European Miocene Hominoids: The Missing Link?
Table of Contents
Hominoids and the Missing Link
Ouranopithecus:
Location and Description
Synapomorphies
of Ouranopithecus and Later
Dryopithecus
Sites and Fossil Descriptions
Dryopithecus
Cranio-Dental Features
Dryopithecus
Postcranial Features and Mobility
The
study of human evolution generally focuses on the hominids beginning with the
australopithecines, but to truly understand where humans came from it is
necessary to look even earlier to the point when the human-great ape clade
diverged from other primates. This split most likely occurred in the Middle to
Late Miocene epoch and it is in this temporal location that we must search for
our hominoid antecedents. A plethora of Miocene hominoid fossils have been
discovered in both Africa and Eurasia, but surprisingly, a majority of the
Middle and Late Miocene hominoids are found in Eurasia and not in Africa. This
paper will discuss two of these European Miocene hominoids, Dryopithecus and Ouranopithecus, and their possible places in hominid and human
evolution. Though the focus will be on these genera, for comparative purposes
mention will be made of other Miocene hominoids, such as the Early Miocene hominoid
Proconsul and the Middle Miocene
hominoid Sivapithecus.
Hominoids and the Missing
Link
Hominoids
are a superfamily under the suborder Catarrhini and include many extinct and
extant genera and species. The extant species included under the hominoids are
the lesser apes (gibbons and siamangs of eastern Asia), the great apes
(chimpanzees and gorillas of Africa and the orangutan of Southeast Asia), and
humans (Conroy 2005:98). The two Miocene hominoids discussed in this paper, Dryopithecus
and Ouranopithecus are only two of many extinct hominoids. These two
are particularly important to human evolution because they are classified under
the family Hominidae (hominids), which includes gorillas, chimpanzees, and the
genera Australopithecus and Homo. The study of hominoids enables us to make
connections between the predecessors of Homo and modern humans and it helps us to understand our
evolutionary relationship with the other extant primates (particularly gorillas
and chimpanzees).
Human
beings did not simply split off from the apes and become human though. The
search for Miocene fossils is focused on finding the Òmissing linkÓ between
fossil apes and early Australopithecus. The fact that this missing link may have evolved in Europe rather than
Africa has been seized by the racial and nationalistic undertones present in
the paleontology field. Thus, Dryopithecus and Ouranopithecus have become very important to many in the search for
our earliest hominid ancestor.
The
Miocene Epoch began around 24 mya and lasted until around 5 mya. It is
generally divided into three periods: the Early Miocene, which spanned from 24
mya to around 16 mya, the Middle Miocene, from 16 mya to 11.5 mya, and the Late
Miocene, which lasted from 11.5 mya to 5 mya. It was during the Early/Middle
Miocene, about 16.5 mya that African hominoids first immigrated to Eurasia and
led to the genera of Dryopithecus and
Ouranopithecus (Conroy 2005:99).
It
is important to understand what the environment was like in the Miocene to
understand the ecological adaptations of the evolving hominoids. The Miocene
was a time of great change and it was during this time that the foundations for
the climate, geography, flora, and fauna of the modern world were set in motion.
The Early Miocene had a much warmer climate, an abundance of forests and
woodlands with fewer grasslands, and in general, was very different from the
way it appears today. As a result of continental shifting and the collision of
Africa and Eurasia, several major mountain systems erupted, including the
Himalayas, Tauros, Zagros, and various European Alpine mountain chains (Pilbeam
1979:334). As a result of these erupting mountain systems and the recession of
the Tethys Sea, which divided the northern supercontinent (including North
America, Europe, and Asia) from the southern supercontinent (including Africa,
South America, India, Australia, and Antarctica), Miocene hominoids began to
appear in Europe around 16-17 mya.
The
environment plays a large role in affecting evolution. In the Middle Miocene,
habitats were changing from less tropical and subtropical forest into more
woodland and savannah. This is likely a result of a large cooling period,
which began about 15 mya, during which hominoids were being established in
Europe. Undoubtedly, it is during this time when hominoids were becoming habitually
less arboreal and spending more time on the ground in these new habitats,
though by no means were they fully bipedal at this time. In addition to moving
out of the trees, the Miocene hominids would have experienced a changing diet,
which may manifest itself in the cranio-dental morphology (which will be discussed
in greater detail in a later section). "Enamel thickness is linked to
diet. A thin enamel corresponds to a diet of soft food such as fruits, buds,
or young leaves that are typical of tropical, aseasonal rain forests"
(de Bonis and Koufos 1994:80). The increasing thickness of enamel in Dryopithecus
and Ouranopithecus indicates a movement away from the tropical
rainforests and into woodlands, grasslands, and savannahs that would contain
coarser rougher food, such as seeds, nuts, roots, and tubers. This is further
supported by the faunal evidence, which includes horses, antelopes, and other
fauna associated with open country and grasslands.
