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Australia: The Land Where Time Began |
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Dmanisi, Georgia, A Complete Skull, and
the Evolutionary Biology of Early Homo An impressive sample of cranial and postcranial
remains have been recovered from the site of Dmanisi in Georgia (the
country) which demonstrate that
Homo was present outside
Africa around 1,8 Ma. In this paper Lordkipanidze et
al. present a new cranium
from Dmanisi (D4500) that, together with its mandible (D2600),
represents the first known completely preserved adult hominid skull from
the Early Pleistocene. A
small braincase (546 cc) with a large prognostic face represents close
morphological affinities with the earliest known
Homo fossils from Africa.
Direct evidence for wide morphological variation within and among early
homo palaeodemes is
provided by the Dmanisi sample, which now comprises 5 crania. The
existence of a single evolving lineage of early
Homo, with physiographic
continuity across continents is implied by this. Hominid fossils, a rich
vertebrate fauna and Mode I (Oldowan) stone artefacts that co-occurred
in geological and taphonomic contexts that were well understood, were
uncovered by excavations at Dmanisi, Georgia. A short time after 1.85
million years ago (Ma) occupation of the site began and is documented
until 1.77 Ma (Gabunia et al.,
2000; Vekua et al., 2002;
Lordkipanidze et al., 2005;
Lordkipanidze et al., 2006;
Lordkipanidze et al., 2007;
Ferring et al., 2011
). The preservation, as well as the temporal and geographic
setting of at least 5 hominid individuals make Dmanisi a site that is
crucial in understanding the variation patterns, biogeography, and
evolution within emergent
Homo. The Dmanisi sample
is linked to the largely contemporaneous fossils from Africa that have
been attributed to
H. habilis and/or
H. erectus, by the
generalised (primitive) craniomandibular morphology, small cranial
capacity, moderate body size and a mosaic of primitive and derived
postcranial features. It is indicated there are affinities to
H. erectus in East Asia
by various features of the cranial vault and base (Lordkipanidze et
al., 2006; Lordkipanidze et
al., 2007; Rightmire,
Lordkipanidze & Vekua; 2006; Rightmire & Lordkipanidze, 2010). In this paper Lordkipanidze et
al. present a complete
cranium that expands the range substantially of variation that is
observed in the Dmanisi sample, with implications of earl history of the
genus
Homo. In 2005 the
specimen D4500 was recovered from layer B1y in Block 2 of the Dmanisi
excavation area (Supplementary materials available on Science Online).
The same individual as mandible D2600 is represented by D4500, which was
found 5 years earlier (Gabunia et
al., 2002). The association of D4500 and D2600 (“skull 5”)
constitutes the most complete adult skull that is known from
Homo in the Pleistocene
(“early
Homo”; the definition of
Pleistocene follows the new terminology). According to Lordkipanidze et
al. skull 5 is probably
associated with the postcranial material of an adult individual that has
body proportions that are almost human [(Lordkipanidze et
al., 2007); supplementary
text S1 and Fig. S1)]. It is now possible to document the morphology of
an entire cranium with its associated mandible and teeth and to study
anatomy that has not previously been known for early
Homo, as skull 5 is
intact and free from taphonomic deformation.
Cranium D4500 – description As the result of a small braincase with a large
prognathic face the morphology of skull 5 stands apart from any other
known specimen of fossil
Homo.
Braincase The endocrinal volume (ECV) that was evaluated
from computed tomography (CT) data is 546 ± 5 cm3. The
braincase, coupled with its small ECV the braincase is low, though
comparatively wide and basically elongate. The frontal squama is
moderately sloping. The parietals are bossed, exhibiting only a slight
midline keeling in their posterior half. The zygomatic arches are
massive, and spacious temporal fossae indicate voluminous masticatory
muscles. Temporal lines are well separated from each other (minimum from
cranial midplane 12 mm behind bregma: 20 mm).Prominent supramastoid
crests are exhibited by the temporal bone and porial saddles which give
the neurocranium a squat, bell-like shape when viewed from the rear. There is a marked protuberance on the occipital
bone immediately below its lambdoid margin, which extends across the
midplane by ~30 mm on either side. The prominence of this “lambdoid
bump,” together with a massive protruding inion, lends the upper
occipital plane an almost vertical orientation, whereas the nuchal plane
is inclined only moderately (21o), relative to the Frankfurt
horizontal (FH) plane. The occipital transverse torus in ruggedly built
and the nuchal region is deeply sculpted. A strong nuchal ligament is
suggested by a bilaminar link that links the inion with the opisthion.
