The common ancestors of modern apes--gibbons, orangutans,
gorillas, chimps, and humans--evolved a number of new features such as
the loss of the tail, a pelvic diaphragm, and a larger brain.
THE SKELETAL SYSTEM
As apes became larger and spent at least some time erect (either walking
or hanging), the pull of gravity on the urinary and reproductive organs
became problematic: there was originally no reinforcement of the body
wall in this region. Apes solved the problem by tucking their tail between
their legs (refer to the following photo of the human pelvis). The tail
was reduced to form a bone a fused vertebrae called the coccyx (in humans,
the tail is much more significant earlier in development) and positioned
to help support these pelvic organs. The muscles which used to move the
tail are still present but they now reinforce the body wall, forming the
pelvic diaphragm. The apes developed a sphenoidal sinus, a prevomer, and
a broad, flat nasal floor.
THE NERVOUS SYSTEM
Apes have a significantly larger cerebellum (its relative size) compared
to that observed in monkeys. (The relative sizes of the cerebellum in
Old and New World monkeys are roughly equivalent). This increase is due
to an expansion of the cerebellar hemispheres, rather than the vermis.
In apes, the relative size of the cerebellum is about 45% greater than
that of other primates.
Proconsul, the first ape in the fossil record, possessed a larger brain
than Old World monkeys. Apes increased the development of the posterior
The thalamic pulvinar nucleus increased in size in primates and is the
largest thalamic nucleus in apes. Apes possess a gyrus transverses, gyrus
longus anterior, and a sulcus principalis/frontal-marginal fissue. The
sulcus intercalates and genualis form the sulcus cinguli , and the anterior
and posterior calcarine sulci fuse (but then divide into 2 caudal branches.
In apes, the rootlets of the facial nerve originate more caudally and
lack the "knee" in the path of the inferior facial root fibers.
Ancestral apes lost the external granular layer of the dorsal cochlear
nucleus in embryonic development in apes.
Apes share the ability to recognize mirror reflection as representing
self and the ability to learn sign language.
THE CARDIOVASCULAR SYSTEM
In apes, the first gamma globin gene is the primary gamma gene expressed
in fetal development.
THE MUSCULAR SYSTEM
Ancestral apes underwent a number of muscular changes. The caudal muscles
were converted into the pelvic diaphragm. Vestiges of the muscles which
are used in tailed primates to flex the tail usually exist (such as the
sacrococcygeus anterior in humans). The ancestral pubo-iliocaudalis is
attached to the visceral organs and becomes the levator ani.
The flexor digitorum brevis of primitive apes is intermediate between
humans and lower primates. Apes usually lack the epitrochleo-anconeus
of lower primates, which was originally derived from the flexor carpi
ulnaris. Apes developed a deep head of the pronator teres and a radial
origin of flexor digitorum sublimis. In apes, the pectoralis minor inserts
onto the coracoid process instead of the arm.
Ancestral apes evolved an appendix. Most apes have a menstrual cycle of
about 30 days (gibbons 30; orangutans 31; gorillas 31; chimps 28-37, humans
28. In ape reproduction, there was a reduction in the number of intromissions/copulation.
The ape ancestor evolved the N blood antigen, a 51 base pair deletion
in exon 8 of the MHC-G gene, amino acid substitutions occurred at positions
121, 151, 155, and 156 of myoglobin, an amino acid substitution at position
19 of alpha hemoglobin, and 3 Alu sequences inserted into globin cluster,
and the genes COX4-1, COX8L, and ISP underwent positive selection.