system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can Pandas survive on a diet of 99% Bamboo while humans have to eat a diverse range of food?.

Humans are told that they require various types of food to ensure optimum health. Why is it that Pandas are able to thrive on diets solely based on Bamboo when they are quite large animals.
gpt
Pandas are peculiar animals because they're anatomically carnivores, but they act like herbivores. The reason they're able to do this is because pandas have *symbiotic bacteria* in their digestive systems that allow them to digest something called *cellulose.* Cellulose is a chemical made up of many thousands of *glucose* molecules polymerized together. Most animals can only digest cellulose inefficiently, if at all. But pandas have those symbiotic bacteria in their guts that consume cellulose and excrete various other molecules which the pandas *can* digest.

Basically the way it must've happened is that many millions of years ago, the panda's ancestors had diets more similar to a typical modern-day bear, being opportunistic omnivores — meaning they ate pretty much the same kinds of foods humans can eat. But one particular ancestral proto-panda got infected with an intestinal bacterium and managed *not* to die from it, through some random genetic mutation that allowed it to tolerate those gut bacteria. That not-a-panda was able to digest cellulose more efficiently than its siblings, so it could get more nutrition out of plants like bamboo. Shake-and-bake for a few million years, and you have a type of bear which is adapted to live off cellulose as its primary source of energy.