An important recent question for anthropologists and evolutionary biologists is why the human brain has shrunk over the last 20,000 years. In the last 20,000 years the average volume of the human brain has decreased from 1,500 cubic centimeters (cc) to 1,350 cc1,2 and based on existing evidence appears to be conserved throughout the globe (North America, Asia, Africa, etc.).2 Despite the relative speed of the shrinking, this issue has not received much attention. This lack of attention can be explained through two different rationalities. First, most individuals understand that evolution proceeds at a crawl and by the time any shrinkage becomes relevant from the standpoint of potential significant intelligence loss those living today will have died long, long ago thus there is little reason to work on such an issue. Second, a number of individuals believed that the encephalization quotient (EQ) indicates the shrinking isn’t anything to worry about in the present or the future.
EQ looks at the ratio between brain volume to body mass. Cro-Magnons from approximately 20,000-25,000 years ago, the species that is widely considered to have the largest brains, also had large bodies and even jutting jaws with large fang-like teeth.3 The general understanding is that such a pairing makes sense because the larger an individual’s body the more neurons are required to control the various and larger parts (muscle, etc.), thus the larger the brain. When looking at modern man the overall body type is smaller, thus following the above understanding it is not surprising that the brain of modern man is also smaller. One piece of early evidence that seems to support such a contention is that the EQ for both Cro-Magnons and modern man were calculated to be very similar.3,4
While supporters may want to close the book on the issue based on that evidence they is a large problem. Anthropologists narrowing the time range of study have drawn the conclusion that the brain shrank more quickly than the body. This is an important distinction because it makes little evolutionary sense for the brain to shrink before the body because such action would lead to the abandonment of physical elements prior to their supposed lack of need. Actually the exactly opposite should be favored, if near simultaneous change did not occur, as the body changes and certain physical features depress the areas in the brain responsible for controlling those areas would also change, typically dying (shrinking in area) in response.
Another problem occurs when looking at research from the Bronze Age. Some research has demonstrated that from the Bronze Age to Medieval Times (4,000 years ago to 600 years ago) the brain shrank much more quickly than the body among other problems with timing correlations.1,5 The shrinkage was so great that the researchers drew an analogy that for the EQ of modern man to be equal to that of Cro-Magnon the body of modern man needs to be the size of a pygmy.1 Thus somebody is not correct pertaining to the equality of modern vs. Cro-Magnon man. If EQ theory is not correct than brain shrinkage cannot be dismissed so easily.
Assume for the sake of argument EQ values of Cro-Magnon and modern man are not equal or even close. Such a conclusion would mean that the human brain is shrinking faster than the human body in a potentially unnatural way. What could be the cause of such a biological change? Researchers believe that the three most probable possibilities are: climate change, brain die-off or assimilation into structured agrarian societies.
The idea that climate change influenced brain size stems directly from how the body manages heat. Bulky bodies are better at conserving heat, thus in the colder climates of 20,000+ years ago it would be a selective advantage to have such an attribute (less likely to die from the cold). However, as the temperature rose leading into modern times the selective advantage to having these larger frames fell allowing for greater survival rates of smaller bodied individuals. There are some problems with this mindset. First, it is difficult to conclude that the selective advantage of a bulky body would disintegrate over the course of approximately 20,000 years with such a small overall surface temperature shift. What happened to the selective advantage? Did bulky bodied individuals start dying of heat stroke or did non-bulky bodied individuals have an inherent advantage that was somehow derailed by the cold because most were unable to adapt to it?
Another possible problem for this theory as the driver of brain shrinkage is that the brain shrunk faster than the body leading to modern times, but based on the fact that a majority of the heat loss for a human occurs through the head it is somewhat confusing to conclude that as temperatures increased evolution would lead to a smaller more heat retaining head. Also such shrinkage behavior did not trend with other warming periods from 2 million+ years ago to 20,000 years; instead both body and brain sizes continued to increase.1
The second possible driver for head/brain shrinkage is humans are getting stupider, thus they do not need as many neurons as their ancestral brethren. The theory behind the ‘stupification’ of the human species relates largely to population density and the formation of human societies. Researchers identified a general global trend in that when the global population density was low, as it was during a vast majority of human evolution due to behavior of nomadic tribes, the cranium (and assumed brain) continued to grow. However, once population densities started to significantly increase cranial size declined significantly starting from about 15,000 to 10,000 years ago.6 The principle reason behind this decline is attributed to specialization displacing generalization.
