Aquatic ape theory, the brain cortex, and language origins
Marc Verhaegen - ReVision 18:34-38, 1995
Symposium "Wetland Apes: The Missing Link? A Fresh Look at Aquatic Ape Theory"
When the human brain is compared with that of apes, several differences are obvious: the brain centers for scent sensation and foot control are smaller than in chimpanzees but those for hand control, airway control, vocalization, and language and thought are larger. In this paper, I will attempt - insofar as the limited data permit - to describe the most striking differences of brain size and brain centers between humans and their nearest relatives (the African apes and especially the chimpanzee), to compare these with other mammals, and to draw conclusions concerning the peculiar evolutionary history of humans.
Humans and chimpanzees are biochemically (DNA and proteins), and hence probably also phylogenetically (evolutionary relationship), more alike than are chimpanzees and gorillas (Goodman et al. 1994). But the brains of humans and chimps differ morphologically (in size and anatomy) much more than those of chimps and gorillas. Most probably, the brains of chimps and gorillas underwent few evolutionary innovations, since they generally resemble other ape and monkey brains. This implies that the human brain changed drastically after the human/chimp split. With the exception of the olfactory bulb (scent registration), most brain structures are larger in humans than in apes. The human neocortex (the most recently evolved parts of the cerebral cortex), for instance, is at least three times as large as that in chimps, although both species are of about equal body weight (Stephan, Frahm, and Baron 1981).
Each hemisphere of the mammalian brain is divided by the central sulcus into a posterior sensory half with incoming information from the sense organs, and an anterior motor half with outgoing information for the muscles (Thompson 1975). Just behind the central sulcus lies the post-central cortex, where the tactile information from the receptor cells of the skin of the opposite body half reaches the brain cortex (via the spinal cord or medulla and via the thalamus in the midbrain). Just in front of the central sulcus lies the pre-central cortex, where the information for the voluntary movements of the muscles in the opposite body half leaves the brain cortex. This pre-central area is called primary motor cortex, or Area 4 in primates.
Human/Chimp Cortex Differences
In humans, Area 4 is almost twice as long as it is in chimpanzees. The part of Area 4 that commands the movements of the leg, foot and toes is reduced in comparison with that in apes (see figure l). This smaller area is more than compensated for by the expansion of the part for the hand, fingers, and thumb. Even more expanded is the lower (inferior) part of human Area 4, related to the lips, tongue, soft palate, throat, vocal cords, and breathing musculature (Holloway 1974; Washbum 1974; Thompson 1975; Bock, O'Connor, and Marsh 1987; Lemon 1988). The human post-central cortex is enlarged in the same way as the pre-central Area 4. The sensory part for the mouth and airways is situated above the primary auditory cortex, where the information from the ears reaches the brain cortex.
Just in front of the primate Area 4 lie the cortex areas (the pre-motor areas) that coordinate the activities of Area 4 (Holloway 1974; Washburn 1974; Thompson 1975; Bock, O'Connor, and Marsh 1987; Lemon 1988). Just in front of the enlarged inferior part of Area 4 in humans lies (in the dominant, usually the left, hemisphere) the typically human Area of Broca, the so-called motor-speech center which coordinates the refined mouth and breathing musculature. In the posterior half of the same hemisphere lies Wernicke's Area, the sensory speech center. This too is a uniquely human brain structure. It is situated behind the post-central sensory cortex for the airway entrances and behind the primary auditory cortex. It has direct neural connections with Broca's Area through the arcuate fasciculus, a neural pathway which seems to be absent in apes (Geschwind 1972; Damasio and Geschwind 1984).
The major difference between the human and the nonhuman primate cortex is, of course, the huge expansion of the association or integration areas, which occupy a great part of the human brain (Thompson 1975; Stephan, Frahm, and Baron 1981). In the anterior (motor) half of the human cerebral cortex, strongly enlarged association areas are found in front of the pre-rnotor cortex and Broca's Area (the frontal association areas). In the posterior (sensory) half, these areas lie between Wernicke's Area and the post-central sensory area and the visual area (the parietal and pre-occipital association areas), as well as around the auditory cortex (the temporal association areas) (Thompson 1975).
