Brain and Evolution

Topics

• Biological fitness and the reason for brains
• Body size and brain size across species: allometric relationships
• Hominin brain evolution
• What factors facilitate and what factors constrain brain size
  • These include hypotheses concerned with ecology and diet, cognitive buffering, sexual selection, bipedalism, climate change, altricial birth, and the cognitive demands of living in social groups.
• Relating brain size to function: morbidity, memory, intelligence.
• Are human brains special?

Preparation for class

The topic of the first lecture is not covered in Purves Neuroscience 6th edition. I have therefore assigned a paper by Dunbar entitled The social brain hypothesis and its implications for social evolution. This will introduce some of the issues I will discuss in lecture. In reading this paper, do not concern yourself with details, but rather get a gist of the main points.

I have linked the Dunbar article here and have also uploaded it into the resources section of Canvas.

Topic slide

Alison Jolly (1937-2014) was a primatologist and conservationist who received her Ph.D. from Yale University in 1962. She worked primarily with lemurs on Madagascar, and was perhaps the first to suggest that social interactions were a driving force for the evolution of primate intelligence. She wrote several books including "Lucy’s Legacy: Sex and Intelligence in Human Evolution". Interested students can read her NY Times obituary and a remembrance from the Duke Lemur Center.

Robin Dunbar (b. 1947) is a professor at Oxford University and Director of the Institute of Cognitive and Evolutionary Anthropology. Dunbar built upon the work of Alison Jolly, Nicholas Humphrey, Byrne and Whiten, and others to develop the Social Brain Hypothesis. This hypothesis asserts that it was the evolutionary pressure to keep track of the social interactions among individuals within a group that provided the fitness advantage for larger brains.

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Evolutionary perspective for brains

Evolutionary biologist Theodosius Dobzhansky (1900-1975) famously opined:

Nothing in Biology Makes Sense Except in the Light of Evolution.

Seen in the light of evolution, biology is, perhaps, intellectually the most satisfying and inspiring science. Without that light it becomes a pile of sundry facts — some of them interesting or curious but making no meaningful picture as a whole.

Evolution provides a perspective for understanding the brain. We should consider what ‘problem’ is solved by having a brain, and how does having a brain, or a larger brain, improve biological fitness.

Biological fitness refers to reproductive success; i.e., an organism passing its genes on to another generation. From the perspective of evolution, this is the only meaningful outcome measure of the success of an organism. Of course biological fitness presupposes that organism’s survival — at least long enough for that organism to reproduce. So the abilities to obtain nutrients and escape predation are generally associated with improved biological fitness. However, when there is a conflict between survival and reproductive success, the latter dominates. Indeed, in many species, mating is fatal (typically for the male).

We will discuss evolution throughout the semester, and we will examine the evolution with respect to function. But we should be careful not to think of changes in brain over the course of evolution as for a particular function. Evolution does not plan or anticipate. Rather, we will consider the environmental pressures experienced by individual members within a species that might select for particular structural/functional changes should those changes occur.

Taxonomies

Although not a recurring theme in this class, I will occasionally make reference to taxonomic nomenclature for life forms. For convenient reference (for you and for me), the standard hierarchy is presented below:

• Kingdom
• Phylum
• Class
• Order
• Family
• Genus
• Species

There are subclassifications and sub-subclassifications for many of these descriptors. In the example below for humans, these are indented.

• Kingdom (Animalia)
• Phylum (Chordata)
  • Subphylum (Craniata; i.e., Vertebrata)
• Class (Mammalia)
• Order (Primates)
  • Suborder (Haplorhini)
    • Infraorder (Simiiformes)
• Family (Hominidae)
  • Subfamily (Homininae)
    • Tribe (Hominini)
• Genus (Homo)
• Species (Sapiens)
  • Subspecies (Sapiens)

So, we see that a human is a chordate, a vertebrate, a mammal, a primate, and a hominid (Great Apes). We are also the only living species of the hominins, and the only living species of the genus Homo. As we will see below, this was not always the case.

In a later lecture, we will discuss the commonalities of the vertebrate brain plan. For future reference, vertebrates are subdivided into five major classes:

• fishes
• amphibians
• reptiles
• birds
• mammals

Genotypes and phenotypes

Let’s review the distinction between genotypes and phenotypes. Changes in an organism’s DNA or genes (its genotype) can cause changes the organism’s structure or morphology (its phenotype) which is reflected in function and behavior. Mutations are changes in an organism’s DNA and can occur for a number of reasons such as exposure to radiation, exposure to mutagenic chemicals, DNA copying errors, and viral infections.

Imagine that a genetic mutation within some individuals of a species causes the size of the brain to be slightly larger. As a consequence, the species would exhibit variation in brain size. If having a larger brain confers an advantage that enables the larger-brained individuals to survive and reproduce more successfully than smaller-brained individuals, then those larger-brained individuals will be more successful in passing their genes to progeny (i.e., it would improve biological fitness). In that case, the percentage of the population with larger brains would increase. However, if the larger brain required more energy and nutrients, then individuals with a larger brain might be at a disadvantage and thus the percentage of individuals with a larger brain might decrease in the population. Having a larger brain may not confer any advantage to individuals within the species until and unless there is an environmental challenge that makes having a larger brain beneficial.

