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
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.
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