Spondylolysis and spinal adaptations for bipedalism: The overshoot hypothesis

Background and objectives

The study reported here focused on the aetiology of spondylolysis, a vertebral pathology usually caused by a fatigue fracture. The goal was to test the Overshoot Hypothesis, which proposes that people develop spondylolysis because their vertebral shape is at the highly derived end of the range of variation within Homo sapiens.

Methodology

We recorded 3D data on the final lumbar vertebrae of H. sapiens and three great ape species, and performed three analyses. First, we compared H. sapiens vertebrae with and without spondylolysis. Second, we compared H. sapiens vertebrae with and without spondylolysis to great ape vertebrae. Lastly, we compared H. sapiens vertebrae with and without spondylolysis to great ape vertebrae and to vertebrae of H. sapiens with Schmorl’s nodes, which previous studies have shown tend to be located at the ancestral end of the range of H. sapiens shape variation.

Results

We found that H. sapiens vertebrae with spondylolysis are significantly different in shape from healthy H. sapiens vertebrae. We also found that H. sapiens vertebrae with spondylolysis are more distant from great ape vertebrae than are healthy H. sapiens vertebrae. Lastly, we found that H. sapiens vertebrae with spondylolysis are at the opposite end of the range of shape variation than vertebrae with Schmorl’s nodes.

Conclusions

Our findings indicate that H. sapiens vertebrae with spondylolysis tend to exhibit highly derived traits and therefore support the Overshoot Hypothesis. Spondylolysis, it appears, is linked to our lineage’s evolutionary history, especially its shift from quadrupedalism to bipedalism.

Lay summary: Spondylolysis is a relatively common vertebral pathology usually caused by a fatigue fracture. There is reason to think that it might be connected with our lineage’s evolutionary shift from walking on all fours to walking on two legs. We tested this idea by comparing human vertebrae with and without spondylolysis to the vertebrae of great apes. Our results support the hypothesis. They suggest that people who experience spondylolysis have vertebrae with what are effectively exaggerated adaptations for bipedalism.

The ancestral shape hypothesis: an evolutionary explanation for the occurrence of intervertebral disc herniation in humans

Background

Recent studies suggest there is a relationship between intervertebral disc herniation and vertebral shape. The nature of this relationship is unclear, however. Humans are more commonly afflicted with spinal disease than are non-human primates and one suggested explanation for this is the stress placed on the spine by bipedalism. With this in mind, we carried out a study of human, chimpanzee, and orangutan vertebrae to examine the links between vertebral shape, locomotion, and Schmorl’s nodes, which are bony indicators of vertical intervertebral disc herniation. We tested the hypothesis that vertical disc herniation preferentially affects individuals with vertebrae that are towards the ancestral end of the range of shape variation within Homo sapiens and therefore are less well adapted for bipedalism.

Results

The study employed geometric morphometric techniques. Two-dimensional landmarks were used to capture the shapes of the superior aspect of the body and posterior elements of the last thoracic and first lumbar vertebrae of chimpanzees, orangutans, and humans with and without Schmorl’s nodes. These data were subjected to multivariate statistical analyses.

Canonical Variates Analysis indicated that the last thoracic and first lumbar vertebrae of healthy humans, chimpanzees, and orangutans can be distinguished from each other (p<0.028), but vertebrae of pathological humans and chimpanzees cannot (p>0.4590). The Procrustes distance between pathological humans and chimpanzees was found to be smaller than the one between pathological and healthy humans. This was the case for both vertebrae. Pair-wise MANOVAs of Principal Component scores for both the thoracic and lumbar vertebrae found significant differences between all pairs of taxa (p<0.029), except pathological humans vs chimpanzees (p>0.367). Together, these results suggest that human vertebrae with Schmorl’s nodes are closer in shape to chimpanzee vertebrae than are healthy human vertebrae.

