Oligoocene Epoch

33-23 million years ago


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.

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.

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 parietal lobe.
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.

In apes, the first gamma globin gene is the primary gamma gene expressed in fetal development.

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.