Once upon a time, there was a incredibly brilliant but very naive scientist. He had a sincere love for animals, was a vegetarian, and objected to any and all animal testing.
He came up with what he thought was a great idea to end all animal cruelty, meat eating, and other such horrible things.
Over the course of most of his life, he invented a computer the size of a refrigerator that would calculate any input given to it, and alter reality to make it fit.
Yes, he invented a machine that could rewrite reality, and his ambition was limited to ending meat eating and animal abuse. He was also very narrow when it came to vision.
He wouldn't rewrite people's minds to fit his view, "I'm not evil," he would have defended.
Instead, he would make every animal everywhere have human intelligence... there was no way any human being would continue to eat meat of something intelligent... or animals would rise up and overthrow humanity, either way it was a improvement.
He inputted his instructions to the computer he had built by himself that could change reality... and maybe if he'd had an engineer look at it, they would have told him the thing was inevitably going to explode the moment it was used. Which it did, blowing itself to atoms and the scientist along with it.
But before it did, in the nanoseconds between the input and the explosion, it carried out its instructions. But the first rule of building an AI was not making sure it had a restriction not to kill you, but understand it was not bound by human morals or logic. If you told it make paperclips, it would turn the whole planet and people into paperclips.
And... humans were animals too. This was never a factor when the scientist inputted the instructions, but it was factored in by his creation. And ironically, the conclusion the computer came to... did not effect the mundane animals of the world in any shape or form, they continued acting on blind instinct, with no true comprehension or understand of the world around them. They day passed for them like any other. They didn't notice nor cared. The scientist's fantasy world was not to be.
Instead... over the course of a day... every human on the planet, from infants to elderly, transformed into a humanoid animal... and while they kept their intelligence and memories... instincts came to the forefront, human scruples for all intents and purposes vanished. Hunt/gather food, eat, mate, rear young, came all consuming drives. Along with 'protect territory,' 'stay with herd' 'serve the pack' and other baser animal mentalities. Modestly, shame, embarrassment, would all vanish from their minds. For example, naked humanoid wolves would be marking territory, and taking dumps in the street without a second thought.
Politics, laws, philosophy, art and culture, would end.