Masticatory-stress hypotheses and the supraorbital region of primates

The purpose of this study is to test various masticatory-stress hypotheses about the evolution and function of well-developed browridges of higher primates. This was done by measuring and analyzing patterns of in vivo bone strain recorded from three-element rosette strain gages bonded to the supraorbital region and to other portions of the bony face of Macaca fascicularis and Papio anubis during mastication and incision. The magnitude and direction of the principal strains recorded support Endo's hypothesis that the supraorbital region during mastication and incision is bent in the frontal plane (Endo, 1966). Our data do not, however, support his hypothesis that the supraorbital region is bent more during incision than during mastication. The data also demonstrate that overall levels of supraorbital strain are not larger in more prognathic subjects. Most importantly, the data indicate that the supraorbital region of nonhuman catarrhines is strained very little during mastication and incision. This indicates that there is much more supraorbital bone than is necessary both to counter masticatory loads and to provide an adequate safety factor to failure for these loads. This in turn suggests that the macaque and baboon browridges can be considerably reduced in size and still maintain these required structural characteristics. Thus, our experiments provide no support whatsoever for those hypotheses that directly link browridge morphology to masticatory stress (cf. Endo, 1966; Russell, 1983, 1985). A recent review of Endo's original work indicates that this latter statement is also true for humans (Picq and Hylander, 1989). We conclude, therefore, that there is no good reason to believe that enlarged browridges in living and/or fossil primates are structural adaptations to counter intense masticatory forces. The evolution of browridge morphology in primates is best explained on the basis of factors related to the position of the brain relative to the orbits (Moss and Young, 1960). When these structures are widely separated, as in gorillas, the large intervening space must be bridged with bone. In addition, enough bone must be present within the supraorbital and bridged regions to prevent structural failure due to non-masticatory external forces associated with highly active primates (e.g., accidental traumatic forces applied to the orbits and neurocranium). This requirement results in both pronounced browridges and in much more supraorbital bone than is necessary to counter routine cyclical stress during mastication and incision. This in turn explains why bone strains recorded from the supraorbital region are extremely small relative to other portions of the primate face during mastication and incision.     

Ontogenetic study of the supraorbital region in modern humans: a longitudinal test of the spatial model

The structural significance of the hominid supraorbital torus and its morphological variation have always been a controversial topic in physical anthropology. Understanding the function of browridge variation in living and fossil human populations is relevant to questions of human evolution. This study utilizes radiograph images to evaluate the spatial model in modern humans during ontogeny. This structural model attributes variation in the supraorbital region to the positional relationship between the neurocranium and the orbits. The relationship between measurements of the antero-posterior supraorbital length and the factors specified in the spatial model were assessed by correlation and partial correlation analyses. Growth rates were also examined to study ontogenetic trajectories and infer aspects of developmental relationships between critical variables. Results agree with previous research supporting the existence of spatial influences between the neural and orbital-upper facial regions on browridge length during ontogeny.     

Dangers in testing statistical hypotheses

Most experiments are intended for the estimation of the size of effects rather than for the testing of a hypothesis of whether or not an effect occurs. Hypothesis testing is often inapplicable, is over-used and is likely to lead to misinterpretations of results. The two types of error possible in hypothesis testing are discussed. Whereas Type I error is usually examined as a matter of course, Type II error is almost always ignored. Investigations in which zero differences are important should recognise the possibility of Type II error in their interpretation. A nonsignificant result should not be interpreted as evidence of a lack of effect. Statistical significance is not synonymous with economic or scientific importance. The importance of choosing the most appropriate design is emphasised and some suggestions are made as to how important sources of error can be avoided.     

Homeostatic non-shivering thermogenesis in humans facts and hypotheses

This review considers current research of different forms of non-shivering thermogenesis related to thermoregulatory and substrate homeostasis. The term “homeostatic non-shivering thermogenesis (HNST)” is proposed for explanation of facultative heat production stimulated by exposure to cold, food intake and accumulation of lactate during intensive muscle loading. Similarities and differences in physiological activity are displayed in three HNST types. Existence of a number of common points makes it possible to propose common physiological mechanisms of HNST realization. Among other candidates for HNST location, the brown adipose tissue (BAT) fits best as its function is specifi between thermogenic function in cold environment and diet-induced thermogenesis that makes it possible to link these two HNST types with BAT activity. Here we present the data indirectly confirming BAT functioning in processes of homeostatic normalization not related to cold acclimation or food intake. We also consider new data about BAT functional activity, its topographic body location, mechanisms of uncoupled respiration in different tissues in adult humans and about methods of BAT diagnostics which include the use of molecular markers. We list a number of facts confirming our suggestion about BAT activity being related to homeostatic normalization after physical loading. In conclusion, we propose an experimental research program for the testing of our hypothesis regarding BAT universal homeostatic function in humans.     

Problems with the statistical testing of panbiogeographic hypotheses

Abstract

Croizat regarded generalised tracks as having a statistical basis, their degree of justification being directly related to the number of individual tracks consistent with them. In order to be logically valid, however, such an approach needs to have an explicit statistical basis. Page (1987) attempts to provide this, proposing the following protocol. Tracks are treated as geographic minimum-spanning trees which in turn are represented numerically as binary connectivity matrices. The statistical significance of similarities between two connectivity matrices is assessed using a permutation-test of association, a test that was developed for comparing distancematrices. The null hypothesis for this test is defined by the set of alternative connectivity matrices corresponding to all possible permutations of the track-vertices. There are however, a number of problems with this statistical test when applied to connectivity matrices derived from panbiogeographic tracks, and these render invalid the procedure advocated by Page.     

The use of parsimony in testing phylogenetic hypotheses

With the advance of cladistic theory differences in principle between it and other systematic techniques are few but of fundamental importance. In the mechanics of classification they are confined to ranking and the rejection of paraphyletic taxa. In cladistic analysis, leading to cladograms, trees and phylogeny reconstruction, inconsistencies in apparent synapomorphies are said to be resolved using Popper's hypothetico-deductive method together with the principle of parsi However, not only do cladists not use Popper's methodology, which is inconsistent with parsimony, but their use of parsimony is invalid as a test of phylo The only accepted extrinsic test of a classification is that enunciated by John Stuart Mill. It has been claimed that cladistic classifications yield the best results when judged by Mill's criteria, but this is only possibly the case with analytic classifications produced by numerical techniques. No satisfactory test exists in normal (synthetic) cladism for distinguishing synapomorphy from homoplasy. The effects of this are particularly dire in cladograms and classifications involving fossils in which a Stufenreihe arrangement is adopted.     

Evolution of the supraorbital region in Upper Pleistocene fossil hominids from South-Central Europe

South-Central European fossil hominids dated to the Upper Pleistocene exhibit a distinct morphological and metric continuum in supraorbital form from early Neandertal (Krapina), through late Neandertals (Vindija), to early Upper Paleolithic hominids. The supraorbital morphologies pertinent to this continuum are documented, and the alterations in size and morphology are discussed ralative to the function of supraorbital superstructures and their relationship to overall craniofacial form. It is concluded that this continuum most likely reflects localized transition between Neandertals and modern man in this region of Europe.     

Archaic lineages in the history of modern humans

An important question in the ongoing debate on the origin of Homo sapiens is whether modern human populations issued from a single lineage or whether several, independently evolving lineages contributed to their genetic makeup. We analyzed haplotypes composed of 35 polymorphisms from a segment of the dystrophin gene. We find that the bulk of a worldwide sample of 868 chromosomes represents haplotypes shared by different continental groups. The remaining chromosomes carry haplotypes specific for the continents or for local populations. The haplotypes specific for non-Africans can be derived from the most frequent ones through simple recombination or a mutation. In contrast, chromosomes specific for sub-Saharan Africans represent a distinct group, as shown by principal component analysis, maximum likelihood tree, structural comparison, and summary statistics. We propose that African chromosomes descend from at least two lineages that have been evolving separately for a period of time. One of them underwent range expansion colonizing different continents, including Africa, where it mixed with another, local lineage represented today by a large fraction of African-specific haplotypes. Genetic admixture involving archaic lineages appears therefore to have occurred within Africa rather than outside this continent, explaining greater diversity of sub-Saharan populations observed in a variety of genetic systems.     

Evolution of female colour polymorphism in damselflies: testing the hypotheses

The existence of several female colour morphs is a conspicuous characteristic of many damselflies that show one male-like (androchrome) and several nonmale-like (gynochrome) morphs. We tested several adaptive hypotheses and the null model for the maintenance of female polychromatism (one androchrome and two gynochromes) in the damselfly Ceriagrion tenellum. We tested the null model by comparing the degree of genetic differentiation between the colour locus and a set of 19 neutral RAPD loci in five populations. Our results indicate that selection is acting to maintain similar frequencies between populations at the colour locus. Using mark–recapture techniques we found that mating success is not dependent on female coloration. We tested the mimicry hypothesis by presenting live and dead models to males. Dead models were highly attractive irrespective of coloration. In contrast, with live models males could not distinguish between androchromes and other males, and were more attracted to gynochrome females. Despite this, within populations morph frequencies remained constant over time and mating was at random with respect to female coloration. However, there was a positive relationship between male density and androchrome frequency in a comparative study of eight populations. We discuss our results in the framework of sexual conflict theory and suggest that andro- and gynochrome females are using different strategies to control their number of matings. The different morphs might be maintained in a balanced polymorphism by a combination of density- and frequency-dependent mechanisms.Copyright 2002 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour     

Endo's stress analysis of the primate skull and the functional significance of the supraorbital region

A review of Endo's experimental and theoretical procedures and data indicates that the magnitude of the principal strains in the glabella region of both humans and gorillas are low as compared to other parts of the face. Therefore, his data do not provide support for the hypothesis that the glabella region is a highly stressed region during biting. In addition, increased levels of strain in the supraorbital region are directly related to increased levels of masticatory muscle and reaction forces, and not necessarily to anterior tooth loading as opposed to posterior tooth loading. His data also indicate that the supraorbital region in extant humans cannot be accurately modeled as a beam. These conclusions either differ from those of Endo or are not clearly presented or emphasized throughout any of Endo's papers. Therefore, we suggest that a number of investigators have made unsupported or erroneous conclusions based on Endo's work. This is particularly true for those studies that have emphasized the existence of powerful bending stress in the glabella region during incisor biting in both humans and non-human primates.     

Microsatellite variation and the differentiation of modern humans

This study presents an analysis of 20 tetranucleotide microsatellites in 16 worldwide human populations representing the major geographic groups. Global Fst values for the 20 microsatellites are indicators of their relative validity as tools in human population genetics. Four different measures of genetic distance (Fst, D_SW, δμ ² and Rst) have been tested and compared with each other. Neighbor-joining trees have been constructed for all the measures of genetic distance and populations. Measures of genetic distance such as Fst, which does not consider different mutational relationships among alleles and has a known relationship to differentiation by drift, and to some extent D_SW, reflect what is known of human evolution, while mutation-based distances such as Rst and δμ ² give very different results from those recognized from other sources (genetic or archaeological). When the genetic relationship between human populations is analyzed through allelic frequencies for microsatellites, the choice of distance may be a key issue in the picture obtained of genetic relationships between human populations. The results of the present study suggest that genetic drift played the main role in generating the present distributions of microsatellite alleles and their variation among human populations; the role of mutation must have been less important owing to the time constraint imposed by the small timescale in which most human differentiation has occurred. Moreover, the results support the theory of a recent origin of modern humans, although the existence of strong bottlenecks in the origin of the various human groups seems unlikely. Received: 20 March 1996     

Beringia and the global dispersal of modern humans

Until recently, the settlement of the Americas seemed largely divorced from the out‐of‐Africa dispersal of anatomically modern humans, which began at least 50,000 years ago. Native Americans were thought to represent a small subset of the Eurasian population that migrated to the Western Hemisphere less than 15,000 years ago. Archeological discoveries since 2000 reveal, however, that Homo sapiens occupied the high‐latitude region between Northeast Asia and northwest North America (that is, Beringia) before 30,000 years ago and the Last Glacial Maximum (LGM). The settlement of Beringia now appears to have been part of modern human dispersal in northern Eurasia. A 2007 model, the Beringian Standstill Hypothesis, which is based on analysis of mitochondrial DNA (mtDNA) in living people, derives Native Americans from a population that occupied Beringia during the LGM. The model suggests a parallel between ancestral Native Americans and modern human populations that retreated to refugia in other parts of the world during the arid LGM. It is supported by evidence of comparatively mild climates and rich biota in south‐central Beringia at this time (30,000‐15,000 years ago). These and other developments suggest that the settlement of the Americas may be integrated with the global dispersal of modern humans.     

Postcranial remains and the origin of modern humans

The nature, timing, and location of the origin of modern humans has been the subject of intense controversy for the last 15 years.^1–4 Genetic data and new radiometric dates for key fossils that lie beyond the range of radiocarbon dating have substantially added to the knowledge derived from the fossil evidence documenting the transition from archaic to modern humans. These new data, however, have failed to resolve the problem in its entirety. Most authorities now accept that Africa played an important, and probably central, role in the origin of modern humans.^7–13 The genetic evidence seems to be particularly emphatic that an African population that existed between 200,000 and 100,000 years ago (100 ka) is ancestral to all living humans.^6,7 Controversy still surrounds the question of how much, if at all, archaic humans from outside of Africa, such as Neandertals, late archaic Chinese hominins such as Jinniushan, and the Indonesian Ngandong hominins, may have contributed to the morphological and genetic diversity present in living populations and the morphology of the earliest fossils of modern humans.¹⁰     

Morphometric integration and modularity in configurations of landmarks: tools for evaluating a priori hypotheses

SUMMARY Identifying the modular components of a configuration of landmarks is an important task of morphometric analyses in evolutionary developmental biology. Modules are integrated internally by many interactions among their component parts, but are linked to one another only by few or weak interactions. Accordingly, traits within modules are tightly correlated with each other, but relatively independent of traits in other modules. Hypotheses concerning the boundaries of modules in a landmark configuration can therefore be tested by comparing the strength of covariation among alternative partitions of the configuration into subsets of landmarks. If a subdivision coincides with the true boundaries between modules, the correlations among subsets should be minimal. This article introduces Escoufier's RV coefficient and the multi-set RV coefficient as measures of the correlation between two or more subsets of landmarks. These measures can be compared between alternative partitions of the configuration into subsets. Because developmental interactions are tissue bound, it is sensible to require that modules should be spatially contiguous. I propose a criterion for spatial contiguity for sets of landmarks using an adjacency graph. The new methods are demonstrated with data on shape of the wing in Drosophila melanogaster and the mandible of the house mouse.