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Homo sapiens first appeared in sub-Saharan Africa perhaps 250,000 years ago. There is no evidence that humans have changed significantly for perhaps 70,000 years when their skull capacity reached modern levels, therefore discussion of hunter-gatherers has contemporary relevance.

They probably lived in groupings averaging approximately 50 individuals. Their diet was largely animal based. Large piles of smashed bones from large grazing animals found near settlements of hunter-gatherers along with tools used to crack them suggests that their bone marrow was the staple food of human hunter-gatherers. Analysis of bone from these piles indicate they consisted of a selection from the animal skeleton with the thickest cortex which no animal could crush without tools, therefore only humans could exploit this resource, therefore they were largely scavengers living off leftovers that more able predators could not use.

Natural selection has provided humans with low satiation which leads to obesity if nutrients are abundant. This adaptation is present in a minority of primates all of which experience seasonal nutritional scarcity which requires reliance on adiposity as energy reserves for survival. Decline in mobility from obesity had less selective disadvantage to human hunter-gatherers than death from periodic starvation.

Harvesting of marrow involved finding the abandoned animal carcass, separating target bones from it, transporting them to a central location for cracking and extraction of marrow. All but the last step was mobility intensive. Mobility is used here to signify all forms horizontal displacement of the human center of mass when weight bearing, which occurs when walking, running, and even maintaining stable equilibrium which is also relatively energy intensive. When nutrients were in abundance the life of human hunter-gatherers was a Sisyphean attempt to satisfy their unrelenting hunger. 

To actuate the intense mobility essential to reach adequate adiposity, humans and other animals rely on addiction to endogenous opioids that are secreted in relation to mobility amplitude. Addiction to either exogenous or endogenous substances is used here to indicate a gradual tolerance from repeated exposure to a psychoactive substance which produces either positive emotions or reduction of negative sensations such as pain, or both, and an intensely dysphoric abstinence syndrome in response to a sudden decline in substance exposure that encourages continued presence of these substances. Endorphin is a word created by combining “endogenous” and “morphine.” It signifies groups of peptides produced by humans and other animals that are similar to morphine in that they can strongly control behavior through their addictiveness and have some of the physiological effects of morphine. In high concentrations they raise pain thresholds and produce a state of euphoria, whereas in extremely low blood concentration humans and other animals become anxious and depressed. This may represent an abstinence syndrome when low endorphin levels are reached or another adaptation that encourages amplified mobility by another means.

Endorphins are released in response by many stimuli but most predictably and intensely in relation to prolonged mobility of moderate intensity. This suggests that endorphins amplified mobility in hunter-gatherers both by euphoria and high pain thresholds when intensely mobile and a dysphoric withdrawal syndrome and pain when mobility attenuates. The endorphin system was probably the intense mobility actuator that was needed to raise food acquisition behavior to the point of adiposity.

Endorphins aided existence of hunter-gatherers in other ways. Hunter-gatherers not only survived but thrived considering they increased their numbers while living unclothed, relatively unprotected from the elements and essentially incapable of understanding the world around them, which by contemporary standards would be extremely physically and psychologically harsh conditions. This was made tolerable because high endorphin levels raised pain thresholds and left them relatively euphoric.

Endorphins also likely helped to maintain high fertility rates in hunter-gatherers. High infant mortality, substantial maternal mortality, predators, periodic starvation and no health care as we know it made it essential that hunter gatherers maintained near optimal reproductive rates where every ovulating female soon became pregnant. Current low fertility rates would have been incompatible with survival of human hunter-gatherers.

The relation between human emotional states and fertility in humans is so strong that fertility rates can be used as a surrogate measure of the emotions of a population and vice-versa. Anxiety and depression suppress fertility whereas positive emotional states amplify it. The euphoric state brought about by high endorphin levels probably amplified fertility well above rates seen today in rich countries. The somewhat euphoric state of hunter-gatherers likely promoted fertility. The high level anxiety and depression in contemporary societies probably accounted for by low levels of endorphins brought about by limited mobility. It is probably responsible for the constantly declining fertility in the past century and attenuation in sexual behavior for the past 40 years since it first was reported.

