[evan_au (OP)]

Background
On this week's Naked Scientists Podcast, a quizz answer suggested that humans are the only primate (and perhaps the only animal) with a chin.
- In all other living species, the jawline recedes in a straight line from the lips to the throat.
- Even in our cousins, the (now extinct) Neanderthals, there is little chin.

The Question
So what does a chin do for us humans?
- Does it give us an advantage over all other non-chinned animals?
- Is it an accident?
- Is it a byproduct of something else?
- Does it promote movie stars with a prominent jawline?
- Did it co-evolve with the napkin industry?

[evan_au (OP)]

Pondering this question....

Question 2: Could there be a gene/promoter region in DNA that controls how far forward the lips are?
- Most dogs would have this turned on strongly, producing a long snout extending in front of the eyes, with its nose just over its lips (in contrast, the Pekingese has been bred for a short snout)
- A chimpanzee or bonobo (our closest living genetic relatives) would have this turned on weakly, with lips projecting moderately far in front of the eyes, with their nose just over their lips
- Humans would have this gene turned off, so that our lips are basically in line with our eyes - and in fact our nose projects in front of our lips (which is a somewhat unusual arrangement)

Presumably, similar genes/promoter regions would exist in birds, so that some birds (like the ibis) can have very long beaks, while other birds have very short beaks.

Question 3: Could this shape of the human face be related to our high forehead, which fits in a larger prefrontal cortex?
- While squashing in the lips, other parts bulged outwards - like the forehead and chin?

The Neanderthals, despite their larger brain volume, had a receding forehead above their prominent brow ridges - and a receding jawline below their lips.
See: https://en.wikipedia.org/wiki/Neanderthal#Anatomy

According to this speculation, the chin would be an indirect byproduct of our larger prefrontal cortex...

[evan_au (OP)]

Question 4: Could the chin form a pocket to hold our tongue?
- This could make a larger resonant cavity for speech
- This could allow the tongue to move more freely, allowing a greater range of speech sounds

[chiralSPO]

I would guess that it might be related to the fact that we walk on two legs. The chin offers pretty good protection for our faces and heads as we learn to walk and run and jump (I say this having gotten 7 stitches in my chin as a 3-year-old).

[Petrochemicals]

I think i has something to do with eating with hands, food being broken up prior lets us drop it into our mouths, extending the jaw forward.

[evan_au (OP)]

Denisovans were one of the ancestors of modern humans in Asia and the Pacific.

This week, a Denisovan jawbone was reported from Tibet. And it has no chin.

However, other Denisovan genetic features like the ability to breathe air at high altitude did make their way into modern Tibetans.

See & Listen: https://www.sciencefriday.com/segments/ancient-human-relatives-at-the-top-of-the-world/