Fascinating. The AAT sounds plausible enough. There are still some things that I don't understand about it though. These are:
Nostrils which cannot be closed. Most aquatic mammals close their nostrils when diving. No animal uses a noseclip or pinches its nose, and few use sinus flooding.
True, we can flare open our nostrils, but can't close them tight. The bell shape of the nose keeps air inside unless tilted, this allows prone shallow diving, but isn't so good for vertical diving.
The following is speculative, but seems to fit the known facts, and answers why our ancestors didn't develop closeable nostrils:
Before the diving era, about 20 - 5 million years ago, while the tailed monkeys were living in inland forest woodlands, there were hominoids (human and ape ancestors) living along coasts and in wetland forests that waded and floated vertically while gathering foods in and around the water (in addition to climbing and plucking fruits in the forest canopy), with inflateable laryngeal air sacs in the throat keeping the nose and mouth above the water surface (unlike the long-tailed macaque monkeys in the video). Their bodies developed some "amphibious mammalian" features (less fur, more bare skin areas, some skin-fat, complete loss of tail), the nose didn't, since it was kept above water, it was oriented upwards (like gorillas), with just a small fleshy cover (like chimps) in case the face got dunked while plucking plants and snails and floating fruit on the surface and in the shallows.
See Aaron Filler's website uprightape.net , which describes changes in hominoids 20ma including a shift in the spinal septum, which allowed upright locomotion. This fits with both hanging from branches and floating/wading, although he doesn't mention AAT specifically.
The Upright Ape - A New Origin of the Species: A Book that Revolutionizes Human Origins and Updates the Theory of Evolution
Gradually the ancestors of the apes expanded inland along gallery forests, but the ancestors of humans remained near the shores, with the nose becoming a pug nose and gradually enlarging, with a bony base, as surface swimming was increased. After a few million years of this, shallow diving in a prone position and plucking foods below was routine. Internal nasal-oral valves (velum, epiglottis, tongue, lips) kept water out of the lungs, rather than the external nostril valves more typical of aquatics, eventual loss of the air sacs and replacement with more hydrodynamic skin fat and hair at the voids, better breath holding, more flipper-like hands and feet (compared to apes), though never as specialized as seals, which have been more aquatic far longer than humans were and actively chased their mobile prey).
Humans have ventral oriented nostrils, this is only shared with sea otters AFAIK, and likely indicates habitual backfloating between diving, H. neandertals and H. erectus had large air filled sinuses in the forehead and dense keel-shaped occiput bones (skull rear) which suggests backfloating habitually, and indicate a change from prone swimming to more vertical diving.
Did they dive empty lung and/or flooded sinus and/or exhaling with a sunlight induced sneeze? No one knows, but I haven't seen clear indications that they did not, and I think it was indeed part of the daily diving experience, up until the development of simple dug-outs from hollow logs and fiber nets about 100-70,000 years ago, whereupon the diving skills gradually became less important and boating and fishing and larger settlements altered the society in various ways.
Reminder, AAT means human ancestors were the most aquatic ape, not the most aquatic mammal. Even during the most aquatic period, they were still only part-time divers, never completely reliant on seafood, unlike long time marine mammals like dolphins. I'd guess maybe 1/3 of the diet was seafood on a daily basis, sometimes more, sometimes less (fruit season, nut season). But even if it was only 1/10 of the diet, if it happened every other day, over long periods, it would show strong selection eventually.
Poor underwater vision. Although some people are apparently better off than others, including tribes who dive regularly, most aquatic animals have much better underwater vision than we do. Those which do not, or live in murky water, have other adaptations such as long whiskers (seals), echolocation (dolphins and whales), lateral line (aquatic amphibians).
Human ancestors were not as aquatic as full-time specialized marine mammals (dolphins, seals) and fish, they were always partly terrestrial and slightly arboreal. The food the collected in the water was not fast moving, it was sessile molluscs, vegetation and slow moving whelks, crustaceans etc. Most likely they used small simple tools, pebbles (like sea otters), stone blades, empty seashells (like capuchin monkeys do to open oysters at mangrove forests), wood/bamboo spears (as chimps do when hunting bushbabies in hollow trees).
In general, human eyes are like other primate eyes, but are larger, more lateral than vertical oriented (unlike apes), have thicker eyelids (keeping the eyes warmer at cool depths), and have exposed white sclerae (whites of the eyes), all which seem to relate to diving and shore life. They were slow divers that probably dove down from 3 - 20 meters normally, in quiet clear tropical reefs, not murky mudholes. I doubt they dove as much during the rainy season when more mud was in the water, since that is also when occasional crocs and sharks would tend to bite first and check second. More likely they would have been undercover in caves or rockshelters or perhaps, like orangutans do during rain, put leaf lids on their nests (dens) and just sleep, waiting for the sun to return.
I think the modern desire to go to sunny beaches and clear waters has deep roots.
[simple questions but hard to answer without explaining the whole thing]
Tangvoranuntakul,P
(my interpretation in [...])
Humans and chimpanzees share >99% identity in most proteins. One rare
difference is a human-specific inactivating deletion in the CMAH gene,
which determines biosynthesis of the sialic acid N-glycolylneuraminic
Acid (Neu5Gc)...[which in testing showed: diminished acoustic startle response
[the change from gasp/scream & air sac inflation during vertical floating to non-inhaled MDR empty lung diving?], abnormalities of the inner ear [better hearing underwater, poorer in air? -> primitive "sonar"? humming/song?] occurred in adults, which also showed delayed skin wound healing [SC fat (blubber?) development?]. Loss of Neu5Gc in hominid ancestors approximately 2-3 million years ago likely had immediate and long-term consequences for human biology.
DDeden