In the view of many anthropologists, who often work within the framework of the savannah theory of human evolution, the differences between the brains of man and ape are explained by the human capacities for using tools and using language. But although these differences help us to understand why present-day humans are superior tool users or why they can speak, the traditional view cannot explain why only human ancestors developed these technical or linguistic skills, that is, why other savannah mammals and nonhuman primates have failed to develop these capacities.
The solution is obvious in light of the aquatic theory of human evolution, the theory that explains human nakedness, abundant subcutaneous fat, and innumerable other typically human features by a semiaquatic phase in human evolution (e.g., Hardy 1960, 1977; Morgan 1982, 1989; Verhaegen 1985, 1988, 1991, 1993, 1995; Ellis 1986; Morgan and Verhaegen 1987; Cunnane, Harbige, and Crawford 1993). There are indications that the early hominoids (ancestors of apes and man) already lived in mangrove or gallery forests (Verhaegen 1985, 1991, 1993), where they adopted a locomotory behavior not unlike that of present-day proboscis monkeys, who climb and hang in mangrove trees, wade bipedally in the shallow water among the mangrove trunks, and swim at the surface (Ellis 1986; Napier and Napier 1967). In my opinion (Verhaegen 1991, 1993), human ancestors, after they split from the chimp's forebear, instead of staying in the forests elaborated their aquatic skills, developed diving adaptations for collecting seaweeds or bivalves in deeper water, and later became bipedal waders in shallower water and finally bipedal walkers on land.
That the volume of the olfactory bulb is only about 44 percent of that of the chimp (Stephan, Frahm, and Baron 1981) is, of course, incompatible with a savannah life of human predecessors: in that case, a keen olfaction would have been indispensable, as it is in all savannah mammals. But an earlier aquatic phase makes it clear why humans have a rather poor sense of smell: since the mammalian olfactory sense bas been designed to detect scents by inhalation of air, aquatic mammals are unable to smell scents dissolved in water and typically have a reduced or even atrophied olfaction (Morgan 1982).
The human pre- (Area 4) and post-central areas for the legs, feet, and toes are reduced, no doubt because human ancestors left the trees and lost the grasping hind limbs of the arboreal apes. The pre- and post-central areas for the hands and fingers, on the contrary, are larger than in apes. Indeed, the human hand is more mobile than the ape's, in particular the thumb and index finger (precision grip [Hardy 1977]), and the human fingertips have more tactile receptors for fine touch (Meissner's corpuscles) than those of apes (Napier and Napier 1967), and faster-growing fingernails. Hardy (1960, 1977; Morgan 1982) pointed to the enhanced hand motility and sensitivity of raccoons and sea otters and suggested that human ancestors were groping for crayfish and shellfish under water and manipulating pebble stones to remove and crack the shells. Raccoons are good climbers but seek most of their prey in shallow water. They have remarkably humanlike fingers on their forelimbs (Hardy 1977; Forbes, MacKeith, and Peberdy 1984), and their brain cortex shows a comparable enlargement of the areas for the hands and fingers (Thompson 1975). Unlike humans or sea otters or mangrove capuchin monkeys, savannah mammals never use tools (Verhaegen 1993).
All diving mammals perfectly control their airway entrances: under water to keep the water out of their lungs and to regulate the pressures in the different compartments of the airways during descent and ascent, and at the surface to quickly take a large amount of fresh air. This voluntary control of their breathing musculature is necessary because they have to inhale strongly from the moment they only intend to dive, and under water they have to hold their breath until they surface. In land mammals, on the contrary, breathing rhythm and depth increase involuntarily (automatically; i.e., not under cortical control) with lower O2 and higher CO2 concentrations in the respiratory chemoreceptors of the lower brain (medulla oblongata); an aquatic mammal with such a mechanism would inhale strongly when its oxygen need was highest, that is, under water! That was why the diving human ancestor doubled or tripled the part of Area 4 for the mouth and airways and why he evolved a new center (Broca) that coordinated the muscle contractions of the mouth and airways.
