A great variety of apes evolved in the Miocene, including
the bipedal Oreopithecus.
During the Miocene, apes experienced their greatest radiation with as
many as 30 species existed, inhabiting broad regions of Africa, Asia,
and Europe. During the Late Miocene, climatic changes that increased seasonality
(and gradually replaced many forests with grasslands) and competition
from an ever increasing number of monkey species caused a decline in the
diversity of ape species (Gibbons, 1997). The primitive apes Nacholapithecus
(15 million years ago) and Proconsul (18 million years ago) lacked tails,
indicating that the loss of the tail was one of the first modifications
in ancestral apes (Natatsuka, 2004; Ishida, 2004).
Proconsul is an ape known from Uganda and Kenya from 22-17 million years
ago. Postcranially, it was more similar to apes than monkeys and had several
apelike jaw and tooth characteristics. Its thumb was opposable. Some characteristics
were similar to those of the hominids (its P3 tooth, the degree of encephalization,
the head of its humerus, its trochlea, hallux, ulna, and the absence of
a tail). There were a number of species of Proconsul: P. africanus was
baboon sized, P. major was largest species and was gorilla sized, and
P. hamiltoni which is tentatively dated at 24-27 million years which would
make it the oldest ape (Walker, 1983; Caroll, 1988; Andrews, 1992).
Family Proconsulidae includes all Early Miocene apes. It includes the
species Dendropithecus, Dionysopithecus (found in Asia), Limnopithecus
(whose denticia was similar to that of Proconsul), Micropithecus, and
Rangwapithecus. Micropithecus is the smallest known ape (living or fossil)
and its face is similar to modern gibbons (Fleagle, 1978). Proconsul and
Afropithecus were probably arboreal apes. The modified hands of Nacholapithecus
kerioi indicate that it adapted for climbing and was more arboreal than
other Miocene apes. Kenyapithecus was more adapted to terrestrial locomotion
than other Miocene apes (Nakatsukasa, 2003).
Family Oreopithecidae includes the apes Nyanzapithecus from the Early-Middle
Miocene from Africa and Oreopithecus from the late Miocene from Europe.
Oreopithecus is a hominoid from the Late Miocene (7-9 million years ago)
from Italy. It was bipedal and had a number of hip adaptations which supported
this locomotion such as cancellous bone architecture, a long ischial spine,
a prominent anterior inferior iliac spine, a short ischium, and a short
pubic symphysis (Rook, 1999). Given that both early hominids and Oreopithecus
independently evolved bipedality, it is not surprising Oreopithecus hands
were modified to improve their grasping ability in ways similar to those
observed in early hominids (Moya-Sola, 1999).
Turkanopithecus was a medium-sized ape whose relationships to other groups
of apes are not yet clear (Leakey, 1986b).
Some feel that the Family Pliopithecidae is related to the ancestors of
gibbons. Laccopithecus is known from the Late Miocene from Asia. Pliopithecus
was known from the Middle to Late Miocene from Europe. These apes had
7 lumbar vertebrae compared to 5 in humans and had specialized characters
such as lower molar form, sideways orbital openings, and long bones.
Dryopithecus is known from the Mid-Late Miocene from Africa and Europe
23 to 9 million years ago. The legs of Dryopithecus were more derived
than those of Proconsul, Morotopithecus, Equatorius/Kenyapithecus, and
Sivapithecus (Maclatchy, 2001). It is classified in the clade with the
African apes and diverged from the hominid lineage before the last common
ancestor of the African apes and humans. Preliminary evidence suggests
that its brain size was consistent with that of other great apes (Kordos,
2001; Moya Sola, 1993; Andrews, 1976). It had long arms and short hands
and was adapted for swinging at low speeds. Other species in the dryopithecine
tribe included Afropithecus, Heliopithecus, Otavipithecus, Kenyapithecus,
and Griphopithecus (Leakey, 1986; Benefit, 1995; Ward, 1999)
Gigantopithecus is known from the Middle Miocene to Pleistocene from Asia.
It was associated with woodland habitat and apparently adapted to the
Miocene climate change. From its teeth and mandibles, it is the largest
ape that ever existed (since teeth are twice the size of gorilla teeth,
it may have reached 11-12 feet in height). It coexisted in Asia with Homo
erectus (Ciochon, 1996).
Sivapithecus existed in the Mid-Late Miocene from Africa, Asia, and Europe.
Many believe it to be ancestral to orangutans (or at least very closely
related to their ancestors) on the basis of a number of traits such as
maxillary sinus, temporo-mandibular joint, zygomatic bone, temporal bone,
facial profile, orbital shape, and palatal shape. Ramapithecus from Eurasia
was a species very closely related to Sivapithecus (some feel they are
the same genus) (Leakey, 1985; Pilbeam 1977, Pilbeam, 1982; Pilbeam, 1990;
Andrews, 1977; Raza, 1983; Walker, 1973; Benefit, 1995).
Fossils of an ancestral chimp femur and a four-million year old gorilla
tooth are known (First fossil chimp, 2004).
POSSIBLE HOMINID ANCESTORS AMONG MIOCENE APES
In recent years, more complete skeletons of Dryopithecus and Kenyapithecus
have been found which demonstrate closer affinities to the hominids than
had previously been described. If Dryopithecus were an ancestor to modern
apes, its position would be before the split between African apes and
hominids. Kenyapithecus shows signs of being a semi-terrestrial quadruped,
perhaps the first to spend significant time on the ground. Kenyapithecus
diverged from lineage leading to the higher apes after Proconsul and Griphopithecus
but before Sivapithecus. Kenyapithecus africanus, a 27 kg ape from 15
mya, has been renamed Equatorius africanus. It is similar to Proconsul
and less derived than Kenyapithecus wickeri (Benefit, 1995; McCrossin
1993; McCrossin 2001; Ward, 1999). Graecopithecus may have been a member
of the African ape clade although further analysis is required to determine
whether thick tooth enamel is a primitive or derived condition. Humans
possess thick enamel on their teeth unlike other African apes which was
interpreted to be an adaptation of the human lineage. It now appears that
this trait was a primitive condition in higher apes (such as Sivapithecus)
which was secondarily lost in gorillas and chimps (Andrews, 1992; Martin
Morotopithecus was an ape that lived 20 million years ago. Its vertebrae
and glenoid cavity similar to modern apes and humans while its proximal
femur and other characteristics are primitive. It is more derived than
Proconsul and some feel that it is a common ancestor to apes and hominids
before orangutan lineage separated (Menon, 1997;. Gebo, 1997) The lumbar
vertebrae of Morotopithecus are more similar to those of living apes than
are the vertebrae of Proconsul (Sanders, 1994). Morotopithecus seems to
be a primitive great ape, more derived than gibbons. If this is true,
it means that great ape and gibbon lineages had diverged by 20 million
years ago, which is 2 million years earlier that what had previously been
thought (Young, 2004).
Ouranopithecus lived 9-10 million years ago. Its canines are smaller than
any recent or extinct apes. Its tooth enamel, tooth occlusal pattern,
and other characteristics are similar to Australopithecus (de Bonis, 1990;
Ankarapithecus lived about 9.8 million years ago. Some characters such
as the narrow interorbital region, flat zygomatic region, and elongated
premaxilla are similar to orangutans. Some characteristics such as a supraorbital
torus and frontal sinus are similar to gorillas and chimpanzees (Alpagut,
Samburupithecus kiptalami was a hominoid about the size of a gorilla
known from the upper Miocene of Kenya. In some dental and oral features,
it is more similar to primitive fossil apes while in others it is more
similar to modern apes and hominids (Ishida, 1997).