How did modern morphology evolve in the human mandible? The relationship between static adult allometry and mandibular variability in Homo sapiens

Key to understanding human origins are early Homo sapiens fossils from Jebel Irhoud, as well as from the early Late Pleistocene sites Tabun, Border Cave, Klasies River Mouth, Skhul, and Qafzeh. While their upper facial shape falls within the recent human range of variation, their mandibles display a mosaic morphology. Here we quantify how mandibular shape covaries with mandible size and how static allometry differs between Neanderthals, early H. sapiens, and modern humans from the Upper Paleolithic/Later Stone Age and Holocene (= later H. sapiens). We use 3D (semi)landmark geometric morphometric methods to visualize allometric trends and to explore how gracilization affects the expression of diagnostic shape features. Early H. sapiens were highly variable in mandible size, exhibiting a unique allometric trajectory that explains aspects of their 'archaic' appearance. At the same time, early H. sapiens share a suite of diagnostic features with later H. sapiens that are not related to mandibular sizes, such as an incipient chin and an anteroposteriorly decreasing corpus height. The mandibular morphology, often referred to as 'modern', can partly be explained by gracilization owing to size reduction. Despite distinct static allometric shape changes in each group studied, bicondylar and bigonial breadth represent important structural constraints for the expression of shape features in most Middle to Late Pleistocene hominin mandibles.

The revolution that wasn't: a new interpretation of the origin of modern human behavior

Proponents of the model known as the "human revolution" claim that modern human behaviors arose suddenly, and nearly simultaneously, throughout the Old World ca. 40-50 ka. This fundamental behavioral shift is purported to signal a cognitive advance, a possible reorganization of the brain, and the origin of language. Because the earliest modern human fossils, Homo sapiens sensu stricto, are found in Africa and the adjacent region of the Levant at >100 ka, the "human revolution" model creates a time lag between the appearance of anatomical modernity and perceived behavioral modernity, and creates the impression that the earliest modern Africans were behaviorally primitive. This view of events stems from a profound Eurocentric bias and a failure to appreciate the depth and breadth of the African archaeological record. In fact, many of the components of the "human revolution" claimed to appear at 40-50 ka are found in the African Middle Stone Age tens of thousands of years earlier. These features include blade and microlithic technology, bone tools, increased geographic range, specialized hunting, the use of aquatic resources, long distance trade, systematic processing and use of pigment, and art and decoration. These items do not occur suddenly together as predicted by the "human revolution" model, but at sites that are widely separated in space and time. This suggests a gradual assembling of the package of modern human behaviors in Africa, and its later export to other regions of the Old World. The African Middle and early Late Pleistocene hominid fossil record is fairly continuous and in it can be recognized a number of probably distinct species that provide plausible ancestors for H. sapiens. The appearance of Middle Stone Age technology and the first signs of modern behavior coincide with the appearance of fossils that have been attributed to H. helmei, suggesting the behavior of H. helmei is distinct from that of earlier hominid species and quite similar to that of modern people. If on anatomical and behavioral grounds H. helmei is sunk into H. sapiens, the origin of our species is linked with the appearance of Middle Stone Age technology at 250-300 ka.

Middle Stone Age human fossils from Die Kelders Cave 1, Western Cape Province, South Africa

Die Kelders Cave 1 (DK1) preserves a thick series of Middle Stone Age (MSA) horizons that date to a fairly short temporal interval sometime between about 60 and 80 ka ago. Twenty-seven human fossils, comprising 24 isolated teeth, a mandibular fragment, and two manual middle phalanges derive from seven of the 12 layers. The vast majority are children, and all may have come from sub-adult individuals. The entire assemblage may represent a minimum of ten individuals. As might be expected for teeth of such antiquity, most of the DK1 crowns tend to be large in comparison to recent African homologues. They tend to be smaller than, albeit more similar in size to, the teeth of penecontemporaneous archaic populations from Eurasia. The majority of morphological variants displayed by the DK1 crowns characterize the teeth of recent sub-Saharan Africans, and the DK1 crowns resemble those of recent Africans in a number of traits that have been used to define a sub-Saharan African regional complex. The morphological similarities between the DK1 MSA and recent African teeth, however, do not necessarily signify a close evolutionary relationship between them, because these crowns variants appear to be plesiomorphic.