Obviously, this refined airway control was a preadaptation for human speech (Morgan 1982, 1989; Morgan and Verhaegen 1987; Verhaegen 1988, 1995).
Speech and Association Areas
The arcuate fasciculus in humans directly connects the coordination center of the breathing musculature (Broca) with the cortex (Wernicke) behind the sensory areas for the mouth and throat (where we feel the movements our breathing, singing, and speaking organs make) and the auditory areas (where we register the sounds we hear, including the sounds we make ourselves) (Thompson 1975; Geschwind 1972; Damasio and Geschwind 1984). This confrontation of airway sensation with sound registration was a prerequisite for learning to produce voluntary sounds (song or speech). In the primitive Area of Wernicke, the first symbolic interpretations of sounds were made possible through the connections with other nearby cortex areas such as the pre-occipital areas (visual impressions) and the parietal areas (where the intersensory integration takes place - our general "world image" - from the combination of tactile, auditory, and visual impressions [Thompson 1975]), so that the sounds were associated with what the brain was seeing and feeling at the same moment.
Once the connection of Wernicke with Broca through the arcuate fasciculus was made, we possessed a sound analyzing/producing apparatus that could "translate" all the information from the outside world into "words" and feed it into our association areas or communicate it to our group mates. The possibilities of this apparatus could then be enhanced by evolving larger association or integration areas, that is, by a larger computational capacity.
Apes lack these huge association areas. Evolutionarily, any ape could have evolved a greater amount of brain tissue and have developed larger association areas, if that had been advantageous for that ape. However, large association areas were useless without the improved sound-producing areas found in humans (the inferior part of Area 4 and Broca, the arcuate fasciculus, and the improved auditory areas with Wernicke). In other words, a nondiving animal could never have acquired the perfect airway control, which was the prime mover for spoken language.
Voluntary (controlled at free will) and highly variable vocalizations are seen in (semi)aquatics such as otters, seals, sea lions, and toothed whales (Morgan 1982; Forbes, MacKeith, and Peberdy 1984). And large brains are a conspicuous feature of many marine mammals, for instance, seals and toothed whales (Armstrong 1983). The exact relation between aquatic life and brain size and vocal control is not fully clear, however (Verhaegen 1995). Manatee sea cows, for instance, have a small relative brain size (Reep and O'Shea 1990). And many arboreal mammals have a well-developed vocalization (though often involuntary - i.e., under emotional impulses such as those having to do with territory or pair-bonding [Verhaegen 1988]).
Another important difference between a human and an ape's brain is the higher degree of asymmetry in the human brain. Human right-handedness, though highly variable, is more pronounced than dexterity in monkeys or apes (Falk 1987). There is evidence that most mammals and birds show some asymmetry in certain brain functions. Norman Geschwind noticed that in most people the left cerebral hemisphere (which controls the right body half) is larger than the other one. In 65 percent of people, the left planum temporale, where the auditory centers and Wernicke's Area are located, is much larger than the right one (Geschwind 1972; Damasio and Geschwind 1984). Musical training before the age of seven especially seems to induce strong enlargement of the left planum temporale (Schlang et al. 1995). In more than 80 percent, the same hemisphere controls the dominant (right) hand. Why is his so? The right hand is usually the active hand, which makes fine manipulations with an object, often within sight, while the left stabilizes the object or the body. Our left hand holds the shield, stabilizes the billiard cue, fixes the paper while writing, and carries the baby. This fits with the better spatial and geometrical insight of the right hemisphere (left hand) in most humans, whereas the left hemisphere is better in analytic, sequential, and verbal performances (Falk 1987). The right hand is not completely dominant for fine manipulations, however, since several tasks can be done with either hand. Rather there is a partial division of tasks between left and right brain centers for the hands, especially for jobs that must be done with both hands.