In a society of individuals that was both larger and did not have a large turnover rate (decades vs. weeks) more individuals could focus on a specialized area of study forsaking a more broad structure of knowledge. Basically for nomads one may encounter someone who can make fire during travel, but that encounter will be short-lived with a low probability of recurrence in the near future; add to that the higher probability of death and it made sense that almost everyone had to know how to make fire. As populations settled safety and life expectancies rose as well as meeting occurrence a number of individuals no longer had to know how to make fire because ‘John the guy who could make fire’ lived four huts away. Thus, movement away from general experience and knowledge developed neuronal growth in only specialized regions in the brain resulting in overall shrinkage.
Another supporting factor may be routine. Specialization in most fields leads to a followed and generic thinking routine, which could also reduce the probability of brain growth and/or increase brain shrinkage due to degradation of neuronal connections. Basically an administration of the physiological concept of ‘use it or lose it’ where specialization could lead to erosion in certain areas of the brain while expanding network connections in more limited regions. Some parts of the brain contract where a smaller overall brain area is enhanced. Post-tetanic potentiation could also hasten the development of this new brain architecture and lead to gene expression that could be transferred to future descendants favoring certain types of specialization.
While modern individuals may have less overall intelligence than their ancestors, specialization allows for the development of new ideas that those in the past would more than likely not be able to derive. The survival support developed within more permanent societies over nomadic tribes allow for those individuals with innate talents in certain interests to develop those interestes without devoting time and resources to survival or other areas. Due to brain volume having a correlation with intelligence7 in a lot of respects due to this support structure one would say that in the past humans were smarter individuals in dumber societies whereas modern humans are stupider individuals in smarter societies.
The third possible driver for brain shrinkage is a changing diet. Some believe that the advent of agriculture and the rise of the agrarian society created a dietary shift from a mixture of fruits and meats to a more grain and vegetable heavy diet supplemented by fruits and meat. Some researchers believe that this shift in consumption patterns lead to scenarios of protein shortage or even starvation due to ineffective farming techniques, which could have resulted in malnutrition and a corresponding loss of brain size.1,8 The problem with this contention is that it is difficult to reconcile psychologically. Nomads that gained sustenance on meat and fruit in the recent past would not immediately give up such options upon developing a permanent settlement especially if the farming success rate was not high and somewhat random. It is difficult to reason that a new settlement would rather sit and starve in the event of a poor harvest over organizing a hunting party in an attempt to gather meat and pre-existing plant matter.
While malnutrition may or may not be the answer, the change in diet derived from a nomadic group becoming agrarian may have driven the change in another way. As discussed above while meat and fruit would still be consumed, various grains and vegetables would be consumed in much larger quantities than in the past. Brain function progresses along two different pathways: electrical and chemical. The chemical pathway is governed by the release and binding of molecules referred to as neurotransmitters. Neurotransmitter production is largely based on the particular precursors, typically amino acids, which are derived from food. Thus a changing diet would change the amount and type of neurotransmitters synthesized in the brain. Higher neurotransmitter concentrations would lead to higher receptor expression for those particular neurotransmitter and elimination of neuronal connections that do not express those receptors.
This change in neurotransmitter concentration may also explain some of the physical changes and the reason for the delay between brain shrinkage and body shrinkage. Meats are large sources of precursors for both acetylcholine and tyrosine. Acetylcholine is the neurotransmitter most responsible neuron-muscle interaction and both acetylcholine and tyrosine (along with its developmental product dopamine) are thought to aid brain development in the lines of intelligence. Grains and vegetables contain smaller amounts of these precursors, which would more than likely reduce the total concentration of acetylcholine and tyrosine in the body at a relative level, which could affect muscle action, muscle and brain growth and maintenance. Food source stability should have also increased total concentration of neurotransmitters on an absolute level, which could change neurotransmitter receptor expression and neuronal connections. In fact some anecdotal evidence could support this conclusion. During the period of brain shrinkage in the past DNA responsible for brain development and neurotransmitter networking appears to go through numerous adaptive mutations and changes.9,10
Beyond adaptive mutations how could changing neurotransmitter concentrations and type affect brain size? One possibility is through readjustment of synaptic spacing due to changes in post synaptic axonal and pre-synaptic dendritic lengths. For example changing diets could lead to more variety of neurotransmitter synthesis and an expansion in overall neurotransmitter concentrations. This concentration increase would drive a greater level of neurotransmitter receptor expression on pre-synaptic dendrites. A sudden expansion of receptor concentration on static dendrites could lead to inappropriate levels of receptor saturation which would induce dendrite growth towards post-synaptic axons in effort to reduce receptor density and increase neuronal efficiency. If dendritic growth occurred in such a way it could crowd the synaptic cleft leading to axon regression.