Comparison of craniofacial features of major human groups

Distance analysis and factor analysis, based on Q-mode correlation coefficients, were applied to 23 craniofacial measurements in 1,802 recent and prehistoric crania from major geographical areas of the Old World. The major findings are as follows: 1) Australians show closer similarities to African populations than to Melanesians. 2) Recent Europeans align with East Asians, and early West Asians resemble Africans. 3) The Asian population complex with regional difference between northern and southern members is manifest. 4) Clinal variations of craniofacial features can be detected in the Afro-European region on the one hand, and Australasian and East Asian region on the other hand. 5) The craniofacial variations of major geographical groups are not necessarily consistent with their geographical distribution pattern. This may be a sign that the evolutionary divergence in craniofacial shape among recent populations of different geographical areas is of a highly limited degree. Taking all of these into account, a single origin for anatomically modern humans is the most parsimonious interpretation of the craniofacial variations presented in this study.

Metrical reconsideration of the Skhul IV and IX and Border Cave 1 crania in the context of modern human origins

The "out-of-Africa" models for origins of modern Homo sapiens incorporate Skhul as one site documenting that early origination. However, only Skhul V is usually considered in the comparative craniology of the question, neglecting the other substantial crania, Skhul IV and IX. Craniometric comparison demonstrates that IV and IX amplify the picture of continuous gradations of Neandertal-to-modern variations throughout the Levant; much variation is thus represented within this one site, raising serious questions about Neandertals and moderns being discrete and long-separated species. Qafzeh 6 too is craniophenetically closer to Neandertals than to the true anatomically modern people of the European Upper Paleolithic. Proper distance analysis of Border Cave 1 cranium shows it is actually far removed from modern African populations. References to Qafzeh, Skhul, and Border Cave as "fully anatomically modern" require reconsideration.

ESR dating evidence for early modern humans at Border Cave in South Africa

The archaeological and hominid site of Border Cave (KwaZulu, South Africa) has a stratigraphic sequence covering the Middle and Later Stone Ages (MSA and LSA). It has been proposed that four hominid specimens recovered there (BC1 and BC2 of uncertain provenance, and BC3 and BC5 recovered from MSA layers) represent very early examples of anatomically modern humans, supporting an early late-Pleistocene appearance of modern Homo sapiens in Africa. This early appearance, however, has been questioned, largely because of doubts about the stratigraphic positions associated with the specimens and because of the lack of a reliable chronology for the stratigraphic sequence. We now report on the first comprehensive radiometric dating analysis of Border Cave, using electron spin resonance (ESR) on teeth within sediment layers. BC3 is likely to be approximately 70-80 kyr old, and BC5, 50-65 kyr old. BC1 and BC2 are almost certainly less than 90 kyr old. These results, although younger than some age estimates, support the early occurrence of anatomically modern humans at Border Cave. In addition, our results suggest that the Howiesons Poort lithic industry (approximately 45-75 kyr) and the MSA-LSA transition (approximately 35 kyr) are younger than often believed.

Hominid molars from a Middle Stone Age level at the Mumba Rock Shelter, Tanzania

Three hominid molars were recovered from a depth of 7.0-7.1 meters in the Mumba Shelter at Lake Eyasi, northern Tanzania. Geological context of the finds and archaeological data indicate that people with a Middle Stone Age technology were using the Mumba locality intermittently whenever retreat of lake waters allowed access to the site. Uranium series dates suggest an age on the order of 130,000 years bp for the teeth and stone tools. Based on morphological analyses, the dental remains probably belonged to one individual and appear to be the crowns of two upper permanent M2s and one lower permanent M2. Crown areas are very small, even in comparison to the variation exhibited by recent African populations. Crown patterns have no archaic features. These teeth are smaller than any verifiable archaic Homo sapiens examples; thus, they may represent early anatomically modern Homo sapiens.

Paleoclimatic setting for Homo sapiens neanderthalensis

A paleoclimatic hypothesis is presented to account for the evolution and eventual replacement of Homo sapiens neanderthalensis. Neandertal populations in the European Late Pleistocene were largely isolated by geographic barriers. Populations of modern Homo sapiens replaced Neandertals at 34000 years ago, near the end of the relatively cold oxygen isotope stage 3. These population were pushed into Europe by conditions brought on by increasing aridity affecting North Africa and southwestern Asia, and their dispersal was facilitated by lowered sea level.