The mastoid processes are large and steeply inclined medially. Their
inferior portion is compressed mediolaterally to form a distinctive
flange-like structure that extends posteriorly. Overall, cranial
superstructures of D4500 are massive and more prominent than those in
other Dmanisi individuals, which suggests it represents a male.
Temporal bones and cranial bones The right zygomatic arch exhibits in vivo
deformation which indicates a healed though displaced multiple fracture
behind the masseteric origin. The left temporomandibular joint (TMJ) as
well as the left mandibular condylar bar evidence for degenerative
arthritic deformation. The left TMJ and the right zygomatic arch are
unaffected by pathology. There is marked lateral extension of the TMJ
with 61% of the glenoid fossa beyond the external wall of the vault,
below the zygomatic shelf. The fossa itself is shallow, transversely
oriented (tympanomedian angle 3o), and has a low anterior
wall that merges smoothly with the preglenoid planum. The tympanic plate
is massive and exhibits a prominent vaginal process. Medially the
tympanic is expanded to form a robust supratubarius process. The petrous
pyramid closely approaches the wall of the basioccipital, thus reducing
the foramen lacerum to a crevice. The main axis of the pyramid forms an
angle of 34o (petromedian angle) with the midsaggital plane.
The foramen magnum is oval in shape and level (inclination 0o)
with respect to the FH. Basicranial flexion, as measured by the
orientation of the clival plane relative to the sphenoid plane, is low
(128o).
Face The face of skull 5 is among the largest and most
prognathic known from early
Homo, which contrasts
with the small brain. The glabellar region is massive and substantially
overhangs the mid-face. The orbits are overarched by thick bar-like
tori; a shallow post-torus shelf (rather than a sulcus) extends
posteriorly towards the frontal squama. Post orbital constriction is
marked. The zygomatic processes of the maxillae are massive and
laterally flaring. The anterior surface is vertically oriented and
coincides with the orbital plane, thus forming a zygomatic-orbital
plane. As a consequence of the tall and wide midface, the
zygomatic-alveolar crests are markedly angled. The roots of the
zygomatic processes originate between the first and second molars. The
nasal floor is delimited anteriorly by a smooth (rather than sharply
angled) sill. The inclination of the nasal-alveolar clivus relative to
the alveolar plane is 42o. The subnasal portion of the clivus
is slightly concave, whereas its alveolar portion is convex, covering
the large incisor roots. The palate is long and arched; the incisive
foramen is located at the position of the second premolar (P4),
indicating marked prognathism of the maxilla and anterior dentition.
Dentition The upper and lower dentition of skull 5is
heavily worn, and crown morphologies are only partially preserved.
Estimated molar dimensions are comparatively large, the maxillary as
well as mandibular molar sizes increase from M1 to M3. Judging from root
orientation, the maxillary incisors were strikingly procumbent. The open
bite (structural non-occlusion of the incisors), rounded labial wear
surfaces on upper incisors and rounded lingual wear surfaces on lower
incisors, respectively, and labiolinguallly oriented striations on these
surfaces suggest that the anterior dentition was used in paramasticatory
activities such as gripping (Lozano et
al., 2008). The first
maxillary premolars (P3) are situated posteriorly to Cs, such
as massive C/P3 juga square off the front of the maxilla.
Similarly massive C/P3 juga are present in the mandible D2600
(Gabunia et al., 2002). P3s
have 3 roots; P4s are double rooted; with the buccal root
exhibiting 2 canals (3 rooted P3s and double rooted P4s
have also been reported for other
Homo specimens. The P3s
of mandible D2600 have 2 roots (the distal roots and 3 canals (Walker &
Leakey, Eds., 1993). The P4s are not preserved.
Comparative analysis According to Lordkipanidze et
al., skull 5 was the first
complete specimen that provided evidence of how the face (including the
mandible) of adult early
Homo was oriented and
positioned relative to the braincase. Prior to the discovery of skull 5
completely craniomandibular morphology of early
Homo was 2 adolescents,
whose facial skeletons were not fully developed (KNM-WT 15000 and
D2700/D2735) (Vekua et al.,
2002; Walker & Leakey, Eds., 1993), and the Dmanisi individual that was
senile D3444/D3900, the gnathic morphology of which was modified
strongly by alveolar bone resorption after tooth loss (Lordkipanidze et
al., 2005; Lordkipanidze et
al., 2005). Anatomical
connections between preserved neurocranial and maxillofacial parts among
adult early
Homo specimens from
Africa and East Asia are typically incomplete and facial to neurocranial
orientation have remained a matter of debate (Bromage et
al., 2008; Leakey et
al., 2012). Therefore the
current perception of skull morphology of early
Homo is influenced
strongly by adolescent representatives (and the less complete and
taphonomically distorted adult cranium KNM-ER 1813) that exhibit
comparatively orthognathic faces with superstructures that are lightly
built. It is now indicated by skull 5 that small-brain large-faced,
remarkably prognathic and robust morphologies are all within the normal
range of early
Homo. The smallest of the
Dmanisi sample (skulls 1 to 4: 601 to 730 cm3) is represented
by skull 5 (546 cm3), and it is at the lower end of the
variation of the
H. habilis hypodigm [509
to 687 cm3 (Holloway et
al., 2004; Rightmire, 2004)].