The relative euphoric state of hunter-gatherers came at a cost. High endorphins produced a cognitive state that probably resembled contemporary humans under the influence of a substantial dose of morphine, with tranquility and relative euphoria dulling concentration and complex reasoning. "Contemplation" is referred here to the mental state in which humans are inquisitive and capable of maintaining prolonged, complex mental constructs. This is essential in both perceiving patterns of natural events and the using symbols systematically as is required in written language. Life of the hunter-gatherer would have been pleasant in terms of  modest anxiety and physical discomfort by current standards despite unpredictability due to little accumulated knowledge and environmental stress on an unclothed body. But it also  would have been robotic by current standards with minimal intellectual curiosity nor pleasure though innovative solutions. This explains why there is no evidence that over the 250,000 years humans have existed prior to civilizations, there was never a useful written language that allowed them to accumulate knowledge, nor was there more than the most meagre advances in technology.

This suddenly changed with civilizations. perhaps six-thousand years ago when human gave up their nomadic existence, which caused a sharp decline in mobility which resulted  in plummeting endorphin levels. Humans were freed to contemplate for the first time in their history as far as we know. Essentially the totality of human intellectual achievements and technological innovations commence at this recent point in the human story. It is not certain  why civilizations came about. Most think that high densities of human hunter-gatherers in sub-Saharan Africa forced them continually to territory without large animal population and able predators which formed the basis of their nutrition. This probably lead to attempts to use plants out of necessity as a nutritional staple. There is near universal agreement that the rise of civilizations was brought about by agronomy, particularly the growing of grains for nutritional purposes.

With civilizations came diseases from relatively large numbers of humans living in close quarters. Bacteria, viruses and parasites mainly from human-to-human transmission and through contaminated water must have ravaged civilizations since natural selection never provided them with resistance to these threats. New stresses emanated from their change in diet. Human gut anatomy and physiology and endocrine system is so poorly adapted to a diet rich in carbohydrates and cellulose, and ethanol which made its presence with grain, which remain a major source of ill health in contemporary humans. However the selective advantage of being intellectually unrestrained was greater than these disadvantages, and civilizations multiplied. Written language allowed the accumulation of knowledge. Bereft of high endorphins, humans experienced lower pain thresholds which required innovations such as clothing and shelter to overcome. These innovations allowed exploitation of regions in which they never could have existed previously even with high the high pain threshold associated with abundant endorphins.

The change to civilizations was so rapid and recent that it may be useful to visualize contemporary humans as not fully adapted to elements of modern life because they retain essentially the body and brain of the hunter-gatherer. The relatively low and constantly declining fertility in contemporary society may be caused by by amplified anxiety and depression in due to low endorphins levels from low mobility or another adaptation that promotes mobility. Increase in mobility sufficient to raise endorphin levels to change overall emotional seems incompatible with life in a modern industrial state. Even most competitive runners would not be mobile enough.

Perhaps many current social trends, such as the non-medical use of anxiety lowering and euphoria producing drugs and dependance on ethanol are actually a form of self-medication to escape unrelenting anxiety and depressiveness associated with limited mobility. Modestly lowering anxiety and depression in a society may have substantial positive effects on fertility. The present low pain thresholds due to extremely low endorphins helps to explain the persistent state of discomfort in contemporary humans which has lead to obsessive searching for the "perfect" footwear, clothing and bedding, and living at narrow temperature range indoors through heating and air conditioning, and outdoors via a selection of clothes for seasonal variations in temperature.

Perhaps antidepressant medications should be in more widespread use. Antidepressant medications have been used for a half century for management of major depressive disorders. At least the current generation of these medications successfully relieve mild to moderate chronic anxiety and depression that may be below levels typically classified as a mental disorder, a condition that is so prevalent in rich countries that some have considered it universal. Antidepressants are not capable of inducing euphoria except with bipolar disorders, therefore they do not function through raising endogenous opioids. They seem to block the dysphoric state associated with low endorphin levels. They may also block somewhat the euphoric state associated with exogenous euphoria producing substances considering that many individuals addicted to these substances report less euphoria with their use.

Obesity is mistakingly considered a disease rather an adaptation necessary for survival in human hunter-gatherer humans which adversely affects health in the contemporary context. Effective and safe pharmaceutical control of appetite to overcome low satiation inherent to humans is currently unavailable but when it is, it should be considered neutralize this now harmful adaptation.

Glorifying human existence as hunter-gatherers is in fashion. This seems inappropriate even though humans are probably best adapted to this condition.  As stated previously, hunter-gatherers probably lived a robotic life by current standards driven by endorphins but also dulled by them. With lower endorphins humans are capable of achieving longevity well beyond that of human hunter gatherers. Humans appear to gain pleasure from the use of their intellect. This is impossible with high endorphins. However to gain the full benefits of the human intellect there is a need to moderate anxiety and depressiveness in the population.