Ouranopithecus: Location
and Description
Ouranopithecus
macedoniensis is one European Middle
Miocene hominoid that has been the source of much controversy. Ouranopithecus
macedoniensis gets its species name
from the region the fossils are found in, Macedonia of northern Greece. Fossils
were first discovered near the village of Vathylakkos in Northern Greece during
the First World War. They were unearthed by French soldiers under the
supervision of paleontologist Camille Arambourg (De Bonis and Koufos 1994:76). In
the 1970s, Louis de Bonis resumed the search for fossils in northern Greece and
located hominoid remains at the sites of Ravin de la Pluie and Xirochori near
Vathylakkos and another site near the village of Nikiti, which lies east of the
city of Thessaloniki. The specimen from Xirochori is a partial skull with a
nearly complete face. It includes most of the right face with orbit, a large
part of the frontal bone, the nasal bones and the nasal aperture, the maxilla
with the root of the zygomatic bone, and the dentition (de Bonis and Koufos
1994:80). These localities have been biostratigraphically dated to the Late
Miocene, between 9 and 10 mya.
Ouranopithecus is a large Miocene hominoid, being approximately the
same size as a female gorilla. Regarding the size of Ouranopithecus, de Bonis and Koufos write: ÒSize disparity among
the specimens indicates a variation which, in our opinion, clearly corresponds
to sexual dimorphism in body and canine sizeÓ (de Bonis and Koufos 1990:78). Based on dental dimensions de Bonis and
Koufos estimate the male Ouranopithecus body weight to be about 160 pounds. What makes Ouranopithecus so controversial is the fact that its discoverers (de
Bonis and Koufos) have claimed that it is the sister group to Australopithecus
and Homo.
"This
claim is based on several general similarities between Ouranopithecus and
Australopithecus, such as low-crowned and thick-enameled molars; broad,
shallow mandibular bodies; the overall appearance of the rounded and swollen
molar cusps; and the trend toward both canine and P3 honing facet reduction"
(Conroy 2005:118).
It is these morphological traits, among others, that make Ouranopithecus more similar to Australopithecus and Homo
than other Miocene apes, such as Proconsul.
Synapomorphies of Ouranopithecus
and Later Hominines
The following is a list of
synapomorphies for Ouranopithecus and
Australopithecus proposed by de
Bonis and Koufos: very thick cheek teeth enamel; inflated external cusp of the
upper anterior premolar (P³); reduced canines (less reduced than those of Australopithecus, but along the same evolutionary trend); rounded
basal sections of the canine crowns; a broad and shortened P3 without honing
facet; vertical profile of the upper face; small glabellar depression; and a
narrow mandibular condyle (de Bonis and Koufos 1994:81)
The
enamel thickness is indicative of the habitat in which Ouranopithecus was
living in, but it also indicates an evolutionary trend toward Australopithecus
and Homo. The thick enamel on the cheek teeth is a derived trait for
Miocene hominoids, as it was not present in previous primate groups that had
occupied the tropical forests. Furthermore, among extant hominoids, it is
unique to Homo sapiens. Modern gibbons, gorillas, and chimpanzees have
thin enamel on the cheek teeth. "Ouranopithecus has very thick enamel, which suggests a modification
of the occlusal surface consistent with the adaptive dental features present
in the bulk of the Ravin de la Pluie fauna "(de Bonis and Koufos 1994:80).
The reduced size of the canines is
evidence of a clear trend toward Australopithecus. Though sexual dimorphism is present in Ouranopithecus
canines, they are still markedly
smaller than the canines of other hominoids excepting Australopithecus and Homo.
For example, the canine of a male Proconsul is three times larger than that of a male Ouranopithecus.
In
apes, the anterior lower premolars, P3, are high with an elongated, laterally
compressed, and sectorial crown and a honing facet. "The honing facet
is lacking on the P3 of both Australopithecus and Homo. Indeed,
this is one of the derived characters that link these genera. Ouranopithecus
has a short, rounded P3 that lacks a honing facet and thus is more Australopithecus-like
than ape-like" (de Bonis and Koufos 1994:79).
