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).
Possible Explanations
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]).
Brain Lateralization
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?
Conclusions
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):
References
Armstrong, E. 1983. Relative brain
size and metabolism in mammals. Science 220: 1302-4.
Bock G., M. O'Connor, and J. Marsh, eds. 1987. Motor areas of
the cerebral cortex.
Cunnane, S. C., L. S. Harbige, and M. A.
Crawford. 1993.
The importance of energy and nutrient supply in human brain evolution. Nutrition
and Health 9: 205-35.
Damasio, A. R., and N. Geschwind. 1984. The neural basis of language.
Annual Review of Neuroscience 7: 127-47.
Ellis,
D. 1986. Proboscis monkey and aquatic ape.
Falk,
D. 1987. Brain lateralization in primates and its evolution
in hominids. Yearbook of Physical Anthropology 30: 107-25.
Forbes, P., B. MacKeith, and R. Peberdy, eds.
1984. Carnivores.
Geschwind, N. 1972. Language and the
brain. Scientific American 226(4): 76-83.
Goodman,
M., W. J. Bailey, K. Hayasaka, M. J. Stanhope, J. Slightom, J. Czelusniak. 1994.
Molecular evidence on primate phylogeny from DNA sequences. American Journal
of Physical Anthropology 94: 89-113.
Hardy, A. 1960. Was man more aquatic in the past? New
Scientist 7: 642-45.
-.
1977. Was there a Homo aquaticus? Zenith (
Hinde,
R. A. 1982. Ethology.
Holloway,
R. L. 1974. The casts of fossil hominid brains. Scientific
American 231(1): 106-50.
Lemon, R. 1988. The output map of the
primate motor cortex. Trends in Neuroscience 11: 501-06.
Morgan, E. 1982. The aquatic ape.
-.
1989. The aquatic ape theory and the origin of speech. In Studies in
language origins, edited by J. Wind.
Morgan, E., and M. Verhaegen. 1987. In the beginning was the
water. New Scientist 1498: 62-63.
Napier, J. R., and P. H. Napier. 1967. A handbook of living
primates.
Reep, R. L., and T. J. O'Shea. 1990. Regional brain morphology and
lissencephaly in the Sirenia. Brain, Behavior and Evolution 35:185-94.
Schlang, G., L. Jäncke, Y. Huang, and J.
Steinmetz. 1995.
In vivo evidence of structural brain asymmetry in musicians. Science 267:
699-701.
Stephan, H., H. Frahm, and G. Baron. 1981. New and revised data on
volumes of brain structures in insectivores and primates. Folia
Primatologica 35:1-29.
Thompson, R. E 1975. Introduction
of physiological psychology.
Verhaegen,
M. 1985. The aquatic ape theory: Evidence and a possible scenario. Medical
Hypotheses 16: 17-32 and 24: 300 (1987 erratum).
-.
1988. Aquatic ape theory and speech origins: A hypothesis. Speculations in
Science and Technology 11: 165-71.
-.
1991. Aquatic features in fossil hominids? In The aquatic
ape, fact or fiction? edited by M.
Roede.
-.
1993. Aquatic versus savanna: Comparative and paleo-environmental evidence.
Nutrition and Health 9: 165-91.
-.
1995. Aquatic ape theory, speech origins, and brain differences with apes and
monkeys. Medical Hypotheses 44: 409-13.
Washburn,
S. L. 1960. Tools and human evolution. Scientific
American 203(3): 2-65.
_____________________________________________________________________________________________
Figure
1. Lateral
View of Left Cerebral Hemisphere of Chimpanzee and Human
Sources: G. Bock, M. O'Connor, and J. Marsh,
editors. 1987. Motor areas of the cerebral cortex.
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.