system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is bitterness, really?

I've looked through tons of ELI5 questions on bitterness and the majority seem to accept "bitterness" as this abstract concept that simply exists, like if you were trying to explain what wind looks like. As a cook and bartender, I enjoy bitter foods daily - amari, bitter veggies, grapefruit, lemon pith, coffee. I can go on and on. I know what it tastes and feels like.

But what is it, on a chemical level? Why do so many poisonous or even unnutritious things taste bitter? How do our taste buds pick it up and what is the "bitter" signal sent to our brains? Why is it neutralized to a degree by other specific flavors?

For an alternative and extended answer to this question, can you extrapolate on this? If humans publicly and consistently accepted and appreciated bitterness in foods, would we continue to have a nature-based aversion to bitter foods, or would it be seen as a generally desirable characteristic? Sugar and salt are the most obviously sought after, but appreciation for acidity is becoming more and more common and I wonder about the effect of marketing and publicity as a means for changing perceptions of flavor.

**In short, is perception of bitterness as a negative (less desirable) flavor largely or even completely driven by environmental influence?**
gpt
Bitterness is perceived via an assortment of taste receptor proteins, the TAS2R family. It's hard to predict from the structure alone which substances activate those receptors, but generally a lot of secondary plant metabolites do, and a lot of those are a defense system by the plants and thus toxic. At the core, it's an avoidance mechanism to prevent you from eating stuff you shouldn't eat. 

You'll notice that kids don't like bitter stuff, not at all. They are even more sensitive to it, which plays into their dislike for certain vegetables. However, we humans being smart, we figured out that there is bitter stuff we can eat safely and came to like it - but that is pretty much always an acquired taste, learned behaviour overriding the innate preferences.