Tourette syndrome in the context of evolution and behavioral ecology

Tourette syndrome, and the closely related spectrum of tic disorders, are inherited neuropsychiatric conditions characterized by the presence of repetitive and stereotyped movements. Tics are elicited by either environmental experiences or internal signals that instruct the basal ganglia to initiate automatic or procedural movements. In most vertebrates the basal ganglia encode instructions for habitually used sequences of motor movements that are essential to survival. Tic disorders may represent evolved phenotypes with a lower threshold for basal ganglia-directed actions. This may have produced a susceptibility to extraneous tics, but also produced fast-acting tactical solutions to immediate physical problems. During periods of nonstop movement, continual foraging, and sustained vigilance, it may have been advantageous to allow subcortical motor commands to intrude into ongoing motor activities. It is clear that the engrams for individual motor responses held in the basal ganglia are selected by converging cortical and subcortical inputs. This form of convergent action selection results in the selection of the most contextually reinforced actions. Today people with Tourette's have tics that seem arbitrary and inappropriate; however, this may be due to the vast discrepancies in reinforcement between the ancestral environment and the modern one. In prehistoric environments, the motor behaviors of individuals with tic disorders may have been appropriate in environmental context, and had ecological relevance in survival and self-promotion.

Solitary mammals provide an animal model for autism spectrum disorders

Species of solitary mammals are known to exhibit specialized, neurological adaptations that prepare them to focus working memory on food procurement and survival rather than on social interaction. Solitary and nonmonogamous mammals, which do not form strong social bonds, have been documented to exhibit behaviors and biomarkers that are similar to endophenotypes in autism. Both individuals on the autism spectrum and certain solitary mammals have been reported to be low on measures of affiliative need, bodily expressiveness, bonding and attachment, direct and shared gazing, emotional engagement, conspecific recognition, partner preference, separation distress, and social approach behavior. Solitary mammals also exhibit certain biomarkers that are characteristic of autism, including diminished oxytocin and vasopressin signaling, dysregulation of the endogenous opioid system, increased Hypothalamic-pituitary-adrenal axis (HPA) activity to social encounters, and reduced HPA activity to separation and isolation. The extent of these similarities suggests that solitary mammals may offer a useful model of autism spectrum disorders and an opportunity for investigating genetic and epigenetic etiological factors. If the brain in autism can be shown to exhibit distinct homologous or homoplastic similarities to the brains of solitary animals, it will reveal that they may be central to the phenotype and should be targeted for further investigation. Research of the neurological, cellular, and molecular basis of these specializations in other mammals may provide insight for behavioral analysis, communication intervention, and psychopharmacology for autism.

Conceptualizing the autism spectrum in terms of natural selection and behavioral ecology: the solitary forager hypothesis

This article reviews etiological and comparative evidence supporting the hypothesis that some genes associated with the autism spectrum were naturally selected and represent the adaptive benefits of being cognitively suited for solitary foraging. People on the autism spectrum are conceptualized here as ecologically competent individuals that could have been adept at learning and implementing hunting and gathering skills in the ancestral environment. Upon independence from their mothers, individuals on the autism spectrum may have been psychologically predisposed toward a different life-history strategy, common among mammals and even some primates, to hunt and gather primarily on their own. Many of the behavioral and cognitive tendencies that autistic individuals exhibit are viewed here as adaptations that would have complemented a solitary lifestyle. For example, the obsessive, repetitive and systemizing tendencies in autism, which can be mistakenly applied toward activities such as block stacking today, may have been focused by hunger and thirst toward successful food procurement in the ancestral past. Both solitary mammals and autistic individuals are low on measures of gregariousness, socialization, direct gazing, eye contact, facial expression, facial recognition, emotional engagement, affiliative need and other social behaviors. The evolution of the neurological tendencies in solitary species that predispose them toward being introverted and reclusive may hold important clues for the evolution of the autism spectrum and the natural selection of autism genes. Solitary animals are thought to eschew unnecessary social contact as part of a foraging strategy often due to scarcity and wide dispersal of food in their native environments. It is thought that the human ancestral environment was often nutritionally sparse as well, and this may have driven human parties to periodically disband. Inconsistencies in group size must have led to inconsistencies in the manner in which natural selection fashioned the social minds of humans, which in turn may well be responsible for the large variation in social abilities seen in human populations. This article emphasizes that individuals on the autism spectrum may have only been partially solitary, that natural selection may have only favored subclinical autistic traits and that the most severe cases of autism may be due to assortative mating.