Conclusions

The results support the hypothesis that intervertebral disc herniation preferentially affects individuals with vertebrae that are towards the ancestral end of the range of shape variation within H. sapiens and therefore are less well adapted for bipedalism. This finding not only has clinical implications but also illustrates the benefits of bringing the tools of evolutionary biology to bear on problems in medicine and public health.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-015-0336-y) contains supplementary material, which is available to authorized users.

Endless forms: human behavioural diversity and evolved universals

Human populations have extraordinary capabilities for generating behavioural diversity without corresponding genetic diversity or change. These capabilities and their consequences can be grouped into three categories: strategic (or cognitive), ecological and cultural-evolutionary. Strategic aspects include: (i) a propensity to employ complex conditional strategies, some certainly genetically evolved but others owing to directed invention or to cultural evolution; (ii) situations in which fitness payoffs (or utilities) are frequency-dependent, so that there is no one best strategy; and (iii) the prevalence of multiple equilibria, with history or minor variations in starting conditions (path dependence) playing a crucial role. Ecological aspects refer to the fact that social behaviour and cultural institutions evolve in diverse niches, producing various adaptive radiations and local adaptations. Although environmental change can drive behavioural change, in humans, it is common for behavioural change (especially technological innovation) to drive environmental change (i.e. niche construction). Evolutionary aspects refer to the fact that human capacities for innovation and cultural transmission lead to diversification and cumulative cultural evolution; critical here is institutional design, in which relatively small shifts in incentive structure can produce very different aggregate outcomes. In effect, institutional design can reshape strategic games, bringing us full circle.

The facial skeleton of the chimpanzee-human last common ancestor

This review uses the current morphological evidence to evaluate the facial morphology of the hypothetical last common ancestor (LCA) of the chimpanzee/bonobo (panin) and human (hominin) lineages. Some of the problems involved in reconstructing ancestral morphologies so close to the formation of a lineage are discussed. These include the prevalence of homoplasy and poor phylogenetic resolution due to a lack of defining derived features. Consequently the list of hypothetical features expected in the face of the LCA is very limited beyond its hypothesized similarity to extant Pan. It is not possible to determine with any confidence whether the facial morphology of any of the current candidate LCA taxa (Ardipithecus kadabba, Ardipithecus ramidus, Orrorin tugenensis and Sahelanthropus tchadensis) is representative of the LCA, or a stem hominin, or a stem panin or, in some cases, a hominid predating the emergence of the hominin lineage. The major evolutionary trends in the hominin lineage subsequent to the LCA are discussed in relation to the dental arcade and dentition, subnasal morphology and the size, position and prognathism of the facial skeleton.

The serotonin transporter: sequence variation in Macaca fascicularis and its relationship to dominance

Specific genotypes of the rhesus monkey and human serotonin transporter gene (SERT) promoter region are associated with personality traits and serotonergic activity. However, the most commonly studied promoter polymorphism (5-HTTLPR) is monomorphic in many other monkey species. To date, no systematic search for alternative potentially functional polymorphisms across the remaining coding parts of the gene has been undertaken in other primate species, despite the crucial role SERT plays in modulating serotonergic tone. We investigated whether sequence variation in this gene is associated with social rank and serotonin metabolite (5-HIAA) differences in 524 cynomolgus macaques. Sequence variation and extent of linkage disequilibrium (LD) across the regulatory and coding regions were initially characterized in 92 macaques. The exons and promoter contained 28 polymorphisms, more than double that recorded for human SERT. In further contrast to humans, the macaque SERT showed no significant LD. Potentially functional polymorphisms were genotyped in all animals. No individual variants or haplotypes were significantly associated with social rank or 5-HIAA concentrations; however, certain serotonin transporter diplotypes may modulate acquisition of dominance status.

