Evolution, Alzheimer's and Neuroecology

Why does Alzheimer's exist? Was it naturally selected? 

Click on the images below to see the posters that I created for the Alzheimer's Association's Research Update. The posters are based on an article that I wrote about the natural history of Alzheimer's disease that can be found at the following URL:


https://behavioralandbrainfunctions.biomedcentral.com/articles/10.1186/1744-9081-5-13

The article is entitled: "Alzheimer's Disease and Natural Cognitive Aging May Represent Metabolism Reduction Programs"


The paper attempts to reconceptualize the pathological changes that accompany aging. It points out that many vital areas are spared by the senile plaques and tangles and that it is predominately the brain areas associated with learning new, higher-order concepts that are burdened with neuropathological load. These changes uncanilly follow another transition, seen in early adolescence, where rapid learning is slowed down because a large proportion of what a child needs to learn has already been learned. The demands on working memory seem to diminish throughout life and alzheimers may represent a pathological extension of an adaptive age-related decline in working memory function.

 ...Oh, added 12/12/11... The work was mentioned by an excellent article in Alzheimer's and Dementia entitled: "Some evolutionary perspectives on Alzheimer's disease pathogenesis and pathology." It was a very interesting article that should be read because of its insightful take on the history and function of neuritic plaques and neurofibrillary tangles. Here is some of what they had to say about the theory:

"Reser [1] discusses the intriguing possibility that preclinical or prodromal AD itself is an adaptation, a kind of “rescue program” that allows the body to conserve resources in food-scarce environments. Many other body systems downregulate in response to low caloric intake, shunting precious calories from the least crucial areas to the most vital ones, and because the brain is a bioenergetically high-cost organ, it might be expected to do the same. Reser suggests that the parts of the brain involved in attending to and encoding new information are expendable in a natural environment once an animal reaches the age where it has learned all the skills it needs to survive. In such an environment, the loss of the least crucial brain areas in exchange for precious calories is a utilitarian trade. In modern human society, however, those brain regions (the hippocampus and higher-order association cortices) are involved in domains that we now highly value, such as higher-level executive functions, personality, working memory, and episodic memory. Also, contemporary civilization, public health initiatives, and medical advances, especially in infectious disease, cardiovascular disease, and cancer, have extended the life span far beyond what we would assume to be that of Homo in the wild, who generally would have died of predation, starvation, injury, or infectious disease. Therefore, clinical AD, in Reser’s view, is “the unnatural progression of natural brain aging changes.” Circumstances notwithstanding, apoE influences the deposition, aggregation, activity, and neurotoxicity of Abeta, and APOE e4 correlates with increased Abeta load in AD patients relative to the other alleles [16]. Next, we examine how Abeta and neurofibrillary tangles, the two signature pathologies of AD, fit into this integrated evolutionary picture and how evolutionarily informed research should approach them."

The “Solitary Forager” Hypothesis of Autism: www.solitaryforager.com

I created a new web page recently to post my “theory of autism.” The theory attempts to explain why autism exists by elucidating its evolutionary history. Consistent with Simon Baron-Cohen’s systemizing theory of autism and with Temple Grandin’s positions, I see most autistic individuals as intelligent and autism as having compensatory advantages. Clearly, sometimes autism is accompanied by disease states, but the psychological "symptoms" of autism are seen here as attributes that would have benefitted a solitary forager (see the table at the bottom).


"To the extent that neurotypical individuals can be thought of as social foragers, autistic individuals can be thought of as solitary foragers."


There has been a lot of recent speculation and controversy over autism and what its existence "means.” A number of theoretical articles have been written on the subject such as the “Neanderthal theory of autism,” the “neoteny theory of autism,” the “extreme male brain theory,” the “imprinted brain theory” and the argument that autism is the “next stage in human evolution.” These are all interesting interpretations some of which may have some explanatory utility. I think that the “solitary forager hypothesis,” however; explains autism from the perspective of natural history.


Please take a look at:  www.epjournal.net/filestore/EP09207238.pdf


You can also find an early and unabbreviated version here: http://www.solitaryforager.com/
Abstract:This article reviews etiological and comparative evidence supporting the hypothesis that some heritable genes associated with the autism spectrum were naturally selected and represent the adaptive benefits of being cognitively suited for solitary foraging. The systemizing theory of autism is extended here and people on the autism spectrum are conceptualized 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, young autistic individuals 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. 


Individuals on the autism spectrum share a variety of behavioral traits with solitary species. Both solitary mammals and autistic individuals are low on measures of gregariousness, socialization, direct gazing, eye contact, facial expression, 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 social contact as part of a foraging strategy often due to scarcity and wide dispersal of food in their native environments. Similarly, it is known that, due to frequent and prolonged dry spells, the human ancestral environment was often nutritionally scarce 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. This solitary forager hypothesis of autism is explored in the context of
anthropology, comparative neuroscience, epidemiology, evolutionary biology, neuroethology, and primatology.

 

Table 1: Behavior in autism, then and now

Trait or Symptom	Psychological  Consequences	Implications for Moderns	Implications for Solitary Foragers
High systemizing ability	A tendency to systematically explore the laws governing nonsocial processes	Eccentric or narrow but substantial knowledge and skills	An impetus guiding the acquisition of food procurement techniques
Obsessive, repetitious tendencies	Perseveration in behavior and thought	Repetitious play and need for sameness	Order, structure and autonomous self-regulation
Gaze aversion and absence of shared eye contact	Minimal eye contact and diminished attention to the faces of others	Unfortunate social hurdle	Instinctually prepared not to challenge or provoke conspecifics
Low oxytocin	Reduced social interest, learning and expressiveness	Unfortunately hindered social cognition	Programmed for a socially impoverished environment
Anomalies in anterior cingulate cortex, orbito and medial frontal cortex	Reduced social learning, capacity for empathy and affiliative need	Hindered social integration	Decreased reliance on others
Amygdala hyperactivity	Potentiation of innate and conditioned fears	Excessive anxiety and withdrawal from social world	Healthy caution, and fear of unfamiliar conspecifics
Hippocampal hyperactivity	Hyperaccessibility of contextual and episodic information	Proficiency with spatial tasks and contextual memory	Skill in tests of spatial intelligence