all intensive purposes be) completely normal. The new being itself could be anything and everything, depending on which path reality itself takes, from a feral, mundane animal, to prehistoric beasts, to anthropomorphic creatures, or monsters of myth and legend, to the truly alien. Even the supernatural can not be ruled out. And what was once the Human race will be nothing more than parts of their bodies, their behinds, their sex... And yet they will still be fully conscious. Most will have no memory of their former lives, of the reality that came before, of what it was like to do, to walk, to breathe, to make decisions for themselves. They will only be able to recall their servitude, the existence as a piece of another being's anatomy. And they will be content to serve, knowing no other way of life, and blissfully unaware of how far they've fallen.
Those who kept their awareness of the past reality, however, will not be quite so lucky. Their body wracked by a horrific transmutation, forced to watch and endure as their very body, as well as their lives, are gradually stripped away from them, increasingly falling under the control of a foreign creature, which they will have to spend the rest of their existence bound to, nothing more than a piece of it's flesh, to be used however it desires, with their new Master potentially not even cognizant of their presence. They will be silent witnesses, recording the apocalyptic cascade in reality, and their memories of Humanity and the life before, but unable to even share this tale.
The time is almost at hand, dear Human. The shift in reality is coming. There is nothing special to protect you from fate. There are no laws of reason or logic that can stop this. Only endless possibility as nature itself is remade. There are no entities guiding the circumstances at work here that might take pity in you. This is just a sort of "hiccup" in the universe itself. You are just like all of the rest, and you will be reformed too, just like the rest of them. There is nothing that you can do to prevent this. But who are you?