system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do we have to brush our teeth but cavemen got along fine without doing so?
gpt
They did not get along fine. Many cavemen died of dental abscesses.

A couple factors include that we live much longer so our teeth have to last longer, and we eat a diet that is much richer in sugars and oils that promote tooth decay. 

We're also in a society that is much closer-packed, so there's less room for bad breath and there's much better odds of picking up the particular acid-secreting bacteria that causes tooth decay and can be reduced in concentration through brushing and flossing (people with perfect teeth usually don't have it). 

We also place a high value on the cosmetic appearance of perfect teeth. Visible cavities are a huge turn-off, both when seeking a mate/date, and when seeking a job later in life. 

By the way, flossing is way more important than brushing.