A change in diet could further play a role in this axon regression due to instability. As mentioned above it is difficult to think that permanent settlements would result in less food vs. a nomadic group due to nomadic strategies still being available; however, due to limited farming experience the certainty of crop number would be in question changing the available food supply at any given harvest cycle. For example a nomadic tribe may average 1300 calories a day with a low standard deviation. Upon settlement food consumption would become more unstable, but with a higher minimum range like 1350 to 1900 calories a day. This unpredictable calorie range could drop neurotransmitter concentrations over a long enough period of time where some expanded dendrites would retract due to lack of extended neuronal activity perhaps creating more void space in the brain resulting in overall shrinkage.
On a side note it is interesting to question whether or not the shrinking brain and body have anything to do with one another. The time separation between their changes in size could be due to slow evolutionary change driven by multiple breeding generations and slow moving adaptive mutations or could have different causes. For example body size could have changed due to the reduced probability of death brought on due to reduced reliance on wild animals for food. Thus, smaller bodied individuals were less likely to be killed by wild animals increasing their survival and breeding probabilities. So one could argue that societal formations and increasing population densities affected both body and brain size, but neither one directly affected the other.
One of the more popular areas regarding is brain shrinkage is domestication and how it has lead to a selection against aggression. Rates of aggression are viewed to correlate well with brain size in a general manner; basically the larger the brain the greater the probability that a particular person will be aggressive. One of the biggest elements of support for the domestication influence argument is that of all the non-human animals that have been domesticated on a significant level each species has demonstrated a reduction in brain volume in those that have been domesticated (over suitable generations) vs. their feral brethren.11,12 Not surprisingly domesticated animals have more flatter faces, smaller and more concave teeth, gracile builds and in the case of some floppy ears and curly tails.1,11,12
Some believe that this domestication methodology also works because any required mutations are more straightforward than potential neurotransmitter or neuronal derived mutations.1 This mutation ease comes from the theory that the easiest way to naturally select for a reduced aggressiveness is to slow brain development, so fully grown animals have more juvenile brains, a feat that can be accomplished with changes to a small number of regulatory genes. A more juvenile brain would probably lead to a reduction in survival instincts, which for most animals rely on aggression possibly stemming from fight or flight.
One common piece of evidence that most proponents of domestication point to is the work of Dmitri Belyaev with silver foxes. In 1958 Dmitri Belyaev started raising silver foxes in captivity and began selecting for the animals that took the most time to snarl at humans (i.e. least aggressive). After about 12 generations the foxes began to demonstrate physical and psychological traits associated with domestication including smaller craniums (and presumed smaller brains).13 The problem with this experiment is that the foxes were directly selected for lack of aggression, leaving the lingering question of how is lack of aggression naturally selected for in non-deterministic environments. Three possibilities come to mind.
First, perhaps ‘domesticated traits’ including less aggression are actually genetic dominant, but in the wild these less aggressive individuals are killed in greater percentages before they are able to breed. Domestication vastly reduces the threat of death allowing more of these individuals to breed creating a greater overall percentage of non-aggressive individuals over aggressive individuals. Second, early in the domestication process more aggressive individuals were killed, viewed as a threat to the rest of the herd leaving only non-aggressive animals.
An alternative option in this ‘selective kill’ vein for less overall aggressive animals is that animals with domesticated traits appear to be ‘cuter’ than those with wild traits and for humans cute is always better than ugly, so the ugly ones (more wild) would have a higher rate of selection for death. Third, domestication was selected indirectly due to opportunity reduction/routine changes in the brain. Domesticated animals have less living options (things to do with their time) vs. their feral brothers and sisters, especially when it comes to different opportunities of action engagement and feeding themselves, thus it would be easier for domesticated animals to develop brain alterations similar to those mentioned above with respects to generality/specialization which would shrink the brain creating less aggressive tendencies.
The above discussion of domestication was largely assigned towards non-humans, so how does the domestication theory apply to humans? The most understandable argument is that the advent of permanent societies reduced the need for individual aggression in favor of unified or group aggression as a means for protection. With this change individual aggression received less of a ‘free-pass’ when violating the norms of society and needed to be punished. Think of it like a superstar football player having a different set of disciplinary rules over a journeyman veteran.
While the application of the death penalty did occur in hunter-gather societies it is reasonable to believe that without the need for aggressive individuals its application increased during the initial stages of developing and expanding permanent societies. This application could provide support for domestication being the driver for brain reduction. However, there is a problem in that from a standpoint of population proportions the number of individuals put to death due to ‘overly aggressive’ behavior does not even come close to that which is required to have any significant influence on selectivity, especially in more modern times (1100s and beyond) as killing moved from brute force with fists, club or sword to simply pulling the twine on a bow or trigger on a gun.
