Introduction: When comparing the human brain to that 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 this paper, I will describe the most defined differences in brain size and centers between humans and their closest relatives, chimpanzees. I will also compare them with other mammals and draw conclusions about the evolution history of humans.
II. Brain Evolution: Humans and chimpanzees are biochemically similar (DNA) and, therefore, probably phylogenetically related. However, 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 olfactory 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 divided by the central sulcus into independent 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).
In front of the central sulcus lies the pre-central cortex where the information for voluntary movements leaves the brain. The pre-central area is called the primary motor cortex, also known as Area 4″ in primates. 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 in apes. This leaves more room for the part that controls the hand, fingers, and thumb. Even bigger is the lower part of the human Area 4, related to the mouth, breathing, and vocal cords. The postcentral 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 that controls the breathing muscles.
Above Area Broca is Wernicke’s Area, the speech center, which is a uniquely human brain center along with Area of Broca. Wernicke’s Area has direct connections to Broca’s Area through the arcuate fasciculus, a neural pathway that apes do not have in their brain. The major difference between the human and ape cortices is the enlargement of the hand and mouth integration areas, which 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 areas are Wernicke’s Area, the visual area, and the auditory cortex.
Many anthropologists believe that the differences between human and ape brains are shown through man’s ability to use tools and language. However, this traditional view cannot explain why only human ancestors developed these motor skills and language abilities. It cannot explain why nonhuman primates and other savannah mammals didn’t develop these abilities.
The solution may lie in the aquatic theory of human evolution, which explains why humans don’t have fur, have excess fat, and possess many other human features. There are indications that early hominoids, ancestors to humans and apes, lived in mangrove or gallery forests where they adapted to a behavior similar to proboscis monkeys, climbing and hanging in mangrove trees, wading into water, and swimming on the surface. In my opinion, human ancestors split from chimpanzees and other apes and, instead of staying in forests like chimps, progressed with their water skills, such as diving and collecting seaweed. They then adapted to wading in shallow water and finally to bipedal walking on land. The fact that human olfactory bulbs are only 44% of the chimpanzee bulb is not compatible with African savannah life. All savannah animals have a good sense of smell. However, 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. The human Area 4 is for the legs, feet, and toes.