There
are other synapomorphies aside from the dentition that link Ouranopithecus with Australopithecus and extant hominoids. Ouranopithecus is the first Miocene hominoid to possess a
supraorbital torus, an apomorphy found in gorillas, chimpanzees, Australopithecus, and Homo. The brow ridge in Ouranopithecus is large but not
projecting, as it is found in gorillas and chimpanzees, indicating that this
was a newly derived trait that would become more fully developed in future
hominoids. Furthermore, the eye sockets of Ouranopithecus are wider than they are tall and rectangular, like
they are in African apes and humans (Benefit and McCrossin 1995:242). These
synapomorphies clearly show that the placement of Ouranopithecus in with the human-great ape clade is correct. No
other Miocene hominoids, with the exception of Dryopithecus, have as many shared derived traits with hominids as
Ouranopithecus seems to.
Dryopithecus Sites and Fossil
Descriptions
Like
Ouranopithecus, Dryopithecus is a genus of Miocene hominoid that is found in
Europe, though unlike Ouranopithecus, which is found only in Greece, Dryopithecus is found widespread in Europe. Many of these
specimens are from Hungary (Rudabanya), Spain (Can Llobateres, Can Ponsic), and
France (St. Gaudens, La Grive). These specimens include both upper and lower
teeth and jaws, reasonably complete crania, and postcranial material (Conroy
2005:118).
Several
species are attributed to the genus Dryopithecus, the first of which being Dryopithecus fontani, found in St. Gaudens, France in 1856 by Lartet. The
fossils from St. Gaudens include three fragments comprising most of a single
lower jaw and a humeral shaft missing both epiphyses, suggesting that the
remains were those of a young individual with unfused epiphyses (Begun 1992b:315).
A large number of Dryopithecus
fossils have been recovered from Rudabanya, Hungary though they have been
elsewhere identified as Rudapithecus. Among these are several postcranial remains, including a distal
humerus, proximal radial and ulnar fragments, a talar body, a distal first
metatarsal fragment, and a number of phalanges (Begun 1992b:316). Also found at
Rudabanya are many cranial and dental specimens. These include large portions
of two craniofacial skeletons, two additional palatal specimens, four
mandibles, and numerous isolated teeth. Though initially classified as a
separate genus, Rudapithecus hungaricus, is now included within the Dryopithecus genus. David R. Begun explains the reason for this:
"The
gnathic material from Rudabanya shares a number of characters with specimens
attributed to the four species of the genus Dryopithecus. These include
high-crowned, narrow, and thick (labiolingually) upper and lower incisors;
upper lateral incisors robust at the cervix and lacking pronounced cingula;
tall, buccolingually compressed canines that are relatively small compared
to the molars and with thick, rounded distal cingula; reduced lower premolar
cusp heteromorphy; broad lower third premolars (P3) often with well-developed
mesio-lingual beaks and small metaconids; elongated lower fourth premolar
(P4) with high talonids; reduced molar cingula; elongated lower molars with
tall, peripheralized cusps, broad basins, and relatively early dentine penetrance;
and reduction in lower third molar (M3) size. For these and other reasons,
the Rudabanya fossils can be attributed to the genus Dryopithecus"
(Begun 1992a:1929).
Therefore, there are many fossils representing Dryopithecus that have been found already and likely more to
come.
Dryopithecus Cranio-Dental
Features
Dryopithecus
shares many cranial features with great apes and humans that are not present
in most other Miocene hominoids. Like Ouranopithecus, Dryopithecus
has a poorly developed, though still present, supraorbital torus. This
is a character that is found among African great apes and Australopithecus.
Other synapomorphies present in Dryopithecus include a shallow sulcus supratoralis, a prominent glabella, and an increase
in the anteroposterior development of the frontal bone in the temporal fossa.
The presence of a supraorbital torus, a shallow sulcus supratoralis, and a
prominent glabella are all related to increased ventral flexion of the face
relative to the cranial base, known as klinorhynchy (Begun 1992a:1930). "The
anteroposterior increase in the frontal contribution to the temporal fossa
also appears only in more klinorhynch apes (African apes) and may also be
directly related to the anterior and ventral rotation of the klinorhynch face"
(Begun 1992a:1930). These four traits, which Dryopithecus also possesses (from
sites RUD 44 and RUD 77), indicate that Dryopithecus was also klinorhynch, suggesting an evolutionary link
between Dryopithecus and the extant African apes.
