About 7 million years ago, the lineage of the hominids
separated from that which would lead to modern chimps. Although this lineage
would produce a variety of fossil species over 7 million years, the only
modern survivors of this lineage are humans. During this time, a number
of changes evolved which separate humans from other animals, including
their closest relatives, the chimps.
CHANGES IN THE HUMAN LINEAGE:
The ancestors of hominids were already capable of some bipedal locomotion.
Chimps use a bipedal posture when feeding on elevated foods, carrying
objects, gathering fruits from ground, and when standing on branches
In hominids, the foramen magnum was positioned more ventrally as an adaptation
for upright posture. Although the skulls of the earliest hominids are
very similar to those of apes, they were modified to allow hominids to
walk erect. In many early hominids, the occipital bone formed a shelf
which is not present in modern humans.
THE NERVOUS SYSTEM
The human cerebellum is about 2.8 times the size of a chimp's cerebellum
and the relative size of the cerebellar hemispheres to the total brain
distinguishes humans from other apes.
While the lateral posterior nuclei of the thalamus have kept the same
number of neurons in humans as in apes, the human pulvinar nucleus has
twice the number of neurons. Humans have 1 ½ times the number of
neurons in the ventrolateral complex of the thalamus as found in great
apes. The size difference between superior and inferior colliculi (the
superior colliculus is the larger of the two) is greater in humans than
in Old World monkeys.
In humans, axons from the motor cortex proceed to the nucleus ambiguus
(the site of laryngeal motor neurons). There are no cortico-ambigual connections
known in Old World monkeys, New World monkeys, or other mammals. This
tract was probably a recent evolutionary step which was critical in the
development of speech.
Although the human brain is about 3 times the size of that apes, ape brains
would be expected to reach comparable sizes if their brains grew at the
same rate of the human brain after birth. The rate of growth of the human
brain after birth is similar (although slightly higher) to that observed
in chimp brains after birth but the period of rapid growth lasts considerably
longer in humans. The human neocortex is 3.2 times bigger than that of
chimps and the size of the human cerebellum 2.8 times the size of the
cerebellum of chimps.
Brain size increased in australopithecines but the external morphology
of the brain remained apelike. Cerebral reorganization in hominids predated
the increase in size according to A. afarensis and A. africanus endocasts.
The Hadar endocast does not show an expansion of parietal and occipital
regions. In H. habilis, the cerebrum significantly increased in size in
its frontal lobes and parietal lobes. The cerebrum had become taller but
not longer and the pattern of gyri and sulci was like humans and unlike
apes. Frontal lobes were narrower in archaic Homo sapiens. Modern humans
have developed the parietal lobes. The occipital region was shortened
and rotated under the parietal and temporal regions. Broca's area and
Wernicke's areas were enlarged and the structure of the meninges was like
that of modern humans. The development of the inferior parietal lobule
was like humans and unlike apes.
The encephalization quotient (the ratio of brain size to body size) of
Australopithecines is intermediate between that of modern apes and that
of species of the genus Homo. Humans have an EQ of 7.4-7.8, that of a
chimp is 2.2-2.5, gorilla 1.5-1.8, gibbon 1.9-2.7, old world monkeys 1.7-2.7,
capuchin monkeys 2.4-4.8 (and white fronted capuchin 4.8), squirrel monkey
2.3; marmoset 1.7; whales 1.8 and bottlenose dolphin 5.3.
In individuals affected by microcephaly, the brain may only reach a third
of its normal size (about the size of that found in early hominids) and
the gyral pattern is less complex than normal. These individuals have
reduced cognitive ability but are otherwise normal. There are two genes
whose mutations are known to cause microcephaly.
Abnormal spindle-like microcephaly associated ASPM is a large protein
which interacts with microtubules and is expressed in areas where new
neurons are produced. Its homolog in flies is known to function in the
organization of microtubules during cell division.
Microcephalin (MCPH1) is related to topoisomerase II-binding protein and
BRCA1. It regulates chromosome condensation in mitosis and DNA repair.
Homologs exist in bilateran animals. In ape lineages, both of these genes
have undergone positive selection unlike that of other mammalian lineages,
suggesting that they have contributed to brain growth in apes.
Neanderthal brain shape was consistent with that of archaic humans while
the change in modern human brains has been caused by growth of parietal.
The brain mass of neanderthals was only slightly less than that of modern
humans. Representatives of Pleistocene humans actually had a larger brain
mass than a sample of modern humans, perhaps due to a decrease in body
size since the Pleistocene.
A. africanus and H. habilis retain the transverse-sigmoid sinus drainage
system most common in apes, including humans. A. afarensis and boisei
have enlarged occipital-marginal sinuses. Occipial-marginal sinus systems
do occur in modern populations and in Pleistocene Homo. O/M drainage in
A. afarensis may have been linked to upright posture.
In humans, the jaw protrudes less, affecting shape of pharynx and sound
production. In the throat, the pharynx lengthened which enhances sound
production but makes choking a greater hazard.
The HLA-H pseudogene is homologous to a functional gene in gorillas. Human
pseudogenes HLAS-COQ and HLA-DEL also are homologous to functional genes
in gorillas. KRTHAP1 is a keratin pseudogene in humans but it is an active
gene in chimps and gorillas. Its loss of function is the result of one
base pair change.