"Evidence is accumulating that suggests that the large human brain is most likely to have evolved in littoral and estuarine habitats rich in naturally occurring essential fatty acids. This paper adds further weight to this view, suggesting that another key human train, our bipedality, might also be best explained as an adaptation to a water-side niche."
"Evidence from apes in the wild show that though preferring to keep dry, they do go into water when driven to do so by hunger and tend to do so bipedally. A new empirical study of captive bonobos found them to exhibit 2% or less bipedality on the ground or in trees but over 90% when wading in water to collect food."
"The paleo-habitats of the earliest known bipeds, and all the evidence reviewed here (as is that of the newly discovered Sahelanthropus), is consistent with the hypothesis that wading contributed to the adaptive pressure towards bipedality."
"One tricky question remains: If a water-side habitat was the original driver both for a bipedal way of locomotion and for an increase in brain size, why did bipedality evolve so much earlier than large brains? The large brain capacity of Homo is a, relatively, recent phenomenon according to the fossil record and only really begins with Homo erectus and then accelerates more recently with Homo sapiens. If hominids have been living in water-side habitats all along, why did larger brains not evolve long ago?"
The aquatic ape theory (AAT) of Sir Alister Hardy (1) states that a few million years ago human ancestors spent a considerable part of their day swimming and diving in a river, lake or sea, and, at least partially, consumed aquatic food. The AAT is supported by the presence of our thick subcutaneous fat layers, by our lack of body hair and by several other features that are absent in non-human primates, but widespread among aquatic mammals (1-13).
The ability to speak is a uniquely human characteristic. Innumerable attempts to explain it have been made but the question of how language emerged is not yet solved. Recently, it has been suggested that the origin of speech was facilitated by our aquatic past (5,14). All aquatic mammals "voluntarily" control their breathing. When surfaced they open the airway passage whenever they want to inhale air, and they can hyperventilate and then close the airway passage when they intend to dive. The subtle "voluntary" control of breathing and airway closure in mammals in general is a pre-adaptation for speech (15,16).
Aquatic mammals can close the airway entrances much more completely than land mammals, thus avoiding being drowned by water entering the lungs, and they have a very refined voluntary control of mouth, nose and throat passages.
Modern man has a very special anatomy of the airway entrances that is not incompatible with a previous semi-aquatic lifestyle. He has a smaller mouth which can be closed more efficiently (24) and, presumably, the wet mucosa of our fleshy lips allows a better fit than the dry skin of the lips of non-human primates. In other primates, the tongue is generally flatter and somewhat less mobile than in humans (ref. 16, p.625). Our nasal cavity is elongated by an external nose (ref. 25, Figure 159) and narrowed by strongly developed inferior conchae, which often cause even complete obstruction in some humans (11,26,29). The nasal cavity can be disconnected from the throat by muscles that raise the velum (probably also in apes) (5).
Our primary motor projection cortex is much larger than that of apes, mostly due to the expansion of the areas for the musculature of mouth, throat and breathing, i.e. the latero-inferior section of Area 4 (see Figure 1). Just in front of that enlarged Area 4 lies Broca’s Area. It is a typically human structure indispensable for speech generation, and can be distinguished histologically from all other human cortical areas (ref. 30, pp.5-12).
In order to use the voluntary airway control for the vocal apparatus, our ancestor must have been able to register and interpret his own sound production (feedback, cf. motor theory of speech production) (31,32). This was certainly improved by the evolution of the arcuate fasciculus (see Figure 1), a typically human neural pathway between Broca’s Area and Wernicke’s Area (33).
Compared with a chimpanzee’s brain, our association areas are enormously large. These areas are found in the temporal, preoccipital, parietal and inferior frontal lobes (see Figure 1). The cortex of these areas can be distinguished histologically from the other cortical areas and even from Broca’s Area (ref. 30, pp.5-12). This suggests that Broca’s Area and the association areas evolved separately (respectively in Phases II and IV?). In my interpretation, most association areas evolved after the breathing and air-holding function of the enlarged Area 4 and Broca’s Area had been integrated with sound generation (Phase III). The new association areas amplified the possible applications of the sound-producing apparatus.
There are indications, I think, that our ancestors returned to a more terrestrial habitat not earlier than two million years ago (in a cooler and drier period of the Pleistocene? see ref. 11). In the hominid fossil record, the great expansion of the association areas seems to begin about two million years ago, with the genus Homo (34,35). The limited brain enlargement of Homo habilis could correspond broadly with the enlargement of Area 4, Broca’s area (34), the arcuate fasciculus and Wernicke’s Area; that of Homo erectus with a further association cortex enlargement.
The relatively small size of the brain of the australopithecines (possibly without a real Area of Broca (34)) could be explained by their dwelling or having dwelt in inland semi-aquatic habitats (e.g. gallery forests), and not in littoral habitats (11). If early Homo lived at the sea coasts, he had to dive deeper and longer than his freshwater cousins, so the voluntary control of this airway muscles became more important. Brain enlargement is a striking feature of many cetaceans. Conceivably, the support of the body (and brain) weight by the surrounding water allowed sea mammals to obtain large brains.
Concerning the relation of language and thought, I assume that a simpler (non-verbal) sort of thinking already existed in our pre-aquatic ancestors, but the great unfolding of human cognitive abilities became possible only after the acquisition of proper input/output organs for the brain. Hence, our great communicational capacities may not have evolved thanks to our large brain; rather the opposite seems true: large association areas only became usable with our voluntary sound production.