Expanding the death penalty argument the inclusion of incarcerated individuals may help stem the shortfall in population selection. Clearly people in prison have a lower probability of passing on genetic material vs. free individuals. However, the problem with this expansion is that most of the aggressive crimes throughout history do not result in complete exile from the population only temporary exile which while may interfere with breeding does not eliminate it. Thus the question falls to self-suppression and how it may influence genetic behavior.
Suppose for a moment that most potential aggressive behavior in society is prevented not through a sense of morality or ‘good-citizenship’ but through fear of consequences. To wit if an individual were told that he/she could take an action and not receive any detrimental consequence for that action then that individual a vast majority of the time would take that action. Therefore, the question is does an aggressive individual not acting aggressively in open society lead to a genetic change in that individual for more inherent passive behavior? The individual in question is not less aggressive in his/her mindset, but is simply refraining from acting out that aggression in public. The answer to this question should be an important issue in whether domestication influenced brain shrinkage or if brain shrinkage influenced domestication.
One lingering question with regards to brain shrinkage is whether or not Cro-Magnons were actually smarter than modern man. Most researchers seem to shy away from answering this question with a mixture of uncertainty and nervousness deferring to an almost stock answer of ‘not smarter, but think differently’. One of the methodologies for judging this change comes from work comparing the cognitive abilities of wolves and dogs. Wolves, with larger brains, have a higher probability of solving problems without assistance both through perseverance and insight vs. dogs that are better able to mimic then learn from someone or something that already knows the solution to the problem.14 From this research one could assign that wolves are better imaginatory/creative learners while dogs are better visual learners. The ability of wolves to better solve new problems than dogs does support the idea that wolves are more intellgent than their domesticated breathren.
With respect to humans the issue swings around this type of learning to develop and expand different subjects. For what can be assumed about specialization it appears that basic to intermediate understandings would be easier for larger brained individuals where complex understandings would be easier for individuals with specialization in that given understanding; the issue is that a smaller brain may not be inherently required for a complex understanding in a given field. So there may not be any inherent advantage to having a small brain, outside of energy concerns, for there is no unique architecture that only a small brain could support over a large brain. Thus, individuals in the past had more raw intelligence than those in modern day.
Another interesting development in the issue of brain size is that some reports have brain size in very modern times (last 200-250 years) rebounding and growing again.1 So assuming for the moment that these conclusions are correct what is driving this brain resurgence? Some conclude that this increase is derived from continued evolution of greater food security. For example even in the 1400s famines were still somewhat common and people with unusually large brains would have had a higher probability of death due to the greater energy demands made by those brains over smaller brains. Recall that the brain consumes a disproportionate amount of energy relative to its size. Due to the apparent speed associated with the change in brain sizes (only hundreds of years not ten thousands of years) it is difficult to conclude that there is an underlying genetic component, thus changes in nutrition is certainly a candidate.
Unfortunately the nutrition explanation does run into a stumbling block from the past. From above it was generally thought that food security increased over time, only sometimes dipping due to rapid population increases, after the formation of agrarian societies over nomadic hunting tribes; however, there appears to be a contradiction, in the past when food security increased brain sizes decreased, but now in modern times increasing food security leads to an increase in brain size. If one accepts the neurotransmitter argument presented above as a factor in the shrinking brain then one could argue that this contradiction is moot because in modern times there is a greater variety of food available over just sheer amount, thus there would be more of a balance of neurotransmitters.
Another option that could explain the increase in brain size is the introduction of modern education. Potentially the greatest invention ever, the printing press, was developed in 1440 by Gutenberg which allowed non-aristocratic children and adults the opportunity to learn a wide variety of subjects. Before the printing press most education, if children actually received one vs. simply working in the fields, was specialized towards a given single profession where the child would typically enter apprenticeship at 12-14 years of age and devote all of his/her time to learning that trade. However, modern education brokered by the printing press created a tiered structure of learning where general knowledge was taught first and students were exposed too much more information, similar to nomadic tribes, and then students who were able could advance further into a more specialized field. This general education, which really began to permeate society in the mid 18th century, could demand brains to grow larger than those only exposed to specialization. The exposure to this ‘new’ generalized method of instruction could provide an explanation for the small rebound to brain size in very modern times.
Overall while the brain has shrunk considerable over the last 20,000 or so years if very recent growth trends are to be believed there is hope that expansion of general education and general problem-solving techniques can put humans back on the course of expanding their brains. The key seems to be maintaining a challenge while not falling into routine. For example even though some would say that doing crosswords expands the brain doing hundreds+ seems to strip their positive influence due to routine thought application. Thus humans must focus on learning in new methodologies in order to continue any reversal of brain loss in the future.
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