I. Introduction

When the human brain is compared with the brains of apes there are several obvious differences; the centers for the sense of smell and foot control are larger in apes than in humans, but the centers for hand control, airway control, vocalization, language and thought are larger in humans. In my paper, I will describe the most defined differences of brain size and centers between humans and their closest relatives, chimpanzees, to compare them with other mammals and to draw conclusions about the evolution history of humans.

II. Brain Evolution

Humans and chimpanzees are biochemically (DNA) and therefore probably phylogenetically (evolution relationships), more alike than chimps and gorillas. But the brains of chimps and humans differ in size and anatomy more than gorillas and chimps. The brains of chimps and gorillas probably didn't go through many evolutionary innovations, because they generally resemble other ape and monkey brains. This implies that the human brain changed a lot after the human/chimp evolution. With the exception of the olferactory bulb (scent), all brain structures are larger in humans than in apes. The neocortex (part of the cerebral cortex), for instance is over three times larger than in chimps, even though chimps and humans are pretty close to equal in body weight.

Each side of the brain is diveded by the central sulces into independant halves. Just before the central sulcus lies the post-central cortex, where the opposite body half (right side for left brain, left side for right brain). Just in front of the central sulcus lies the pre-central cortex where the information for the voluntary movements leave tthe brain. The pre-central area is called primary motor cortex, and also "Area 4" in primates.

III. Human and Chimp Cortex Differences

In humans Area 4 is almost twice as large as it is in chimpanzees. The part of Area 4 that commands the movement of the leg, foot and toes is smaller in humans than apes. This leaves more room for the part that controls the hand, fingers and thumb. Even bigger is the lower part of human Area 4, related to the mouth and brething and vocal cords. The post central cortex is enlarged the same as Area 4.

In front of the primate Area 4 lie the cortex areas (pre-motor) that tell Area 4 what to do. In front of the enlarged part of human Area 4 is the Area of Broca, the motor-speech center which controls the breathing muscles. Above Area Broca is Wernicke's Area, the speech center, a uniquely human brain center along with Area of Broca. Wernicke's Area has direct connections to Broca's Area through arcuate fasciculus, a neural pathway that apes don't have anywhere in their brain.

The major difference between the human and ape cortex's is the enlargement of the hand and mouth integration areas. These areas occupy a large part of the human brain.? In the motor half of the cerebral cortex, enlarged areas are in the pre-motor area and Broca's Area. In the sensory half, the enlarged ares are Wernicke's Area and the visual area as well as the auditory cortex.?

IV. Explanations

Many anthropologists believe that the differences between human and ape brains are shown through man's ability to use tools and language. This traditional view cannot explain why only human ancestors developed these motor skills and language abilities, that is, why nonhuman primates and other savannah mammals didn't develop these abilities.

The solution may lie in the aquatic theory of human evolution, the theory that explains why humans don't have fur, and why we have excess fat, and many other human features.(4) There are indications that the early hominoids (ancestors to man and ape) lived in mangrove or gallery forests(5), where they adapted to a behavior like proboscis monkeys, climbing and hanging in mangrove trees, wading into water and swimming on the surface. In my opinion human ancestors, split from chimpazees and other apes and, instead of staying in forests like chimps, progressed with their water skills, like diving and collecting seaweed, then adapted to waders in shallow water and finally to bipedal walkers on land.

The fact that human olfactory bulbs are only 44% of the chimpanzee bulb?, is not compatible with African savanah life. All savanah animals have a good olfaction. But an aquatic evolutionary phase would explain why humans have a poor sense of smell. Water animals typically have a reduced or even non-existent sense of smell.(4)

The human Area 4 for the legs, feet and toes are reduced, because human left the trees and lost the grasping hind limbs of apes. Area 4 for the hands and fingers are larger than apes. The human hand is much more mobile than an apes, the thumb and index finger in particular, the human fingertips are more sensetive, we have faster-growing fingernails. All of these enhancements point towards the enhanced hand mobility and sensitivity of raccoons and sea otters, which suggests that human ancestors groped for crayfish and shellfish underwater, also the mobility was needed to remove the shells of the food. Raccoons are good climbers but seek most of their prey in shallow water. They have human like forelimbs and fingers?. And their brain cortex shows the same types of enlargements as humans. Sea-otters, humans, and mongrove monkeys all use tools, unlike savannah mammals.(5)

Like humans, all diving mammals have excellent airway control, to keep the water out of their lungs. This voluntary control of breathing is necessary because they have to inhale strongly before they dive, and under water they have to hold their breath until they surface. In land mammals, however, exhaling and inhaling breathing rythms change involuntarily. with lower oxygen and higher carbondioxide. An aquatic mammal with that mechanism would inhale strongly when its need for oxygen was the highest, in other words, it would inhale involuntarily while underwater. That's why the human ancestor tripled the part of Area 4 for the mouth and airways, and why he evolved the Broca area which coordinates the muscles of the mouth airways. This refined airway control was a preadaptation for human speech.(4)

