You wake up lying on the ground in what seems to be a park, or maybe farmland. You pull yourself up, and realize you're naked. You feel like a lot of things should weird you out, but in truth you're more weirded out that a lot of things aren't weirding you out. You don't find it weird that you're standing around naked outside, you don't find it weird that you slept in a field, you don't find it weird that you don't seem to be able to talk. As you leave where you were sleeping, you see other people walking around, most of them on all fours.
Slowly it dawns on you- this is a world where humans act like animals. They never evolved language or societies, don't have any taboos about nudity or sex, and live off of food they find or kill. Consequently, almost all humans are fitter than in your reality, but also way less intelligent.
You realise that when you find your girlfriend, you're guaranteed to get laid, and wonder how you can find her...