Common Descent 2 by DDeden

I think human ancestors a million years ago dove daily for food (clams etc.) at tropical seashores, which is why we don't look like our closest genetic cousins the chimps. These human seashore ancestors did not hyperventilate before diving, did not pack or wear fins, masks or
scuba. They dove alternatively in male-female diving teams; while one backfloated above in the very warm sunlight lagoon surface water, the other dove down into the cool dark blue along reefs collecting sessile shellfish, repeating this over and over, switching roles, until they got their fill of seafood, then went back to the beach for a coconut cocktail.

This would require that at depth, the water was cool, not hot, otherwise the [MDR] Mammalian Divers Reflex would not fully engage, which means oxygen would not be conserved well. The MDR was an essential energy-saving part of the dive cycle; whereas during back-floating at the surface the opposite occurred, "wasting" [O2] oxygen in order to remove excess [N2] Nitrogen and [CO2] Carbon Dioxide and accumulated waste products. [SEE NOTE AT POST BOTTOM] Urinating during the dive, and sweating at
the sunny surface removed urea (which contained Nitrogen), and humming at the surface removed [NO] Nitric Oxide from the sinuses (killing waterborne bacteria in the water which was allowed into the nasal/sinus/middle ear cavities during the dive to avoid equalizing constantly). This humming at the surface by the backfloater (likely with a chest-held infant that was clinging to the long head hair or beard) was the first lullaby, the NO-enriched air was inhaled by the infant
(who could not hum (until later first saying "mama" and could not yet shed tears, so the parent's NO provided antibiotic protection to the infant's face), and the humming could be heard by the dive partner below (not distracted by noisy scuba bubbles), who then made tongue clicks which was heard by the backfloater, this "hydrosonic" communication was the original form of sustained speech, (which later derived into the vowels and consonants) allowed non-visual contact between the partners, the sound waves carried efficiently through water and the very dense occiput (the skull plate on back of head which in ancient humans was the most dense bone in the body, perfect for sound transmission via bone conduction as found in dolphins and sea cows).

So, "too deep. too often, too soon", according to the archaic diving cycle was not an issue, as long as the diving partners alternated consistently, allowing gas exchange at the surface. I assume max depth was 100 m, but much more likely 30m max for adult males, 15m for females repetitively, and probably often less. No diving in fast current, cold surface water, hot depth water, big surf or rainy weather. [They don't teach all this in kindergarten or sunday school, but I think it's accurate.] :-) DDeden

KP @ Deeperblue.net) i'm buying most of it but since i can't make loud clicking sounds with my tongue while having the ease of air around - i can't imagine being able to do it loud enough underwater [UW] for someone's skull plate to feel the reverberation. Perhaps those (archaic) folks had larger sinus cavities and mouths as well? kp

(DD) Neandertals living on the coasts had huge sinuses. According to some anthropologists, their voices (according to fossil anatomy, hyoid bone structure) probably had somewhat high-pitched during speech compared to modern folks.

Perhaps their mouths were larger (they lacked chins), I don't think that would matter so much. I think their visible external ears (pinnae) were smaller than ours, maybe even smaller than gorilla or gibbon ears, but pinnae don't fossilize so it's unknown.

Still water carries sound far far better than air does, that's why both dolphin clicking and humpback whale song can be heard at long distances. For the same reason, solid flat ground (savanna) carries sound. Bull elephants make thunderous bass trumpeting sounds that carry for miles, the females hear the sounds through their FEET via bone conduction from the solid ground, NOT primarily through their huge external ears via air conduction. Air simply doesn't carry sound very effectively. Human hearing is 50% air conduction, 50% bone conduction. In water, only bone conduction functions.

As long as the backfloater is on the still surface with the ears and occiput in the water, and the entire ear (inner, middle and outer ear) is fluid filled (in a lagoon, not in open sea with loud surf), and the diver's ears are also fluid filled, any clicking or humming sounds will carry a good distance. In air, clicking doesn't carry very far, (though the Khoisan people retain clicks in their language), but dolphins prove that UW clicking is both effective and efficient communication.