This individual is placed within the range of variation that has been
estimated for early
Homo and at the lower end
of variation of African
H. erectus and modern
humans (Holliday, 2012), by the stature and body mass estimates that
were obtained from the postcranial elements that are probably associated
with skull 5 [146 to 166 cm, 47 to 50 Kg (Lordkipanidze et
al., 2007). The skull 5
individual has provided the first evidence that early
Homo comprised adult
individuals with small brains, though with body mass, stature and limb
proportions reaching the lower range limit of modern human variation
(Lordkipanidze et al., 2011;
Holliday, 2012). The encephalisation quotient (EQ) of the skull 5
individual was estimated as ~2.4, which is within the range of variation
for
Australopithecus
(Holloway, Broadfield & Yuan, 2004. Lordkipanidze et
al. suggest the larger ECV of
early
Homo as compared to
Australopithecus africanus
[340 to 515 cc (Holloway, Broadfield & Yuan, 2004)] and
Au. Sediba [420 cc
(Holliday, 2012)] might therefore reflect primarily an evolutionary
increase in body size rather than increased encephalisation. Skull 5 has allowed the scope of comparisons to
be widened, though previously similarities between Dmanisi and eastern
African fossils that were attributed to
H. ergaster and
H. erectus have been
documented (Lordkipanidze et al.,
2006; Rightmire, Lordkipanidze & Vekua, 2006; Rightmire & Lordkipanidze,
2010; Gabunia et al., 2002;
Baab, 2008). Skull 5 exhibits midfacial architecture which has
morphological affinities with contemporaneous African specimens SK 847
and OH 65 (Blumenschine et al.,
2003), as well as with early representatives of
Homo in eastern Africa
such as KNM-ER 1470 and KNM-ER 62000 that dates to ~ 2.0 to 1.9 Ma
(Leakey et al., 2012). The
naso-alveolar is moderately sloping and laterally wide curving around
the buccal roots of P3s, in all these specimens. The Chemeron temporal fragment, which dated to
2.4 Ma, that was suggested to be the earliest fossil evidence of
Homo (Hill et
al., 1992; Sherwood, Ward &
Hill, 2002), though this assignment has remained inconclusive given the
wide variation in TMJ position and morphology in the Dmanisi and early
Homo samples. The maxilla
of skull 5 is, however, similar in overall form and dento-alveolar
features to the well-preserved early
Homo maxilla A.L. 666-1
from Hadar, Ethiopia, with a geological age of 2.3 Ma (Kimbel, Johnson &
Rak, 1997). Recently, the generic and chronological status of A.L. 666-1
has been questioned, and it has been proposed that
A. Sediba (1.977 Ma) is
instead, a potential ancestor of the
Homo lineage (Pickering
et al., 2011). Such a
scenario has, however, been falsified by the close morphological
affinities between A.L. 666-1, additional early
Homo specimens from
eastern Africa such as OH 65 and KNM-ER 1470, and now skull 5. It has now been established by the enlarged
sample of Dmanisi variation is crucial to identifying patterns of
variation among and between palaeospecies of early
Homo. Evidence has been
provided by several studies for variation along and across a single
evolving lineage of early
Homo (Asfaw et
al., 2002; Suwa et
al., 2007), with differences
between fossil specimens reflecting, primarily, intraspecific variation
at a population level and evolutionary change within a species over
time. However, other studies interpret variation in small samples of
early
Homo from Africa as
reflecting taxonomic species diversity (Leakey et
al., 2012; Wood, 1991; Wood,
1992; Spoor et al., 2007) and
posit 3 or more largely contemporary palaeospecies (H.
