Wikipedia and the scientific community
Platycephaly (flat skulls)
One of the features of semi-aquatic Carnivora (otters, seals) is flattened skull-caps, presumably for more streamlined diving. In the fossil record, only Homo (erectus, neanderthalis, sapiens) has/had flat skulls. This feature along with others mentioned below, is not seen in apes-austrolopiths, which suggests the main diving phase began under two million years ago in Homo.
The following extracts are from:
"The Aquatic Ape evolves: Common Misconceptions and Unproven Assumptions about the so-called Aquatc Ape Hypothesis"
AAH is not about ‘aquatic apes’ or even australopiths, but about archaic Homo.
"Most or all traits that can possibly be explained by a (semi)aquatic past seem to be absent in apes and australopiths, and appear in the fossil record mostly or exclusively in the genus Homo: the spectacular brain enlargement (arguably through the abundant DHA in aquatic foods, see Crawford et al. 2002), an external nose (strongly projecting nasal bones, as in some semi-aquatic mammals), pachy-osteo-sclerosis (POS, very thick and dense bones, as in slow and shallow diving tetrapod species), platycephaly (flattened skull-caps, as in semi-aquatic Carnivora, presumably for hydrodynamic streamlining, see Curtis et al. 2012), platymeria (dorso-ventrally flattened femora, as in Pinnipedia), wide and deep thoraxes (as in most shallow-diving endotherms), ear exostoses (as in human divers in cold water), etc. In the malacological record, marine mollusc species in combination with hominid fossils are not seen with australopiths, but appear together with Homo erectus and relatives (Munro 2010). This does not mean that our ancestors’ littoral adaptations could not have begun prior to that time, only that to date we have no evidence of littoral adaptations before the Pleistocene (this absence of Pliocene evidence might or might not be due to the fluctuating sea levels of the Ice Ages). Since littoral adaptations seem to be more prominent in Homo erectus than in Neanderthals, later Homo populations might gradually have ventured more and more inland along the rivers, where their remains have been found in oxbow lakes at the time (e.g. Mauer in Germany, Lynford in the UK), frequently in paleo-landscapes with reeds and beavers. The Neanderthal diet seems to have been remarkably varied (Hardy & Moncel 2011) and probably included salmon (Bocherens et al. 2013). How aquatic the non-archaic Homo fossils were, is less clear. Which of the different so-called Homo habilis fossils were close relatives of archaic Homo (e.g. many O.H. fossils?) and whether some of them might have belonged to the australopiths (e.g. KNM ER-1813?) might be difficult to answer with the present knowledge. In any case, Sahelanthropus, Orrorin, Ardipithecus and australopith fossils have to be studied on their own, apart from Homo’s littoral past: even if they can provide information on how our ancestors before their most-aquatic phase might have looked and lived, they have in my opinion little bearing on AAH in the strict sense."
"In archaic Homo skulls, the frontal brain-case was placed behind the eyes rather than above as in sapiens, and the inferior part of the brain skull was relative wider (notably in neanderthalensis). The skull-cap was remarkably flattened and ventrodorsally long (platycephaly), with a heavy eye-protecting ridge (torus) above the orbits (in neanderthalensis largely filled by frontal air sinuses, though not in erectus), and sometimes with parasagittal ‘keeling’ (especially in erectus)."
* AAH is less about bipedal wading (except in later phases <200 ka?) than about slow and shallow diving.
"As explained [in the previous paragraphs], but not commonly acknowledged by AAH opponents and some proponents, archaic Homo fossils displayed a number of features that are often seen in shallow-diving mammals: brain expansion, ear exostoses (in some erectus and many neanderthalensis), POS, platymeria, platycephaly and keeling, relatively wide bodies and extremities, midfacial prognathism with projecting nose, etc., whereas in H. sapiens (in the fossil record after ~200 ka) these possibly-littoral features disappear or become reduced. This does not imply that most or all archaic Homo populations did not frequently wade or walk bipedally, only that we do not have enough evidence to make firm conclusions about how often they waded. Although archaic Homo had relatively larger femoral heads than apes and australopithecines, which suggests more frequent bipedalism (standing, wading or walking), they had very heavy skeletons, and at least some of them (e.g. heidelbergensis and neanderthalensis, especially the males) seem to have had larger bodies than most extant humans. Thick and dense skulls (POS), on the other hand, are exclusively seen in slow and shallow diving tetrapods (e.g. Laurin et al. 2011): there is no reason—apart from conservatism—why archaic Homo should be unlike other animals with POS (Munro & Verhaegen 2011, Verhaegen & Munro 2011). POS, ear exostoses, abundant edible shellfish, human slow-diving skills (Schagatay 2010) etc. all independently point into the same direction: our Pleistocene ancestors were no cursorial runners, but—at least parttime—littoral divers."
"Regular slow and shallow littoral diving parsimoniously explains many other ‘odd’ features seen in Homo—fossil (e.g. ear exostoses, projecting nasals and mid-face, low and long braincases with pronounced frontal ridges, flattened femora, huge brain size) and living (e.g. fur loss, SC fat, head–spine–legs in one line, and in human newborns vernix caseosa and reniculi). The fossil Homo traits that are more typical of diving species (e.g. POS, platycephaly, platymeria, ear exostoses, external nose) apparently did not appear before the Pleistocene epoch: arguably, our ancestors’ most-littoral phase began with the Ice Ages, when Homo during glacials could colonize the drying continental shelves."
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