Nuclear insertions help and hinder inference of the evolutionary history of gorilla mtDNA

Numts are fragments of mitochondrial DNA (mtDNA) that have been translocated to the nucleus, where they can persist while their mitochondrial counterparts continue to rapidly evolve. Thus, numts represent 'molecular fossils' useful for comparison with mitochondrial variation, and are particularly suited for studies of the fast-evolving hypervariable segment of the mitochondrial control region (HV1). Here we used information from numts found in western gorillas (Gorilla gorilla) and eastern gorillas (Gorilla beringei) to estimate that these two species diverged about 1.3 million years ago (Ma), an estimate similar to recent calculations for the divergence of chimpanzee and bonobo. We also describe the sequence of a gorilla numt still possessing a segment lost from all contemporary gorilla mtDNAs. In contrast to that sequence, many numts of the HV1 are highly similar to authentic mitochondrial organellar sequences, making it difficult to determine whether purported mitochondrial sequences truly derive from that genome. We used all available organellar HV1 and corresponding numt sequences from gorillas in a phylogenetic analysis aimed at distinguishing these two types of sequences. Numts were found in several clades in the tree. This, in combination with the fact that only a limited amount of the extant variation in gorillas has been sampled, suggests that categorization of new sequences by the indirect means of phylogenetic comparison would be prone to uncertainty. We conclude that for taxa such as gorillas that contain numerous numts, direct approaches to the authentication of HV1 sequences, such as amplification strategies relying upon the circularity of the mtDNA molecule, remain necessary.

A natural diet versus modern Western diets? A new approach to prevent "well-being syndromes"

Obesity is the most common nutritional disorder in the Western world. Actually, 250 million adults are obese, and 500 million adults and 22 million children under 5 years of age are overweight. Obesity is a complex trait, depending upon interactions between multiple genes and the environment, but its recent rise and "epidemic proportions" are, above all, the consequences of dramatic changes in lifestyle, socioeconomic progress, and political and cultural trends. Eating behavior has strong extraphysiological determinants, being influenced by neuroendocrine, nutritional, environmental, and cognitive stimuli, able to modify the body weight set-point. Health care professionals should be concerned about obesity, because of the well-established relations between excess body weight and pathologies such as type II diabetes, hypertension, atherosclerosis, osteoarthritis, dyslipidemia, and cancer, which afflict more and more people in the Western world--sort of "well-being syndromes." An overview of modern Western diets--the American, Mediterranean, Atkins, and Zone diets--reveals the contradictions existing about the correct and healthy approach to human nutrition and suggests a "return to Nature." From the actual artificial nutrition systems, based on cereals, milk, and their products, irrespective of our genome and metabolic attitudes, a simple diet based on natural food can be an ally in health maintenance and restoration.

Multivariate apportionment of global human craniometric diversity

Extensive research in human genetics on presumably neutral loci has shown that the overwhelming majority of human diversity is found among individuals within local populations. Previous apportionments of craniometric diversity are similar to these genetic apportionments, implying that interregionally differing selection pressures have played a limited role in producing contemporary human cranial diversity. Here we show that when cranial variation is independently partitioned using principal components analysis of Mosimann shape variables, some estimates of interregional craniometric differentiation are larger than those for most genetic loci. These estimates are similar to estimates for genetic loci where interregionally differing selection pressures are implicated in producing the observed patterns of variation within and among regions. These results suggest that differences among regions in at least some cranial features, particularly in the nasal region, are in part the product of interregionally differing selection pressures. Moreover, these results have implications for assessing the biological affinities of prehistoric samples that are temporally separated from contemporary human reference populations.

Conceptualizing human variation

What is the relationship between the patterns of biological and sociocultural variation in extant humans? Is this relationship accurately described, or best explained, by the term 'race' and the schema of 'racial' classification? What is the relationship between 'race', genetics and the demographic groups of society? Can extant humans be categorized into units that can scientifically be called 'races'? These questions underlie the discussions that address the explanations for the observed differences in many domains between named demographic groups across societies. These domains include disease incidence and prevalence and other variables studied by biologists and social scientists. Here, we offer a perspective on understanding human variation by exploring the meaning and use of the term 'race' and its relationship to a range of data. The quest is for a more useful approach with which to understand human biological variation, one that may provide better research designs and inform public policy.

