Why Didn't Ancient Humans Die From Drinking River Water?

For most of human history, people drank straight from rivers shared with animal carcasses and stagnant runoff — water that would put a modern person in the hospital within hours. So how did entire populations survive on it for hundreds of thousands of years? This video breaks down the real biology behind it: the gut microbiome differences found in modern foraging populations like the Hadza, the concept of early-life immune calibration, and the brutal survivorship cost that made it possible in the first place. It's not that ancient humans had superpowers — it's that their bodies were built by an environment that killed the ones who couldn't handle it. And it explains something surprising about your own gut today: why a contaminated glass of water on vacation can wreck you for days while it barely touches a local. Ancient survival science, human evolution, and the biology hiding in your own stomach — all in under 6 minutes.