rudolfensis, H. habilis &
H. erectus). According to
this interpretation differences among individual fossil specimens
indicate that there are interspecific differences and concomitant
phyletic diversification. Information Variation within palaeodemes is
depended upon for testing these contrasting hypotheses i.e., within
populations of fossil species at a given point in space
and time (Howell, 1999). It is likely that a fossil sample
represents a palaeodeme if 2 conditions are met. It must come from a
stratigraphic and taphonomic setting [which is the case for Dmanisi
(Lordkipanidze, et al., 2006;
Lordkipanidze et al., 2007]
that is spatially and temporally constrained, and it must be
within-sample variation that is similar in range and mode to be
within-deme variation in extant species that are closely related. It is shown by geometric morphometric analysis
and resampling statistics that shape of craniomandibular variation among
hominids from Dmanisi is congruent with patterns and ranges of variation
in chimpanzee and bonobo demes (Pan
troglodytes,
P. t. verus, P. t. schweinfurthii, and
Pan paniscus and in a
global sample of
H. sapiens. Variation in
cranial shape in all groups is mainly due to interindividual differences
in size and orientation of the face relative to the braincase. The
Dmanisi sample, which includes skull 5, therefore represents normal
within-deme variation, which ranges from small-faced relatively
orthognathic (typically female and/or subadult) individuals, to
large-faced relatively prognathic individuals that are typically male. It is also indicated by nonmetric cranial
characters that there is substantial within-deme variation, and wider
than typically recognised. Variation in maxillofacial features is shown
the Dmanisi palaeodemes, which have been interpreted as evidence for
taxonomic diversity in fossils from eastern Africa (Leakey et
al., 2012; Spoor et
al., 2007). The best current available example of cranial
form and form variation within the early
Homo lineage is
represented by the Dmanisi sample. It can tentatively be characterised
as follows:
·
Face with flaring, vertically oriented zygomatic processes that are
coplanar with the orbital plane;
·
Similar upper and midface widths;
·
Markedly angled zymaticoalveolar crests;
·
Marked alveolar prognathism;
·
C roots that exhibit parallel orientation relative to the cranial
midplane;
·
naso-alveolar clivus sloping moderately and delimited laterally by C
and/or P3 juga and;
·
ECV greater than ~500 cc.
According to Lordkipanidze
et al. these features from
early
Homo from nonrobust
species of
Australopithecus;
·
which exhibit a more protruding though less sloping naso-alveolar
clivus;
·
inclined maxillary zygomatic processes;
·
moderately curved zygomaticoalveolar crests and;
·
superomedially tapering canine juga, which delimit laterally the
anterior surface of the maxilla.
In early
Homo other features, such
as 3-rooted premolars, marked postorbital narrowing, a shallow glenoid
fossa, and a moderate degree of Basicranial flexion, represent
pleisomorphic (primitive character states that are shared variably with
the
Australopithecus species.
Discussion
Recognising diversity of species, as has been noted, comes at the
expense of “admitting much variation” within species [(Darwin, 1869),
p.51). Dmanisi adds to the growing evidence, together with data from
Au. Afarensis,
palaeodemes, such as A.L. 333 (White & Johnson, 1989;
Reno
et al., 2003; Behrensmeyer,
2008),
that variation in intrademic and intraspecific in the Plio-Pleistocene
fossil hominids tends to be misinterpreted as species diversity,
especially in the case of single fossil specimens from different
localities are compared with each other (Gilbert, White & Asfaw, 2003).
It is indicated by skull 5 and the other 4 Dmanisi specimens that
cranial shape variation within early
Homo populations was
similar in mode to that seen in modern
Pan demes. Also, it is
indicated by Dmanisi that a proportion of character state variation in
nonmetric features is a reflection of intrademic variation rather than
real diversity of species. There are several implications of these
findings for the interpretations of morphological diversity in the
fossil record of early
Home. Morphological
diversity, when seen from the perspective of
Dmanisi, in the African
fossil
Homo record about 1.8 Ma
is probably a reflection of variation between demes of a single evolving
lineage, which is appropriately named
H. erectus. It appears
that the hypothesis of multiple independent lineages (palaeospecies)
(Leakey
et al., 2012; Spoor et
al., 2007)
is less parsimonious, especially when there is no empirical evidence for
adaptations to separate ecological niches. A classificatory, though not
an evolutionary dilemma is raised by the hypothesis of phyletic
evolution within a single polymorphic lineage, and it is premature to
describe the rate(s) of evolution in this lineage, given the small
sample that is available. Specimens that were previously attributed to
H. ergaster are therefore
sensibly classified as a chronosubspecies of
H. erectus. Lordkipanidze
et al. suggest it is probable
that the
Dmanisi population from
the Early Pleistocene of
H. erectus lineages from
Africa, so it is sensibly placed within
H. e. ergaster and
formally designated as
H. e. e. georgicus in
order to denote the geographical location of this deme [therefore
retracting the species status given earlier to mandible D2600 (Gabounia
et al., 2002 [Gabunia?]).
