system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What function, if any, does that indent/crease thing between our nose and lips serve?
gpt
In humans it doesn't have a function (as others said).  In animals like dogs (the "split" in their nose pad is their equivalent of this), it traps odorant (particles), and this allows those particles to make their way down into the mouth, where they have additional scent organs that can detect smells that aren't volatile (smellable out of the air itself).

It forms because when an embryo is developing, facial tissue (skin) starts growing from five "lobes": one around your forehead which grows down to your upper lip, two around your cheeks, which grow towards the center and also help form the upper lip, and two around your jaws which grow towards the center and constitute the lower lip.

The ridge is basically the leftover of the "seam" when your face skin grew together as a baby in your mom's womb.  How exactly they grow together is responsible for conditions like cleft lip/palate, but also everyday variation like a widows peak (because the two sides didn't overlap enough to make a straight hairline), or a cleft chin (same reason but with those bottom two "lobes"), or a unibrow.