Conceptualizing the Autism Spectrum in Terms of Natural Selection and Behavioral Ecology: The Solitary Forager Hypothesis

This article reviews etiological and comparative evidence supporting the hypothesis that some genes associated with the autism spectrum were naturally selected and represent the adaptive benefits of being cognitively suited for solitary foraging. People on the autism spectrum are conceptualized here as ecologically competent individuals that could have been adept at learning and implementing hunting and gathering skills in the ancestral environment. Upon independence from their mothers, individuals on the autism spectrum may have been psychologically predisposed toward a different life-history strategy, common among mammals and even some primates, to hunt and gather primarily on their own. Many of the behavioral and cognitive tendencies that autistic individuals exhibit are viewed here as adaptations that would have complemented a solitary lifestyle. For example, the obsessive, repetitive and systemizing tendencies in autism, which can be mistakenly applied toward activities such as block stacking today, may have been focused by hunger and thirst toward successful food procurement in the ancestral past. Both solitary mammals and autistic individuals are low on measures of gregariousness, socialization, direct gazing, eye contact, facial expression, facial recognition, emotional engagement, affiliative need and other social behaviors. The evolution of the neurological tendencies in solitary species that predispose them toward being introverted and reclusive may hold important clues for the evolution of the autism spectrum and the natural selection of autism genes. Solitary animals are thought to eschew unnecessary social contact as part of a foraging strategy often due to scarcity and wide dispersal of food in their native environments. It is thought that the human ancestral environment was often nutritionally sparse as well, and this may have driven human parties to periodically disband. Inconsistencies in group size must have led to inconsistencies in the manner in which natural selection fashioned the social minds of humans, which in turn may well be responsible for the large variation in social abilities seen in human populations. This article emphasizes that individuals on the autism spectrum may have only been partially solitary, that natural selection may have only favored subclinical autistic traits and that the most severe cases of autism may be due to assortative mating.

Sleep apnea, respiratory cooling and thermoregulation

This article will suggest that moderate sleep apnea may have an adaptive benefit, the reduction of nocturnal, respiratory heat loss, that may have caused it to be selected for despite the associated disadvantages. Sleep apnea is caused by the collapse of the throat, specifically the pharyngeal airway, during sleep and can result in discomfort and hypoxia. The large size of the fleshy structures in the pharynx and hypotonicity of the pharyngeal musculature are what cause humans, and many other species of mammals, to be susceptible to apnea. There are many functional and anatomical parallels between sleep apnea and a well known, thermoregulatory response that minimizes respiratory cooling in the lower airways known as reflex bronchoconstriction. Bronchoconstriction involves the constriction of the inner airways, the bronchioles, in response to cold air, a reaction that acts to warm inspired air before it makes contact with the more sensitive lower airways and alveoli. Because air is colder at night and body temperature drops severely during sleep, sleep apnea may represent a similar protective, thermoregulatory adaptation. When the pharynx is collapsed, the diameter of the pharyngeal airway decreases, allowing increased intermolecular collisions between inspired air and the epithelial walls of the upper airway. An increase in the number of collisions facilitates the transference of both warmth and humidity to inspired air before it reaches the more sensitive lower airway resulting in the maintenance of internal, core temperature and alveolar heat retention. In fact, sleep apnea can be conceptualized as a functional reversal of the thermoregulatory process of panting, where the pharynx is highly dilated instead of collapsed. Given the disorder associated with sleep apnea, the absence of a disease state secondary to surgical correction, the fact that the disorder is exclusive to warm-blooded animals, the high proclivity for collapse documented in pregnant mothers, and the functional similarities between collapse and reflex bronchoconstriction, it appears that the phenomenon may represent an evolutionary trade-off that helped to minimize respiratory cooling.

Alzheimer's disease and natural cognitive aging may represent adaptive metabolism reduction programs

The present article examines several lines of converging evidence suggesting that the slow and insidious brain changes that accumulate over the lifespan, resulting in both natural cognitive aging and Alzheimer's disease (AD), represent a metabolism reduction program. A number of such adaptive programs are known to accompany aging and are thought to have decreased energy requirements for ancestral hunter-gatherers in their 30s, 40s and 50s. Foraging ability in modern hunter-gatherers declines rapidly, more than a decade before the average terminal age of 55 years. Given this, the human brain would have been a tremendous metabolic liability that must have been advantageously tempered by the early cellular and molecular changes of AD which begin to accumulate in all humans during early adulthood. Before the recent lengthening of life span, individuals in the ancestral environment died well before this metabolism reduction program resulted in clinical AD, thus there was never any selective pressure to keep adaptive changes from progressing to a maladaptive extent.Aging foragers may not have needed the same cognitive capacities as their younger counterparts because of the benefits of accumulated learning and life experience. It is known that during both childhood and adulthood metabolic rate in the brain decreases linearly with age. This trend is thought to reflect the fact that children have more to learn. AD "pathology" may be a natural continuation of this trend. It is characterized by decreasing cerebral metabolism, selective elimination of synapses and reliance on accumulating knowledge (especially implicit and procedural) over raw brain power (working memory). Over decades of subsistence, the behaviors of aging foragers became routinized, their motor movements automated and their expertise ingrained to a point where they no longer necessitated the first-rate working memory they possessed when younger and learning actively. Alzheimer changes selectively and precisely mediate an adaptation to this major life-history transition.AD symptomatology shares close similarities with deprivation syndromes in other animals including the starvation response. Both molecular and anatomical features of AD imitate brain changes that have been conceptualized as adaptive responses to low food availability in mammals and birds. Alzheimer's patients are known to express low overall metabolic rates and are genetically inclined to exhibit physiologically thrifty traits widely thought to allow mammals to subsist under conditions of nutritional scarcity. Additionally, AD is examined here in the contexts of anthropology, comparative neuroscience, evolutionary medicine, expertise, gerontology, neural Darwinism, neuroecology and the thrifty genotype.