The problem with hearing UW is presence of air in bone cavities, once air is replaced with water, sounds carry well, no need for loud clicks, even soft clicks carry a ways. Simply ticking the teeth together works too for a short distance. Some shrimp, crabs and fish make clicking sounds in various ways. There's no loss of air in clicking, unlike vocal speech or humming.

Caution, this stuff is potentially DANGEROUS if you don't know what you're doing. It's still just theoretical, I haven't tested it yet, being poor and stuck in Nor Cal by cold water, I need to get down to the tropics to do empirical research.---

MV: Makes a lot of sense to me, except the surface sweating: do we have more sweat glands on the abdomen?

DD: No, fewer. Eccrine sweating is a whole body phenomenon. However, the top 6 inches (15cm?) of sunwarmed lagoon equatoreal water is very warm at midday. Therefore the whole body sweats AFAIK, but the body is not appreciably cooled unless a breeze is present. Do you think a backfloater would benefit from having many sweat glands on the abdomen? UV light is anti-biotic, it can damage eccrine sweat glands even in melanized people IIRC. Do Phocids have sweat glands more dorsally or ventrally on abdomens or flippers?

btw, I just read (Discover or Nature mag.) that urea (fresh) is anti-fungal and anti-biotic. That confirms my earlier speculation (w/Elaine etc.) that UREA, in addition to the cathlicidens (spelling), defensins and dermcidin in eccrine sweat, ALL are components of the biochemical antibiotic armor in humans. What I don't know is whether urea converts back to ammonia in the presence of air or seawater. I know that urine smells ammonia-like after mixing with air, but is that uric acid or urea converting to ammonia? Ammonia is highly toxic both to humans and microbes.

the reduction of the olfaction in hominoids and further in Homo, was it associated with the reduction of the bony conchae/turbinates as well? I would expect so, since semivertical floating would select for lightening of the protruding nose, while backfloating would also select for lightweight nose as long as some structural integrity was present. I'd expect that the LCA Hominoid - LCA Hominid had a relatively low density skull and or skull bone thinning as opposed to modern chimps & gorillas. This only changed with Homo erectus. (So apiths would not be
expected to have thick skulls except the mandibles for chewing). Significantly, those animals with large protruding noses (probosis monkey, tapir, elephant) do not have hyperventilatory air sacs but do have tails, they employ a different form of vocalization resonance than tail-less apes, and do not float semivertically, but rather swim or wade. Macaques seem split, with long tail divers and short tail waders/floaters? AFAIK only the floaters have enlarged lar air sacs
(need to verify). I think the pinnae were smaller, and this was retained in gorillas,
gibbons, but later enlarged in the chimps, humans due to heavier predation pressure later on (more savanna type environments).

NOTE: DT: Do you mean that the NO and urine remove excess Nitrogen that was absorbed from air?
If so then I don't see how that is possible, N2 is inorganic and needs to be "fixed" inorder to be metabolized, as far as I know mostly unicellular organisms have Nitrogen fixation capabilities and we definitely don't... and I doubt symbiotic bacterias can produce enough during diving for it to make any difference....

DD: thanks much for checking.

I erred, mixing (apples) inert N2 from air, and (oranges) Nitrogen compounds from high protein molluscs. I'd thought that N2 was not easily absorbed into blood, that scuba at depth increased absorption, but that a significant part came from high-protein foods, and that adding N2 at depth was just a part of it, the straw that broke the camel's back.

Do you know whether, upon switching from inhaling air to O2, does blood N2 drop down to zero? If so, would digestion of hi-protein food rich in nitrogen compounds (molluscs) cause an increase in N2 in the body, especially in the blood? That is what I thought happened, which is why I thought of excretion. I was thinking of other chemicals which end in the bloodstream (drugs etc.).
Anyway, I'll review and edit my post, for now i'll just note it.

Note on NO production sources: From Wikipedia: (KM at AAT)

"In the body, nitric oxide (the 'endothelium-derived relaxing factor',
or 'EDRF') is synthesized from arginine

Arginine - Wikipedia, the free encyclopedia and oxygen
Oxygen - Wikipedia, the free encyclopedia by various nitric oxide synthase
Nitric oxide synthase - Wikipedia, the free encyclopedia (NOS) enzymes
Enzyme - Wikipedia, the free encyclopedia and by sequential reduction of inorganic nitrate."