Miocene Epoch

23-5 million years ago


The common ancestors of chimps and humans evolved a number of traits.

In ancestor of humans and chimps, the maxillary nerve supplied the subscapularis and in the foot, digit II was innervated by superficial peroneal.

Tool usage is especially common in chimps: sticks for termites, wad of chewed leaves to remove water from tree holes, stout sticks to dig up ant, bee, or termite nests, leafy branches for sandals or gloves, leaf cushions to protect from thorny branches, bone picks to extract bone marrow, leaf napkins to clean themselves and infants, leaves to scoop water, natural objects to carry water, mortar and pestle to smash palm. The use of signs to communicate in wild has been observed in chimps. After chimps were taught sign language, it became evident that chimpse could paint representationally. It can be argued that wild chimp populations possess rudiments of culture since not all chimp populations have the same practices/tool usage and that these practices are passed down in each population as a learned behavior.

The ancestor of humans and chimps developed an origin of extensor digitorum on antebrachial fascia, an origin of lateral head of triceps from lateral intermuscular septum, a fusion of the proximal portion of tensor fascia lata to the gluteus maximus, an origin of the extensor digitorum on crural fascia, and an origin of the flexor digitorum brevis on the plantar aponeurosis. The extensor carpi ulnaris sometimes extends to proximal phalanx V, the teres major and minor share an origin from intermuscular septum, the clavicular origin of pectoralis major was reduced. The insertion of the abductor hallucis may include medial cuneiform and there is frequently a coronoid origin of the flexor digitorum sublimis.

There were also modifications of blood vessels. The interosseous artery arises from common interosseous artery, the superficial palmar artery passes over thenar muscles, the medial femoral circumflex sometimes originates from profunda femoris, and the hamstrings receive musclular branches of profunda femoris artery. Humans and chimps share dorsalis pollicis vesssels.

In male humans and chimps, the scrotum located more postpenially and testes increase in size.
Some alleles of the HLA-A and HLA-B genes are shared between humans and chimps and thus arose before the split between their lineages. The polymorphism of the HLA-DRB locus predates the split of human and chimp ancestors and gives evidence for multiregional evolution of human evolution. About 135 alleles are known (of which over 90% evolved after the split between human and chimp ancestors); some alleles offer resistance to malaria.
Not only are the -globin (eta) sequences of chimps and humans more similar to each other than to the sequence of gorillas (and humans, chimps, and gorillas are much more similar to each other than any are to orangutans), there are changes from the ancestral pattern that humans and chimps share: transitions at positions 1338 and 4473, transversions at positions 560, 5480, and 6971, deletions at positions 1287 and 3054, and one insertion at position 3272.
Humans and chimps also share a deletion in globin and a 6 base pair deletion in intergenic DNA between and globin. Human and chimp recognition systems are clearly similar in that NK receptors of one species can recognize the MHC proteins of the other.

At functionally important DNA sites, humans and chimp sequences are 99.4% identical while at less important sites, the homology is 98.4% homologous (Wildman, 2003).