Evolutionary neuropathology and Down syndrome: An analysis of the etiological and phenotypical characteristics of Down syndrome suggests that it may represent an adaptive response to severe maternal deprivation

This paper will suggest that the Down syndrome phenotype would have been well suited, physiologically, for a deprived environment and that it may represent a predictive, adaptive response to severe maternal deprivation. A trisomy of the 21st chromosome, prior to, or at conception is responsible for Down syndrome and is known to increase in incidence with advanced maternal age. One out of 11 mothers over the age of 50 conceives a Down syndrome baby, compared to one in one thousand at age 30. This article emphasizes that an older mother is more likely to die before she is able to provide the parental investment necessary to produce an ecologically self-sufficient offspring. Prolonged maternal investment is known to be essential for hunter-gatherers to master the skill intensive food procurement techniques that they will need in order to become independent of their mothers. Because Down syndrome individuals are much more likely to be born to older mothers, they must have been routinely deprived of maternal investment in the human environment of evolutionary adaptedness. This consistent paring of maternal deprivation to trisomy 21 conceptions, over time, may have caused natural selection to favor genes responsible for the energy conserving traits seen in modern day Down syndrome. These traits include muscle hypotonia, decreased cerebral metabolism, decreased hippocampal volume, a strong propensity for obesity and growth hormone and thyroid hormone paucity. Such a "thrifty phenotype" may have allowed Down syndrome individuals to become independent of their mothers at a far earlier age and allowed them to forgo the skill intensive ecological niche that non-trisomic humans are phenotypically suited for in order to take up a less cognitively and physically rigorous one.

Evolutionary neuropathology & congenital mental retardation: Environmental cues predictive of maternal deprivation influence the fetus to minimize cerebral metabolism in order to express bioenergetic thrift

This article will propose that humans have an adaptive vulnerability to certain forms of mental retardation, specifically, neuropathological disorders that cause decreased energy expenditure in the hippocampus and the cerebral cortex. This hypothesis will be analyzed in terms of the thrifty phenotype paradigm according to which adverse prenatal events can cause differential gene expression resulting in a phenotype that is better suited, metabolically, for a deprived environment. For example, a malnourished mother has an increased propensity to give birth to offspring that feature a "thrifty phenotype" which permits highly efficient calorie utilization, increased fat deposition and a sedentary nature. This article interprets several prenatal occurrences, including maternal malnourishment, low birth weight, multiparity, short birth interval, advanced maternal age and maternal stress--which are currently identified by the epidemiological literature as risk factors for neuropathology--to be environmental cues that communicate to the fetus that, because it will be neglected of maternal investment, developing a metabolically conservative brain will be the most effective ecological strategy. Success in hunting and foraging in mammals, primates and especially humans is known to be dependent on prolonged maternal investment. Low levels of maternal care are known to result in low survivorship of offspring, largely because the offspring are forced to subsist using simple, low-yield foraging strategies. A predictive, adaptive response, marked by cerebral hypometabolism, may produce a level of metabolic conservancy that mitigates the risks associated with low levels of maternal care. This article will suggest that certain, human neuropathological phenotypes would have been well suited for an ecological niche that closely resembled the less skill-intensive niche of our less encephalized, primate ancestors. The forms of congenital neuropathology discussed in this article do not cause damage to vital homeostatic systems; most simply decrease the size and energy expenditure of the cerebral cortex and the hippocampus, the two structures known to show plasticity during changes in ecological rigor in vertebrates. Also, many disorders that present comorbidly with neuropathology, such as tendency toward obesity, decrement in anabolic hormones, hypotonic musculature, up-regulation of the hypothalamic-pituitary-adrenal axis, and decreased thyroid output are associated with energy conservancy and the thrifty phenotype, further implicating neuropathology in an ecological strategy. Determining the relative impact of evolutionary causation on neuropathological disease should prove informative for medical and gene therapeutic treatment modalities. Furthermore, use of the maternal deprivation paradigm presented here may help researchers more precisely identify the risk factors that determine cognitive trajectory.