Yoel Rak

A Complete Skull from Dmanisi, Georgia, and the Evolutionary Biology of Early Homo

A Heady Find In the past two decades, excavations at the archaeological site at Dmanisi, Georgia, have revealed hominin fossils from the earliest Pleistocene, soon after the genus Homo first dispersed beyond Africa. Lordkipanidze et al. (p. 326; see the cover) now describe a fossil cranium from the site. Combined with mandibular remains that had been found earlier, this find completes the first entire hominin skull from this period. An early Pleistocene adult skull illuminates the evolution and morphology of the first hominins outside Africa. The site of Dmanisi, Georgia, has yielded an impressive sample of hominid cranial and postcranial remains, documenting the presence of Homo outside Africa around 1.8 million years ago. Here we report on a new cranium from Dmanisi (D4500) that, together with its mandible (D2600), represents the worlds first completely preserved adult hominid skull from the early Pleistocene. D4500/D2600 combines a small braincase (546 cubic centimeters) with a large prognathic face and exhibits close morphological affinities with the earliest known Homo fossils from Africa. The Dmanisi sample, which now comprises five crania, provides direct evidence for wide morphological variation within and among early Homo paleodemes. This implies the existence of a single evolving lineage of early Homo, with phylogeographic continuity across continents.

The Neanderthal: A new look at an old face

It is suggested that the unique facial topography of the classic Neanderthal be viewed as stemming from a change of much of the infraorbital region from the coronal orientation of the generalized face to a more sagittal orientation. This reorganization renders the infraorbital facial plates more efficient in opposing the rotation of the snout in the sagittal plane, which in turn is generated by the unusual use of the anterior teeth. The triangular shape that the “sagittally” oriented infraorbital plate assumes is considered the result of its greater efficiency in opposing the rotation of the anterior part of the face. Both the shift from the coronal into the sagittal orientation and the reshaping of the bone plate into a triangle with a specific anteroposterior length to achieve optimal proportions act to produce the unusual mid-facial prognathism in the Neanderthal skull.

The Importance of Species Taxa in Paleoanthropology and an Argument for the Phylogenetic Concept of the Species Category

Paleoanthropology attempts to describe the diversity of extinct primate forms, to interpret this diversity in a phylogenetic framework based on the distribution of shared evolutionary novelties, and to explain the emergence and transformation of novelties in terms of a positive causal relationship between changes in structure/function and enhanced organismal fitness (i.e., adaptation). These components may be viewed as a sequence of steps toward a “complete” explanation of evolutionary change, each step logically contingent on those preceding it. Thus, hypotheses seeking to explain the adaptive basis of evolutionary morphological change necessarily depend on the prior acceptance of a hypothesis of vectored phylogenetic change. In turn, a phylogenetic hypothesis must be grounded in some theoretical concept of the units of diversity, among which the pattern of phylogenetic relationships is sought.

Does Any Mousterian Cave Present Evidence of Two Hominid Species

“While the skull is the creation of God, the jaw is the work of the devil.”* Be the source of this quote as it may, it is undoubtedly expressive of the frustration that researchers feel when, as so often happens, they examine a mandible that is disarticulated from the skull. Phrased in scientific terms, this quotation simply states that the diabolic mandible bears fewer taxonomic characters—is less diagnostic—than the cranium. Consequently, it lends itself less readily to taxonomic evaluation, with the result that the ensuing taxonomic decision is less secure. Two mandibles were found in Tabun Cave, both reported as originating in layer C, as described in the field report.

Morphological Variation in Homo neanderthalensis and Homo sapiens in the Levant

It was only the geographic proximity of the Mount Carmel specimens that spared them from being assigned to many different taxa as “separate form[s] of humanity.” Were it not for these circumstances, McCown and Keith (1939) would not have conceived of accommodating such a great range of variation in one taxon. However, a clear morphological dichotomy between the hominids from Skhul and those from Tabun (C-l, the female skeleton, and C-2, the isolated mandible) emerges from their monograph. The existence of two kinds of hominids in a relatively small geographic area of the Middle East has since been confirmed through discoveries at several additional sites. Other hominids have been found, including specimens from Amud (Suzuki and Takai, 1970) and Shanidar (Trinkaus, 1983), which can be grouped comfortably with the Tabun specimens, whereas specimens uncovered at Qafzeh (Vandermeersch, 1981) can be added to the Skhul group. Until quite recently, McCown and Keith’s basic contention—that the Neanderthal-looking Tabun group represented the earlier, primitive anatomy, and the modern-looking Skhul group represented the later, derived anatomy—was generally accepted.

Gorilla-like anatomy on Australopithecus afarensis mandibles suggests Au. afarensis link to robust australopiths

Mandibular ramus morphology on a recently discovered specimen of Australopithecus afarensis closely matches that of gorillas. This finding was unexpected given that chimpanzees are the closest living relatives of humans. Because modern humans, chimpanzees, orangutans, and many other primates share a ramal morphology that differs from that of gorillas, the gorilla anatomy must represent a unique condition, and its appearance in fossil hominins must represent an independently derived morphology. This particular morphology appears also in Australopithecus robustus. The presence of the morphology in both the latter and Au. afarensis and its absence in modern humans cast doubt on the role of Au. afarensis as a modern human ancestor. The ramal anatomy of the earlier Ardipithecus ramidus is virtually that of a chimpanzee, corroborating the proposed phylogenetic scenario.