There
are many other traits that Dryopithecus shares with African apes that support this claim. These include a broad,
flat nasal aperture base, a broad, relatively shallow canine fossa, a stepped
subnasal floor, a biconvex naso-alveolar clivus, a true incisive canal, a
reduced incisive foramen, an ethmoidal frontal sinus, and a broad interorbital
distance (Begun 1992a:1931, Table 1). Some of these traits, such as the nasal
aperture and the subnasal floor, are also present in Ouranopithecus, which indicates that these traits are plesiomorphies
for the great ape and human clade, though they may be synapomorphies in Dryopithecus,
and Ouranopithecus.
Some others of these traits are found in other primitive hominoids, such as
Proconsul but are subsequently lost in Proconsul's supposed descendents in the Pongo clade.
According
to Begun (1992a), many of the traits linking Dryopithecus to Gorilla are not derived traits, but rather they are primitive for the great
apes and even for the hominoids. Some of the characters shared among Dryopithecus,
Gorilla, and other hominoids not
including Pan and Australopithecus, include relatively large incisive foramen,
comparatively short incisive canal, comparatively short premaxilla, broad thick
lateral orbital margin along the frontal zygomatic process, smoothly convex
anterior surface of the frontal zygomatic process, sloped mandibular ascending
ramus, narrow upper lateral and lower incisors, conical, asymmetrical upper
lateral incisors with strongly sloped incisive edges distally, long P4 relative
to M1, and elongated molars relative to breadth (Begun 1992a:1931). These
traits, though not necessarily evidence of a direct ancestral link between Gorilla
and Dryopithecus, do show that Pan and Australopithecus (and subsequently Homo) are more closely related to each other than either
is to Gorilla.
Dryopithecus Postcranial
Features and Mobility
The
discovery of a partial skeleton of Dryopithecus laietanus from Can Llobateres in Spain provided evidence for
Miocene hominoid locomotion that had previously been unknown. Salvador
Moya-Sola and Meike Kohler recovered numerous postcranial elements of Dryopithecus
in 1992 to 1994. The skeleton from
Can Llobateres (CLI-18800) includes the following: well preserved humeral
diaphysis with insertion areas of deltoids, pectoralis, teres major and
coracobrachialis, and other small humeral fragments; an ulna nearly complete
from the middle of the trochlear notch to the distal end of pronator quadratus
insertion; radial diaphysis; triquetal; all metacarpals; all phalanges except
apical ones of the first and fifth fingers; four sesamoids, all of them of the
right side; fragment of the right clavicle, showing the sigmoidal curvature
from the middle of the shaft to conoid tubercle; several rib fragments; three
incomplete lumbar vertebrae; one thoracic vertebra; two neural arches; two
femora, the left one from caput to the distal end of diaphysis, the right one
from caput to the distal third of the shaft; and the distal third of the tibia
(Moya-Sola and Kohler 1996:157). Paleomagnetism indicates the age of the
skeleton to be about 9.5 million years.
The Size and coordination of certain
skeletal elements indicates that Dryopithecus was both orthograde and used to more climbing and
suspension behavior. The evidence of an orthograde posture include: the lumbar vertebrae
are proportionally shorter than those of cercopithecoids and proconsolids; the
transverse processes originate on the pedicle dorsolaterally on the vertebral
body; and the caudally directed spinal processes indicate reduced mobility of
the lumbar region (Moya-Sola and Kohler 1996:157) All of these are characteristics
of more orthograde postures and less like the pronograde posture of monkeys and
other primates. David begun has worked with several humeral bones and other
postcranial elements to determine locomotion activities in Dryopithecus and other Miocene hominoids. The humeral and ulnar
specimens from Rudabanya were especially useful in that they were directly
comparable to material from other European locations. Begun writes:
"The
material from Rudabanya is particularly useful to the analysis of positional
behavior and phyletic affinities in Miocene hominoids for two reasons. First,
it contains forelimb shaft and joint surface specimens that can be compared
directly to other Miocene hominoids, and which are complete enough to provide
definite indications of both positional behavior and evolutionary relationships.
Second, the Rudabanya postcrania occur together with abundant remains of hominoid
cranial and dental material" (Begun 1992b:317).
RUD 53 is a left distal
humeral fragment with a short length of shaft and a nearly complete distal
articular end. The lateral epicondyle is large, rounded proximodistally and
projects laterally. The lateral trochlear ridge runs from between the deep,
well-defined triangular coranoid fossa and the relatively shallow radial fossa
anteriorly, to the lateral border of the olecranon fossa posteriorly. Based on
these and other characteristics of the humerus of RUD 53, begun claims that it
is most similar morphologically to modern extant great apes, and more
specifically, to be metrically most similar to the great apes and humans (Begun
1992b:321).