It bas been argued that our dexterity had its origin in a primate ancestor that picked fruits from a twig with one hand, while stabilizing its body by holding fast a stouter branch with the other. Although individual apes and monkeys often right- or left-handed for a certain task, systematic handedness in the human sense has hardly been demonstrated so far in nonhuman primate species (but studies by different investigators are often difficult to compare) (Falk 1987). One explanation for dexterity's being more pronounced in humans than in apes is that diving hominids used the right hand to remove a shellfish from the bottom while the left was keeping a few bivalves between palm and fingers, or to open a shell with a pebble while the left held the mussel or oyster or snail (the beginning of human tool use? [Hardy 1960, 1977; Morgan 1982]).
Hands are paired organs. Each hand needs its own control center in the brain (in the pre-motor areas). These two centers can be symmetrical or - when, as in humans, each hand bas a different function - more or less asymmetrical. An unpaired organ, however, is better off with one brain center being dominant over the other, so that fine movements would not be disturbed by intervening commands from the other hemisphere. A good coordination of the breathing and airway musculature would be particularly indispensable, hence a tendency for developing only one headquarters. This is true for the breathing control during diving, as well as during sound production.
Fernando Nottebohm showed that song production in birds is strongly lateralized (Hinde 1982). In adult chaffinches, section of the left hypoglossal nerve (the nerve for the syrinx, the bird's vocal organ) leads to loss of most components of the song, but section of the right hypoglossal nerve has only minor effects. If the left nerve is cut in a young bird before song develops, the other brain side and the other hypoglossal nerve take over completely (Hinde 1982).
The human speech centers, too, show a high degree of plasticity (Hinde 1982). The localization of Broca's and Wernicke's Areas in the left hemisphere is more constant than human dexterity: not only right-handers, but also most left- handers, have their speech centers in the left hemisphere. Is there any relation between right- or left-handedness and the localization of the sound-analyzing/producing apparatus? That the control of our dominant hand is usually situated in the hemisphere of the speech centers could perhaps mean that the earliest linguistic activities in human ancestors were the naming of objects that were manipulated or pointed at with the right hand. Or is it simply a coincidence?
The conspicuous changes in human brain anatomy, as compared with apes and monkeys, fit beautifully with the aquatic theory of human evolution and have parallels in aquatic and semiaquatic mammals (see table l):
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Figure 1. Lateral View of Left Cerebral Hemisphere of Chimpanzee and Human
Sources: G. Bock, M. O'Connor, and J. Marsh,
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Note: Illustrations not to scale.
Table 1. Relative Size and Importance of Brain Centers in Different Mammals
Brain center ††††††††††††††††††††††††††††††††††††††† Aquatic††††††††††† Semi-†††††††††††††† Arboreal††††††††† Savannah
or function†††††††††††††††††† Humans†††††††††† mammals††††††††† aquatics ††††††††† ape/monkey†††† mammals
Olfaction††††††††††††††††††††† -††††††††††††††††††††† --†††††††††††††††††††† -††††††††††††††††††††† +†††††††††††††††††††† ++
Foot control†††††††††††††††† -††††††††††††††††††††† -††††††††††††††††††††† -††††††††††††††††††††† +†††††††††††††††††††† -
Hand control††††††††††††††† ++†††††††††††††††††† -††††††††††††††††††††† -/++†††††††††††††††† +†††††††††††††††††††† -
Airway control ++†††††††††††††††††† +/++††††††††††††††† +†††††††††††††††††††† -/++†††††††††††††††† -
Vocalization†††††††††††††††† ++†††††††††††††††††† +/++††††††††††††††† +†††††††††††††††††††† -/++†††††††††††††††† -
Total brain†††††††††††††††††† ++†††††††††††††††††† -/++†††††††††††††††† +/++††††††††††††††† +†††††††††††††††††††† -
Note: In this table, -- denotes smallest or least important; and ++ denotes largest and most important. Symbols divided by a slash show that there are some animals in that group with the first rating and other with the second rating.