V. Speech and Association Areas

The arcuate fasciculus in humans directly connects the coordination center of the muscles needed for breathing (Broca) with the cortex behind the sensory areas for the mouth and throat (where we feel the movements our breathing, singing, talking etc. make) and the audio areas (where we hear and register the sounds we hear)(5). This connection of airway sensation with hearing was the beginning of learning to make voluntary sounds. In the primitive' part of Wernicke, the first interpretations of sound are made possible through connections to the visual and paretial areas, so that the sounds were associated with what we were seeing and feeling when we heard the sound.

Once the connection of Wernicke and Broca was made, we got a device that could both make sound and interperet it. Using that apparatus we learned to communicate with the others living in our group. We bettered our communication abilities by evolving larger areas for the use of our new mechanism.

Apes lack the association ares. Any ape could have evolved a greater amount of brain tissue and developed the larger association areas, if the ape had found a need for the extra brain. But, the larger association areas were useless without the improved sound making/interpereting areas found in humans.

Voluntary and variable sound production seen in aquatic animals like otters, seals, sea lions, and toothed whales. Large brains are a feature of many aquatic animals, seals and toothed whales. The relation between aquatic life, brain size and vocal control is not clear. But even the small-brained sea mammals have fairly well-developed vocalization skills.

VI. Brain Lateralization

An important difference between a human brain and an ape's brain is the larger amount of asymmetry in human brains. Like humans being right-handed, is more pronounced than dexterity of monkeys or apes.(4) Most mammals and birds show small signs of asymmetry in certain brain functions. The left part of the brain, in most people, is larger than the right half. (Remember that the left half of the brain controls the right half of the body and vice versa.) In 65% of people, the left planum template, where the hearing centers are, is much larger than the right one. Musical learning that occurs before the age of seven seems to induce strong enlargement of the left planum temporale. In more than 80% of people the same hemishpere controls the dominant hand (right). Why is that?

The right hand is usually the hand that does things with an object while the other hand holds the object steady. The left hand holds the shield, holds the billiard cue, and holds the paper when writing. This fits with the spatial and geometricality of the right hemishpere. The right hand is not completely dominant, there is a small division of tasks between the left and right brain centers for the hands, especially with jobs that two hands have to be used in.

Some people say that our dexterity came from an ancestor that picked fruit with one hand while stabalizing himself by holding onto a branch with the other hand. Although apes are sometimes right or left-handed for certain tasks, systemetic handedness in the human sense has hardly been demonstrated so far in nonhuman primates. One explanation for humans having more refined dexterity than apes is that diving homonids used the right hand to get shellfish from the bottom of waters while the left grasping something to keep them on the bottom. Or they used a rock to open the shell while the left held the mussel or oyster etc. That could be the beginning of human tool use.

Hands are paired organs. Each hand needs its own control center in the brain. The two centers can be symmetrical -- like apes or, like humans when each hand has a different function -- more or less asymmetrical. However, an unpaired organ works better if it has one brain center dominating over the other, so that fine movements would not be messed up by commands from the other brain center. A good coordination of the breathing muscles would be essential, the need for dominant brain centers made dominant brain centers.

Song production in birds is strongly asymmetrical. In adult finches, section of the left nerve for the syrinx leads to the loss of most of the song, but the right section has only minor effects of song loss. If the left nerve is cut before song develops, the right takes over completely.

Human speech centers, too, show a great deal of plasticity. The localization of Braca's and Wernicke's Area in the left hemishpere is more constant than human dexterity: not only right- handers, but also most left-handers, have their speech centers in the left hemisphere of their brain. Is there any relation between right/left-handedness and the location of the sound- interpereting/making device? The fact that the control of our dominant hand is usually situated on the hemisphere of speech centers could mean that the earliest language use in human ancestors were the naming of objects that were manipulated or pointed at with the right hand. Or is it simply cooincidence.

VII. Conclusions

The changes in human brain anatomy, compared with the brain anatomy of apes and monkeys, fits with the aquatic theory of human evolution and have relationships with aquatic and semiaquatic mammals.

Reduced olfaction is typically seen in aquatic mammals.

Diminished foot control is a feature of nonarboreal (not living in trees) mammals.

Very refined finger control is a feature of shallow-water feeders.

Perfect control of the airway entrances is essential in diving mammals.

Elaborated vocal ability is seen in aquatic mammals.

A large brain is seen in aquatic mammals such as seals and toothed whales.

Brain asymmetry leads to an aquatic ancestor in human evolution history.

Result: Homo Aquaticus? I think so. And I thik I have proved to myself well enough to believe in the theory.
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