The anthropoid postcranial axial skeleton: Comments on development, variation, and evolution

Within-species phenotypic variation is the raw material on which natural selection acts to shape evolutionary change, and understanding more about the developmental genetics of intraspecific as well as interspecific phenotypic variation is an important component of the Evo-Devo agenda. The axial skeleton is a useful system to analyze from such a perspective. Its development is increasingly well understood, and between-species differences in functionally important developmental parameters are well documented. I present data on intraspecific variation in the axial postcranial skeleton of some Primates, including hominoids (apes and humans). Hominoid species are particularly valuable, because counts of total numbers of vertebrae, and hence original somite numbers, are available for large samples. Evolutionary changes in the axial skeleton of various primate lineages, including bipedal humans, are reviewed, and hypotheses presented to explain the changes in terms of developmental genetics. Further relevant experiments on model organisms are suggested in order to explore more fully the differences in developmental processes between primate species, and hence to test these hypotheses.

Using secondary structure to identify ribosomal numts: cautionary examples from the human genome

The identification of inadvertently sequenced mitochondrial pseudogenes (numts) is critical to any study employing mitochondrial DNA sequence data. Failure to discriminate numts correctly can confound phylogenetic reconstruction and studies of molecular evolution. This is especially problematic for ribosomal mtDNA genes. Unlike protein-coding loci, whose pseudogenes tend to accumulate diagnostic frameshift or premature stop mutations, functional ribosomal genes are not constrained to maintain a reading frame and can accumulate insertion-deletion events of varying length, particularly in nonpairing regions. Several authors have advocated using structural features of the transcribed rRNA molecule to differentiate functional mitochondrial rRNA genes from their nuclear paralogs. We explored this approach using the mitochondrial 12S rRNA gene and three known 12S numts from the human genome in the context of anthropoid phylogeny and the inferred secondary structure of primate 12S rRNA. Contrary to expectation, each of the three human numts exhibits striking concordance with secondary structure models, with little, if any, indication of their pseudogene status, and would likely escape detection based on structural criteria alone. Furthermore, we show that the unwitting inclusion of a particularly ancient (18-25 Myr old) and surprisingly cryptic human numt in a phylogenetic analysis would yield a well-supported but dramatically incorrect conclusion regarding anthropoid relationships. Though we endorse the use of secondary structure models for inferring positional homology wholeheartedly, we caution against reliance on structural criteria for the discrimination of rRNA numts, given the potential fallibility of this approach.

Evolutionary Psychology

▪ Abstract  Studying human behavior in the light of evolutionary theory involves studying the comparative evolutionary history of behaviors (phylogeny), the psychological machinery that generates them (mechanisms), and the adaptive value of that machinery in past reproductive competition (natural selection). To show the value of a phylogenetic perspective, I consider the ethology of emotional expression and the cladistics of primate social systems. For psychological mechanisms, I review evidence for a pan-human set of conceptual building blocks, including innate concepts of things, space, and time, of number, of logic, of natural history, and of “other minds” and social life, which can be combined to generate a vast array of culture-specific concepts. For natural selection, I discuss the sexual selection of sex differences and similarities, and the social selection of moral sentiments and group psychology.

Antievolution and Creationism in the United States

Evolution is considered controversial by a substantial minority of Americans. Religious opposition explains this, but this opposition is comprised of a broad continuum of religious views. It runs from “young earth creationism” through “old earth creationism” (including “day-age,” “gap,” and “progressive creationism”) to “theistic evolutionism.” Historically, antievolutionists have attempted to ban evolution and to present it on an equal footing with “creation science.” Scholars largely ignored antievolutionism until efforts to pass “equal time for creation and evolution” laws stimulated both political and scholarly activism. Lately, there are efforts to discourage the teaching of evolution by requiring teachers to read disclaimers before teaching it, to teach it as “theory, not fact,” or to present fancied “evidence against evolution.” Recently, “intelligent design theory,” a restatement of William Paley's Argument from Design, has surfaced. Although rejected by scientists, intelligent design arguments and publications are appearing at the college level (in nonscience courses) as accurate representations of scientific scholarship.