The cranial base of Australopithecus afarensis: new insights from the female skull

Cranial base morphology differs among hominoids in ways that are usually attributed to some combination of an enlarged brain, retracted face and upright locomotion in humans. The human foramen magnum is anteriorly inclined and, with the occipital condyles, is forwardly located on a broad, short and flexed basicranium; the petrous elements are coronally rotated; the glenoid region is topographically complex; the nuchal lines are low; and the nuchal plane is horizontal. Australopithecus afarensis (3.7–3.0 Ma) is the earliest known species of the australopith grade in which the adult cranial base can be assessed comprehensively. This region of the adult skull was known from fragments in the 1970s, but renewed fieldwork beginning in the 1990s at the Hadar site, Ethiopia (3.4–3.0 Ma), recovered two nearly complete crania and major portions of a third, each associated with a mandible. These new specimens confirm that in small-brained, bipedal Australopithecus the foramen magnum and occipital condyles were anteriorly sited, as in humans, but without the foramens forward inclination. In the large male A.L. 444-2 this is associated with a short basal axis, a bilateral expansion of the base, and an inferiorly rotated, flexed occipital squama—all derived characters shared by later australopiths and humans. However, in A.L. 822-1 (a female) a more primitive morphology is present: although the foramen and condyles reside anteriorly on a short base, the nuchal lines are very high, the nuchal plane is very steep, and the base is as relatively narrow centrally. A.L. 822-1 illuminates fragmentary specimens in the 1970s Hadar collection that hint at aspects of this primitive suite, suggesting that it is a common pattern in the A. afarensis hypodigm. We explore the implications of these specimens for sexual dimorphism and evolutionary scenarios of functional integration in the hominin cranial base.

Ardipithecus ramidus and the evolution of the human cranial base.

Significance The Pliocene (4.4 Ma) hominoid species Ardipithecus ramidus has been linked phylogenetically to the Australopithecus + Homo clade by nonhoning canines, a short basicranium, and postcranial features related to bipedality. However, aspects of the foot and pelvis indicative of arboreal locomotion have raised arguments that this taxon may instead exemplify parallel evolution of human-like traits among apes around the time of the chimpanzee-human split. Our investigation of the basicranium shows that Ar. ramidus shares with Australopithecus and Homo a relatively short, broad central cranial base and related modifications of the tympanic, petrous, and basioccipital elements. These similarities support the proposed relationship of Ar. ramidus to Australopithecus + Homo. Reorganization of the central basicranium is among the earliest morphological attributes of this group. The early Pliocene African hominoid Ardipithecus ramidus was diagnosed as a having a unique phylogenetic relationship with the Australopithecus + Homo clade based on nonhoning canine teeth, a foreshortened cranial base, and postcranial characters related to facultative bipedality. However, pedal and pelvic traits indicating substantial arboreality have raised arguments that this taxon may instead be an example of parallel evolution of human-like traits among apes around the time of the chimpanzee–human split. Here we investigated the basicranial morphology of Ar. ramidus for additional clues to its phylogenetic position with reference to African apes, humans, and Australopithecus. Besides a relatively anterior foramen magnum, humans differ from apes in the lateral shift of the carotid foramina, mediolateral abbreviation of the lateral tympanic, and a shortened, trapezoidal basioccipital element. These traits reflect a relative broadening of the central basicranium, a derived condition associated with changes in tympanic shape and the extent of its contact with the petrous. Ar. ramidus shares with Australopithecus each of these human-like modifications. We used the preserved morphology of ARA-VP 1/500 to estimate the missing basicranial length, drawing on consistent proportional relationships in apes and humans. Ar. ramidus is confirmed to have a relatively short basicranium, as in Australopithecus and Homo. Reorganization of the central cranial base is among the earliest morphological markers of the Ardipithecus + Australopithecus + Homo clade.

Suggested guidelines for invasive sampling of hominid remains

The last few years have witnessed remarkable technical developments in paleoanthropology. On the one hand, accurate imaging techniques have limited the need to access actual specimens. On the other hand, direct dating, isotopic studies, and the study of ancient DNA, proteins, and microstructures have experienced great technical improvements but still require a degree of invasive sampling. The power of these invasive approaches for answering important questions in evolutionary anthropology brings forward the question of how to balance preservation of fossil hominid remains for the future against the application of current scientific analyses. In order to address these issues, a workshop was hosted by the Max-Planck Institute for Evolutionary Anthropology in Leipzig on April 26– 27, 2007 where the issues that emanate from the need for sampling of hominid remains versus the need for preservation of specimens for the future were discussed. At the end of the meeting, the participants produced a set of recommendations that might be useful to museums and other institutions as well as scientists that have to make decisions on requests for invasive sampling of hominid remains.

Opposing Extremes of Zygomatic Bone Morphology: Australopithecus Boisei versus Homo Neanderthalensis

The lateral margin of the zygomatic bone of Australopithecus boisei flares both anteriorly and laterally. As a result, the bone loses the suspensory bracing of the facial frame and is transformed into a visor‐like structure that supports itself and gains its rigidity from its shape. The coronally oriented bony plates and the outline of the facial mask help the A. boisei face resist the effect of the visor‐like structure, which tends to pull the bone plates of the face away from the midline. On the other hand, the nearly sagittal orientation of the zygomatic bone in Homo neanderthalensis helps the face resist torque and bending forces, which themselves stem from the positioning of the bite point on the anterior teeth. Although the zygomatic bones of these two taxa are highly specialized, they differ fundamentally from each other. Anat Rec, 300:152–159, 2017.