Questions of early fossil
Homo phylogenetics and
classification remain to be resolved, given the scattered and
fragmentary fossil record in Africa that predates
Dmanisi. It has yet to be
tested whether all of the fossils that are currently allocated to the
taxa
H. habilis and
H. rudolfensis are
actually part of the single evolving lineage of
Homo. There are
alternative scenarios, though Lordkipanidze et
al. regard this null
hypothesis to be parsimonious and fully compatible with new evidence
from
Dmanisi. At present there
is no convincing signature of early
Homo cladogenesis, given
the range of variation that is observed in the
Dmanisi palaeodeme.
Increase in brain size and correlated changes in the craniofacial
morphology within the evolving lineage of
H. erectus are exhibited
by the fossils from Africa that postdate the
Dmanisi ensemble. It is,
moreover, likely that the underpinning of the East African dispersal of
H. erectus, as well as
the roots of the subsequent evolution of
H. erectus in Africa
(e.g., OH 9, Daka), shared greater craniofacial robusticity.
The strong African affinities that were recognised previously for this
early Eurasian outpost of the
Homo genus is reinforced
by the new evidence from the ancient
Dmanisi deme of early
Homo (Rightmire,
Lordkipanidze & Vekua, 2006). Variation between continents may therefore
provide insight into the evolutionary population dynamics of early
Homo. That genetic and
phenetic variation decreases with geographic distance from Africa is
well known from the dispersal in the Upper Pleistocene of modern humans
from Africa due to serial founder effects as well as population
bottlenecks. Modern human phenetic variation in western and eastern Asia
is 85 and 95% of African variation, respectively, e.g. (Manica
et al., 2007).
Direct comparisons must remain tentative as the expansion of early
Homo from Africa might
have occurred at longer time scales. The observation that Dmanisi
conserves a substantial proportion of cranial shape and its variation
among early
Homo from Africa,
nevertheless, speaks for genetic continuity. The effective size of early
Homo in western Asia
might have been similar [Ne
>10,000 (Marques-Bonet, Ryder & Eichler, 2009)], as intrademic variation
in
Dmanisi is similar to
that found in extant and extinct near relatives. Also, the early
H. erectus from Java
(Trinil/Sangiran) (Kaifu
et al., 2005; Kaifu et
al., 2010),
the dentognathic remains of which are remarkably large and robust,
exhibit close affinities with skull 5. Evidence is provided by this for
morphological (and presumably underpinning genetic) continuity across
large geographic distances, and for preservation in East Asia of an
appreciable proportion of the variation that was originally present in
palaeodemes of early
Homo in Africa.
In contrast with continuity of early
Homo across continents,
current palaeontological data indicate a low degree of similarity
between contemporaneous mammalian genera in Africa and at Dmanisi
(Lordkipanidze
et al., 2007),
which implies a generally low rate of faunal exchange between Africa and
Asia before about 1.8 Ma (Agustí & Lordkipanidze, 2011). The
populational, ecological and evolutionary dynamics of early
Homo probably differed
significantly from those of coeval large mammals, which included other
hominid lineages, as has been suggested earlier (Antón & Swisher, 2004).
It is indicated by theoretical considerations that the long-range rate
of dispersal of a population depends mainly on reproduction rates and
habitat expansion (Hastings, 1996) and that intragroup cooperation can
have an important role in population persistence (Hamilton
et al., 2009).
Various features such as:
·
Derived lower limb and foot morphologies (Lordkipanidze et
al., 2007; Pontzer et
al., 2010),
·
Tool mediated widening of the dietary niche towards meat eating
(Lordkipanidze et al., 2006;
Ferring et al., 2011) and,
·
increased levels of intragroup cooperation (Lordkipanidze et
al., 2005),
might have led to increased levels of
reproduction, survival, and mobility in early
Homo and consequently the
establishment of stable populations outside Africa. It is now indicated
by skull 5 and other members of the Dmanisi palaeodeme that early
Homo expanded from Africa
to ultimately establish substantial populations in western Asia. This
earliest hominid dispersal predated any significant increase in brain
size. In order to test and modify the attendant hypotheses further
analyses will be required. Ultimately, the identification of hominid
palaeodemes and assessing within-deme variation (Howell, 1999) will be
key to understanding mechanisms of evolution and geographic dispersal of
early
Homo.
Lordkipanidze, D., et al. (2013). "A Complete
Skull from Dmanisi, Georgia, and the Evolutionary Biology of Early <em>Homo</em>."
Science 342(6156): 326-331.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |