Dental reduction and the probable mutation effect.

A recent test of the probable mutation effect can be interpreted to suggest the operations of mutations under conditions of reduced selection in the late Pleistocene reduction of the human dentition.     

A genetic model of dental reduction through the probable mutation effect

A simulation approach is used in order to elucidate the nature of the hypothesized “probable mutation effect” as it applies to dental reduction in man. Computer-generated simulations of the accumulation of mutations in a human gene pool show the results of the proposed model under the influence of various parameters, as well as illustrating the nature of such genetic change through time. This approach supports a polygenic model of the probable mutation effect as a viable hypothesis for an explanation of the dental reduction which has occurred in some human populations over the last 40,000 years.     

Human dental reduction: natural selection or the probable mutation effect

Dental reduction has been sufficiently widespread among human populations to render the phenomenon of reduced tooth size worthy of scientific explanation. One of the most controversial models invoked to explain structural reduction in organisms is referred to as the "probable mutation effect" (PME). According to this model, structures no longer functional owing to ecological or cultural changes will experience a relaxation of selection pressure, permitting an accumulation of mutations in the population that inevitably will result in the reduction in size or the loss of the concerned structure. Although the PME continues to be offered as a viable explanation of human dental reduction, it is based upon several premises that modern dental clinical experience fails to support. Known enzyme defects resulting from mutations, factors predisposing to dental infections, and the deleterious effects of teeth that are too large or too small reveal that the PME does not logically account for the reduction of tooth size. Given such information, this paper proposes models of dental reduction based upon natural selection, which, unlike the PME, are testable in both modern and archaeological populations. The integration of clinical and skeletal data permits a more thorough understanding of dental reduction in the hominid fossil record.     

The Neandertal genome and ancient DNA authenticity

Recent advances in high‐thoughput DNA sequencing have made genome‐scale analyses of genomes of extinct organisms possible. With these new opportunities come new difficulties in assessing the authenticity of the DNA sequences retrieved. We discuss how these difficulties can be addressed, particularly with regard to analyses of the Neandertal genome. We argue that only direct assays of DNA sequence positions in which Neandertals differ from all contemporary humans can serve as a reliable means to estimate human contamination. Indirect measures, such as the extent of DNA fragmentation, nucleotide misincorporations, or comparison of derived allele frequencies in different fragment size classes, are unreliable. Fortunately, interim approaches based on mtDNA differences between Neandertals and current humans, detection of male contamination through Y chromosomal sequences, and repeated sequencing from the same fossil to detect autosomal contamination allow initial large‐scale sequencing of Neandertal genomes. This will result in the discovery of fixed differences in the nuclear genome between Neandertals and current humans that can serve as future direct assays for contamination. For analyses of other fossil hominins, which may become possible in the future, we suggest a similar ‘boot‐strap’ approach in which interim approaches are applied until sufficient data for more definitive direct assays are acquired.     

Discrete dental trait asymmetry in Mexican and Belizean groups

Discrete dental trait asymmetry was scored on four Mexican Indian and two Afro-Belizean groups. The Mexican populations show significantly higher mean asymmetry than the two populations from Belize even though the Belizean groups have larger teeth. Point biserial coefficients of correlation between asymmetry of discrete traits and tooth size is low and in most cases not significant. This study indicates that populational and ethnic differences exist in the asymmetry of discrete traits, suggesting that these traits may be used to investigate the etiology of dental asymmetry, and the relative roles of genetics and environment.     

Fluctuating dental asymmetry as a measure of odontogenic canalization in man

Fluctuating odontometric asymmetry was evaluated in 202 Lengua Indians and in 125 contemporary caucasoids using Euclidean map analyses, rescaled asymmetry values, and Naperian logarithmic transformations. Both populations showed bimodal distributions of canalizing ability, with significantly more Lengua Indians being less well canalized. Student t-tests indicated that significant interpopulation differences in rescaled asymmetry values centered around maxillary mesiodistal dimensions. Analyses of variance failed to show significant levels of sexual dimorphism in the magnitude of asymmetry for both populations. It is concluded that even in a relatively stress-free population, there exist some individuals who are less well canalized than others. When a population is subjected to elevated levels of parasitic and nutritional stress, the number of less-well-canalized individuals increases significantly.     

Out of the North Sea: the Zeeland Ridges Neandertal

In 2001, a portion of human frontal bone was discovered in sediments extracted from the bottom of the North Sea, 15 km off the coast of the Netherlands. The extraction zone is located in the so-called Zeeland Ridges area located at 51°40′ northern latitude and 3°20′ eastern longitude. The specimen was dredged up from sediments containing Late Pleistocene faunal remains and Middle Palaeolithic artefacts, including well-finished small handaxes and Levallois flakes. The details of the supraorbital morphology, as well as the quantitative assessment of the shape of the external surface of the squama using traditional and 3D geometric morphometrics, unambiguously assign the Zeeland Ridges frontal bone to Homo neanderthalensis. Carbon and nitrogen isotopic analysis indicate that the Zeeland Ridges hominin, like other Neandertals, was highly carnivorous and does not show evidence for the consumption of aquatic foods. A lesion on the outer table and diploic layer of the bone in the area of the supratoral sulcus can be interpreted as the result of an intradiploic epidermoid cyst, a type of neoplasm diagnosed for the first time in Neandertal remains. So far, the Zeeland Ridges Neandertal is the first Pleistocene fossil hominin found under seawater and the first recorded in the Netherlands.     

A Neandertal mandible from the Cova del Gegant (Sitges, Barcelona, Spain)

A human mandible from the site of Cova del Gegant is described here for the first time and compared with other Middle and Upper Pleistocene representatives of the genus Homo from Europe and Southwest Asia. The specimen was recovered from sediments which also yielded Mousterian stone tools and Pleistocene fauna. The preserved morphology of the mandible, particularly in the region of the mental foramen, clearly aligns it with the Neandertals, making the Cova del Gegant the only known site in Catalonia documenting diagnostic human skeletal remains in association with Middle Paleolithic stone tools. This represents an important new addition to the human fossil record from the Iberian Peninsula and joins the Ba?olas mandible in documenting the course of human evolution in the northern Mediterranean region of Spain.     

On the meaning of increased fluctuating dental asymmetry: a cross populational study.

Suarez reports a greater magnitude of fluctuating dental asymmetry for Neandertal sample when compared with a sample of modern Ohio whites. He postulates that this greater antimeric variance could be due to a greater degree of inbreeding in the Neandertal populations. In the present investigation, the magnitude of fluctuating dental asymmetry is evaluated for Eskimo and Pueblo populations. These populations were found to exhibit dental variance of equal magnitude to that of the Neandertal population. As these populations are not highly inbred, a stress related mechanism is suggested to explain these observations and the inbreeding hypothesis is rejected. The implications of this mechanism to Brace's Probable Mutation Effect are discussed.     

A functional test of Neandertal and modern human mitochondrial targeting sequences

Targeting of nuclear-encoded proteins to different organelles, such as mitochondria, is a process that can result in the redeployment of proteins to new intracellular destinations during evolution. With the sequencing of the Neandertal genome, it has become possible to identify amino acid substitutions that occurred on the modern human lineage since its separation from the Neandertal lineage. Here we analyze the function of two substitutions in mitochondrial targeting sequences that occurred and rose to high frequency recently during recent human evolution. The ancestral and modern versions of the two targeting sequences do not differ in the efficiency with which they direct a protein to the mitochondria, an observation compatible with the neutral theory of molecular evolution.     

Ear asymmetry and left-side cradling

Women and girls tend to cradle infants and dolls on the left side of the body. Left-sided cradling is found in chimpanzees and gorillas, is cross-cultural and present in historical works of art, and is transmitted down the human maternal line. One explanation for the left-cradling tendency is that it facilitates the flow of affective information from the infant via the left ear and eye to the center for emotional decoding, that is, the right hemisphere of the mother. We show that the developmental stability of the ear, as measured by ear asymmetry, is negatively correlated with the left-sided cradling tendency. Left-cradling English women holding infants and Jamaican girls holding dolls had a strong tendency to show lower ear asymmetry than right cradlers, whereas no such relationship was found in boys nor for various measures of asymmetry of the hand, with the possible exception of the wrist in Jamaican girls. Degree of handedness, as measured by the Annett peg-moving test, did not predict cradling preference in the Jamaican children, and the relationship between ear asymmetry and cradling preference was independent of hand preference. Our results suggest that developmental instability of the ear (including the pinna, external auditory meatus, and middle ear) may interfere with the flow of affective information to the right hemisphere. Ear asymmetry also showed evidence of strong maternal but not paternal transmission. It is suggested that between-individual variation of in utero stress may explain patterns of maternal transmission of lateral cradling tendencies.     

Sexual dimorphism in the pelvic midplane and its relationship to Neandertal reproductive patterns

The fragmentary nature of the fossil record has limited the analysis of the Neandertal pelvis to the superior pubic ramus and the pelvic inlet. From an obstetric viewpoint, the pelvic midplane or “plane of least dimensions,” defined by the distance between the ischial spines, must be considered in the analysis of hominid reproduction. We examined the relationship between BSD and weight in a mixed sex hospital population undergoing diagnostic computed tomography (CT) scans (41 females and 40 males). Because femoral head diameter squared (FH²) has been used as a proxy for weight in skeletal populations, it was also analyzed with respect to BSD and weight. Bivariate regression analysis of BSD with other body dimensions indicates the presence of significant sex differences. In females, but not in males, weight is a statistically significant predictor of BSD. FH² is an even better predictor of BSD in females while nonsignificant in males. Although weight and FH² are significantly correlated with BSD in females, FH² does not predict weight in females as well as it does in males. The positive correlation between skeletal frame size and BSD in females is indicative of an evolutionary pattern that must take into account the pressures of reproduction. Our results indicate that critical dimensions of the pelvis must increase as the maternal skeleton becomes larger. These results provide a context for the interpretation of the reproductive patterns of a relatively robust hominid population like the Neandertals. © 1996 Wiley-Liss, Inc.     

Sexual dimorphism in the human bony pelvis, with a consideration of the Neandertal pelvis from Kebara Cave, Israel.

Sexual dimorphism of the human pelvis is inferentially related to obstetrics. However, researchers disagree in the identification and obstetric significance of pelvic dimorphisms. This study addresses three issues. First, common patterns in dimorphism are identified by analysis of pelvimetrics from six independent samples (Whites and Blacks of known sex and four Amerindian samples of unknown sex). Second, an hypothesis is tested that the index of pelvic dimorphism (female mean x 100/male mean) is inversely related to pelvic variability. Third, the pelvic dimensions of the Neandertal male from Kebara cave, Israel are compared with those of the males in this study. The results show that the pelvic inlet is the plane of least dimorphism in humans. The reason that reports often differ in the identification of dimorphisms for this pelvic plane is that both the length of the pubis and the shape of the inlet are related to nutrition. The dimensions of the pelvis that are most dimorphic (that is, female larger than male) are the measures of posterior space, angulation of sacrum, biischial breadth, and subpubic angle. Interestingly, these dimensions are also the most variable. The hypothesis that variability and dimorphism are inversely related fails to be supported. The factors that influence pelvic variability are discussed. The Kebara 2 pelvis has a spacious inlet and a confined outlet relative to modern males, though the circumferences of both planes in the Neandertal are within the range of variation of modern males. The inference is that outlet circumference in Neandertal females is also small in size, but within the range of variation of modern females. Arguments that Neandertal newborns were larger in size than those of modern humans necessarily imply that birth was more difficult in Neandertals.     

The effect of sexual harassment on lethal mutation rate in female Drosophila melanogaster

The rate by which new mutations are introduced into a population may have far-reaching implications for processes at the population level. Theory assumes that all individuals within a population have the same mutation rate, but this assumption may not be true. Compared with individuals in high condition, those in poor condition may have fewer resources available to invest in DNA repair, resulting in elevated mutation rates. Alternatively, environmentally induced stress can result in increased investment in DNA repair at the expense of reproduction. Here, we directly test whether sexual harassment by males, known to reduce female condition, affects female capacity to alleviate DNA damage in Drosophila melanogaster fruitflies. Female gametes can repair double-strand DNA breaks in sperm, which allows manipulating mutation rate independently from female condition. We show that male harassment strongly not only reduces female fecundity, but also reduces the yield of dominant lethal mutations, supporting the hypothesis that stressed organisms invest relatively more in repair mechanisms. We discuss our results in the light of previous research and suggest that social effects such as density and courtship can play an important and underappreciated role in mediating condition-dependent mutation rate.     

A twin study of dental dimension. I. Discordance,asymmetry, and mirror imagery

Bilateral tooth measurements in twins were partitioned into three orthogonal contrasts, each associated with one degree of freedom, to estimate three parameters: discordance, asymmetry, and mirror imagery. The probability levels of the within-pair variance ratios were used to test for significance of these estimates. The results provided strong evidences for the existence of significant genetic determinants of almost all of the individual tooth dimensions, but little or no evidence for a genetic basis of asymmetry. The analysis gave no indication that monozygotic twinning was associated with an increased degree of either fluctuating asymmetry or mirror imagery, when compared to dizygotic twins. The data on monozygotic twins further suggested that for most variables examined, the increment of environmental discordance resulting from the twinning phenomena was greater than the developmental noise that caused asymmetry within individual cotwins.     

A note on the effect of asymmetry in suture closure in mature human skulls

Studies on suture closure in mature European and East African Bantu skulls revealed marked differences in the degree of suture closure on the left and right side. Asymmetrical closure of sutures may lead to erroneous conclusions being made in ageing skulls if only one-half of the skull is available for examination. Asymmetry in suture closure was rather common in recent skulls, being particularly noticeable in the bregmatic and complicate parts of the coronal suture and in intermediate portions of lambdoid sutures.     

Age- and sex-distribution of the mutation load

We investigate the age and sex distribution of genetic fitness under mutation–selection balance by means of simple one-locus two-allele models. We find that the extent of age and sex variation in the mutation load is very dependent on the average effect of new mutations. If the average heterozygote selective effect of new mutations is large, then age and sex differences may constitute a significant fraction of the total load, and be significant as compared to standing genetic variation. Whether the mutation load will increase or decrease with age depends on the age- and sex-specific effects of the new mutations, and on the rate of accumulation of mutations in the germ line as individuals age. We argue that the load will most likely increase with age in animals with continuous germ-cell division throughout life, and that this will occur even when mutations have unconditionally deleterious effects. We show that a male-biased mutation rate is likely to result in both a male-biased mutation load and a load that increases with male age. This revised version was published online in July 2006 with corrections to the Cover Date.