Begun
also looks at a proximal end of a right ulna (RUD 22). It is broken proximally
about midway along the trochlear notch, so the proximal part of the articular
surface and the entire olecranon process is missing. "The trochlear notch
is segmented by a prominent median keel or ridge dividing the articular surface
into roughly equal medial and lateral surface areas" (Begun 1992b:321).
By comparing many aspects of RUD 22, Begun concludes that it bears a strong
resemblance to modern hominoid proximal ulnae.
For phylogenetic purposes, Begun
compared the Dryopithecus specimens
from both Rudabanya and St. Gaudens with other Miocene hominoid fossils from
Klein Hadersdorf, in Germany. The three humeral specimens fell into two
morphological categories, according to Begun. The specimens from Rudabanya and
St. Gaudens, the Dryopithecines,
are modern hominoid-like, while the specimen from Klein Hadersdorf is more like
homologues from early and middle Miocene localities.
ÒThe
traits shared among modern hominoids and the Rudabanya/St. Gaudens group, to
the exclusion of Klein Hadersdorf and earlier catarrhines, include the
following: 1) long, slender shaft; 2) shaft anteriorly concave proximally; 3)
gracile deltopectoral crest; 4)flat diaphysis distally; 5)proximal epicondyles;
6) medially placed medial epicondyle; 7) broad symmetrical trochlea; 8) deep
coranoid fossa; 9) distinct radial fossa; 10) diaphysis round; 11) anterior
bicipital surface; 12) reduced brachialis flange; 13) deep trochlear groove;
14) deep lateral keel; and 15) deep zona conoideaÓ (Begun 1992b:327).
The ulnar specimens fall into
the same two morphological categories. The specimen from Rudabanya is modern
hominoid-like and the Klein Hadersdorf sample resembles Proconsul and other catarrhine ulnae that lack hominoid
synapomorphies. The corresponding synapomorphic ulnar traits between Dryopithecus
and modern hominoids include: 1) anteroposteriorly shallow,
mediolaterally robust shaft; 2) rounded anterior shaft crest; 3) shallow radial
facet; 4) laterally facing radial facet; 5) pronounced trochlear keel; and 6)
broad trochlear notch (Begun 1992b:327). These morphological traits and the
presence of synapomorphies supports the placement of Dryopithecus in with the great apes and humans both
evolutionary/morphologically (clade) and behaviorally (grade) concerning
locomotion and the orthograde posture. This also provides even stronger
evidence that it is Dryopithecus,
or some close variation thereof (such as Ouranopithecus), that is the missing link between primitive apes
and hominids.
There
are many questions still to be answered and many more, still to be asked
regarding human evolution. The question of the missing link is still yet
unresolved, and may remain so for a very long time. One of the questions
regarding the possibility of European hominoids being the missing link is, how
and when did they leave Africa and come back to evolve into Australopithecus?
The
most parsimonious answer is that they had only two migrations, one out of Africa,
during the Early to Middle Miocene, when they then evolved within Europe to the
Dryopithecus and Ouranopithecus forms we find in the ground today, and then another
migration back into Africa in the Late Miocene, possibly because of the cooling
temperatures, where they eventually evolved into Australopithecus and eventually Homo.
This scenario is supported by the fossil
evidence, both the presence of more hominid-like Miocene hominoids in Europe,
and the absence of any likely fossil candidates from Africa. If we did not
evolve (at this temporal point) in Africa, then the next logical place to look
is Europe. It is also supported by molecular evidence, surprisingly enough.
Caro-Beth Stewart and Todd Disotell have developed a phylogenetic tree based on
DNA sequence, fossil evidence, and a molecular clock. Their molecular clock
supports the scenario that the Òmissing linkÓ was an unknown ape from Europe or
Asia that dispersed into Africa about 10 million years ago (Gibbons 1998:622).
Could
this ancestor be Dryopithecus or
Ouranopithecus? The answer is still unclear and there is much ongoing
debate over this issue. Though the missing link status may not be applicable
to these hominoids as of yet, it is still reasonable to include them in the
great ape and human clade. The Synapomorphies they possess with Australopithecus, Homo,
and the extant African apes is certainly suggestive that if one of these groups
is not the missing link, then it must be a sister group of Ouranopithecus
and Dryopithecus. As with all unanswered questions in the field of paleontology, the thing
that is needed to provide more answers is more fossils.
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