Unfinished thoughts to be cont'd.
[QUOTE=naiad;673130]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.[/QUOTE]
True. First, our ancestors went through two different stages.
The first (amphibious) was transitional from a tarsier-like mammal with tail to a vertical-oriented part-time awkward bipedal upright wader/walker/floater (amphibious tree-frog-like) which lost the tail due to large laryngeal air sacs that inflated upon cool-water immersion due to a gasp-like reaction, allowing plucking foods on the water surface and shallow snails etc. but which did not swim well and did not dive at all, about 15 to 5 million years ago, along coastal lowland forests often flooded, this included all ape ancestors. But while human ancestors remained there, other apes moved inland into Asia and African into various swamps and riverine gallery forests, moving deeper and deeper into tropical rainforests, and because of large inland predators became more and more tree dependent, and when walking on the ground, tended to knucklewalk for more speed and stability. About 5 million years ago, when the chimps finally permanently split, chimps followed the path that a million years previously gorillas had gone, into the Rift valley and Congo. At this time all hominoids had air sacs, used for floating vertically with the face out of water, and for calling loudly. So now only the human ancestors were left at the seashores, they never knucklewalked (but the infants kneecrawled on the soft sands), they climbed easy-to-climb fig trees, mangroves and coconut palms, but not the tall forest trees. They adapted more to coastal pocket beaches surrounded by cliffs with caves and rockshelters, and slowly became better surface swimmers but not divers, mostly beachcombing and peeling molluscs from mangroves and rocks at low tide, in addition to fruits and seabird eggs. At this time, the nose was shaped like a baby's pug nose, the bell jar shape keeping water out during the rare dunkings. Since the nose was usually kept above the water by the air sacs, there was no natural selection for closable nostrils. This was completely unlike other aquatic animal ancestors, none of which had inflated laryngeal air sacs keeping the nose above water.
The second phase (aquatic) was only human ancestors, not apes, and seems to have occurred about 3 - 1 million years ago, when they gained a fat layer under the skin, the air sac was reduced to a vestige, lost most of the fur coat, but retained hydrodynamic hair in the voids of the body (throat, neck, armpits and pubic areas), and changed from surface floating and plucking to deeper diving, and eventually at some point developed a backfloating-diving cycle somewhat like a sea otter. Up until then, the pug-nose shaped like a bell jar was enough to keep water out of the airways as long as the mouth was closed or the tongue or velum closed the airway, this worked fine at the surface with the inflated air sac just under the throat, but only allowed plucking food below at arm's length. The air sac interfered with face submersion, and was slowly selected against, allowing deeper dipping, while the nose was selected for longer length. so during backfloating breathing was done through the nose when head was tilted back.
Until this time, the nostrils would not have been selected for closing, since the air sacs always kept the face above water. But with deeper diving, came water pressure problems and breath holding, which no ape had ever had before.
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).[/QUOTE]