This is much more analogous to "stretching" by reaching to the sky, or by arching one's back, than to the static muscle stretches discussed in the article. This natural kind of stretching feels good (no need to educate people), which is evidence it's been selected for, and seems to concentrate more on joints than muscles.
I suspect our diet has changed considerably since our ancestors' time. So all the extra sugars and processed food we shovel in our mouths is bound to affect our oral health in ways that might necessitate additional cleaning measures (e.g. flossing).
Well... has flossing been shown to be useful for human health?
ETA: quick Google, found one meta-study. Results:
"Trials were of poor quality and conclusions must be viewed as unreliable. The review showed that people who brush and floss regularly have less gum bleeding compared to toothbrushing alone. There was weak, very unreliable evidence of a possible small reduction in plaque. There was no information on other measurements such as tooth decay because the trials were not long enough and detecting early stage decay between teeth is difficult."
Flossing supposedly helps fight gum disease, halitosis (by removing some of the gunk stuck between the teeth that toothbrushes can't get at) and other mouth afflictions.
How much of that "research" and "science" is actually funded by Colgate, Johnson and Johnson and the like I don't know though...
