Miocene Epoch

23-5 million years ago


The common ancestors of African apes--gorillas, chimps, and humans--evolved a number of changes.

The ancestor of African apes developed a sulcus principalis/frontal-marginal fissue. Changes in muscular innervation occurred: the median nerve usually innervates 3 ½ digits, the femoral nerve innervates the psoas minor, and the flexor digitorum longus is innervated by muscular branches of tibial nerve. Changes evolved in the clusters of spindle cell pyramidal neurons: gorillas possess more than orangutans, chimps possess even more, and humans possess the greatest number.

Ancestral African apes evolved a number of muscular changes. The origin of flexor pollicis brevis was limited to flexor retinaculum and trapezium, the flexor digitorum superficialis originated from the intermuscular septum, the pronator teres was obliquely oriented, the flexor pollicis longus originated from anterior radius and interosseous membrane, the extensor digitorum IV sent slips to digits III and V, the origin of coroacobrachialis occurred on the intermuscular septum, an anterior extension of the coracobrachialis was usually present, the origin of extensor pollicis brevis came from the ulna and interosseous membrane, the extensor indicis usually didn't insert on digit III, the teres minor inserted below the greater tubercle of the humerus, the origin of subclavius occurred on the first rib only, the origin of psoas major reached S1, the piriformis usually fused with gluteus medius, and the insertions of the adductor magnus and quadratus femoris insertions met. Although the peroneus tertius has frequently been described as a muscle which only exists in humans as an adaptation to bipedal locomotion, it occasionally occurs in gorillas and rarely in chimps. The forearm's deep extensor layer was reduced.

The ancestor of African apes evolved novel characteristics of the spleen and transverse rugae in the vagina. There was a gradual reduction of the baculum in higher apes from orangutans to gorillas to chimps to the absence of a baculum in humans. In apes, three was a reduction in the number of intromissions/copulation (usually multiple in OW).
The ABCC13 protein contains only 6 exons of the 28 which are present in the gene due to a frameshift mutation which prevents the other exons from being translated. The complete protein is expressed in Old World monkeys while a shared 11 base pair deletion (which results in the frameshift) exists in humans, chimps, and gorillas. The HLA-Cw*0702 seems to have arisen before the separation of the higher ape lineages, producing alleles in humans, chimps, and gorillas.

African apes reduced the frequencies of several alternate venous drainage channels in the skull including occipital-marginal. The palmar metacarpal artery may be included in origin of radialis indicus, the radial artery enters the palm at the dorsum of the first interosseous space, the superior ulnar collateral artery sometimes originates from brachial artery , and the lateral thoracic artery usually forms an independent branch of axillary artery.