If sneezing evolved only for clearing the nose, which is the typical reason given, then why don't humans have closable nostrils to allow build up of air pressure to remove particles in the nasal cavity?
Why do we sneeze, which is a complex, whole body reaction, only to remove microscopic pollen, when simply blowing the nose is more effective and more efficient?
If sneezing is to remove particles, then why do people often blow their nose AFTER sneezing? And why the runny nose AFTER sneezing, if the particles are supposedly removed already?
It just does not add up to natural selection for fitness.
If sneezing is a former diving exhalation, (completely refilling the lungs instantaneously as dolphins do), and if our ancestors allowed seawater into the nasal, sinus and middle ear cavities to avoid equalizing, then having closable nostrils would NOT be advantageous.
And if sneezing removed this water upon surfacing, backfloating between dives would select for longer noses pointed towards the sky, but with ventral-position nostrils since the occiput was dense and sinuses were lightweight, the head would tilt back further, so ventral nostrils would be higher up than prognathic outward pointing nostrils.
Now we see why the human nose, including He, Hs and Hn, does NOT have dense bone except at the base. The external nose in Homo is cartilage and tissue and fat, because it is lighter weight than solid bone.
The human nose is hollow tetrahedral structure (pyramid like) therefore using the least amount of structural materials (ie. light weight), and dense bone is found only at the foundation as part of the skull.
This was a difficult puzzle, as it made sense for the snorkel to be bony for protection, yet only the base is bony. The solution lies in the need to balance the heavy occiput during backfloating, in order to position the nostrils in the perfect position, like the sea otter. The presence of the keel on the He calvaria fits with this well, the occiput providing ballast for centering thus keeping the nose at the highest position. Presumable the keel shape was slightly advantageous during back stroke and back sculling as well as during diving.
Is the sea otter occiput or spine more dense (pachyostotic) or enlarged compared to the river otter or stoat? Is it keeled? Is the nose thick boned or thin and hollow cartilaginous?
Why do neandertal skulls have a notch in the occiput? One Hs skull also has this notch. (Update: a 2nd one, a small mix skull in Spain 23ka also has it).
Why do humans have white eye sclerae? Because even though humans are very visual oriented, during backfloating the eyes were kept closed generally, with vocal-aural information transmitted via song, clicking and abbreviated calling which evolved into resonance vowels and staccato consonants easily produced while backfloating.
The sunlight which penetrated through the thick fatty eyelids (Khoisan/East Asian type) hit the white sclerae and reflected back out through the lids, rather than being absorbed into the eyes (as with dark eyes) thus preventing damage over the long term due to UV.
Lullabyes and love songs derived from food gift exchange and sharing while diving and backfloating. Probably monogamous diving pairs, but based on size difference of male to female, possibly mini-harem system, 1 male with 1 pregnant and 1 non-pregnant female, so one is watching kids while other is diving with male? Possible similarity to bonobo female-female sharing, to prevent jealousy, better child raising in harsh environment?
Regarding the apparent size dimorphism in He, compare to sea otters, the males are much larger than females, IIRC river otter dimorphism is reduced. So IMO He may have dived monogamously as partners, but not foraged on land as pairs? Males patrolled beaches with sticks (possibly dugouts to get freshwater?), females guarded small kids at shore while wading, male teens in coconut palms & fig trees dropping foods and look out for predator/prey above water and beach and higher ground, female teens babysitting on dry sand with a palm leaf for shade and camoflage?