Why have a brain?

The single-cell amoeba has no nervous system or brain. Nevertheless, it responds to stimuli and moves adaptively. Its surface membrane is in direct contact with its environment and its cell membrane responds to signals from the environment. Thus, there is no need to create an internal representation of the external world.

Adaptive movement

Adaptive movement is a key advantage provided by a brain. Adaptive movement can assist and organism in finding a mate, finding food, and escaping predation. Adaptive movement connects external sensory events with motor commands to move towards sensory events that are rewarding (such as food sources), and away from sensory events that are punishing (such as predators).

Cost of brain?

Let’s consider a simple example. The sea squirt, a marine member of the Phylum Chordata. Although an invertebrate, in its larval stage it has a notochord that resembles a spinal cord and a collection of neurons (a neural ganglion) that constitute a primitive nervous system. Sea squirt larvae are ‘tadpole-like’ and they swim adaptively to find an anchoring spot. However, once anchored, the sea squirt becomes sessile and absorbs its proto-nervous system. An adult sea squirt subsists by drawing water containing microscopic food sources through its internal chambers. It is hermaphroditic and produces both sperm and egg which are both released into seawater. The sea squirt can also reproduce asexually by budding.

Thus, once adaptive movement is no longer required and an anchoring point is found, the sea squirt ganglionic ‘brain’ becomes expendable. This implies that there is a cost to having a brain. Brains consume energy, and devoting a portion of the limited supply of energy available to an organism to a brain without a corresponding increase in biological fitness is not adaptive but rather maladaptive. We will consider the energy costs of the brain further below.

Is the brain entirely devoted to sensation and movement?

As we will learn in detail, the brain has regions devoted to receiving and integrating sensations from the environment, and regions devoted to planning and organizing movements. For example, humans have ~ 20 square feet of skin sending myriad sensory signals to the brain and ~650 named skeletal muscles to control. However, across mammals, the proportion of cortex devoted to primary sensory and motor processes diminishes as one moves from simpler to more complex species. I illustrated this progression in lecture with a slide showing the proportion of the brain devoted to primary sensory and motor functions from hedgehogs to prosimians to humans.

Why have a larger brain?

Brains vary greatly in size – humans have a big brain, but we don’t have the absolute biggest brain, or the most convoluted brain (i.e., by folding the brain's surface, a greater surface can be fit into a given volume. Think of taking a piece of tissue and crumpling it into a smaller space).

Nicholas Humphrey (b. 1943) is a psychologist with a strong evolutionary perspective. He wrote:

Nature is surely at least as careful an economist as Henry Ford. It is not her habit to tolerate needless extravagance in the animals on her production lines: superfluous capacity is trimmed back, new capacity added only as and when it is needed. We do not expect therefore to find that animals possess abilities which far exceed the calls that natural living makes on them. If someone were to argue — as I shall suggest they might argue — that some primate species (and mankind in particular) are much cleverer than they need be, we know that they are most likely to be wrong.

Humphrey’s quote emphasizes that evolution is not likely to bestow costly capacities upon an organism unless those capacities are adaptive (i.e., they increase biological fitness).

Is a larger brain epiphenomenal? Allometry

Perhaps brains get bigger because bodies get bigger and brain size simply scales with body size. Arguably, a larger body has a larger sensory surface to receive sensory input (for example, many more sensory receptors in the skin) and more muscles to control. Is the larger brain thus epiphenomenal (i.e., a secondary effect, not causative)? Gould and Lewontin famously cautioned scientists to be vigilant for epiphenomenal explanations and to resist the follies of overly enthusiastic adaptationism and ‘just so stories’ in their metaphor of the Spandrels of San Marcos.

One way to decide this issue is to see how different body parts (such as the brain) scale against other body parts or other variables. One illustrative example of this approach is the relationship of body size and metabolism that was examined by Max Kleiber. This relationship is best described as a power law; i.e., one quantity is proportion to the other quantity raised to a power. If the quantities are plotted on a log-log scale, this power law relationship is evident as a straight line with the slope equal to the exponent (i.e., the ‘power’).

The scientific approach called allometry studies the relative sizes of body parts. If you plot body sizes vs. brain sizes (on a log-log plot) of different species, you would expect to find a straight line relationship if a larger brain is simply a secondary effect of body size. Such plots do reveal an overall power law relationship between body size and brain size. That is, a large amount of the variation of brain size across species can be attributed to body size.

However, what is most interesting about these plots are the deviations of different animal families, genera, species from the straight line. These deviations can be below the straight line expected by a strict power-law relationship (negative – hypoallometric) or above the line (positive – hyperallometric).

The Wikipedia has a nice summary of this topic reprinted below and which you can read in more detail here:

• Isometric scaling happens when proportional relationships are preserved as size changes during growth or over evolutionary time.
• Allometric scaling is any change that deviates from isometry.
• A classic example is the skeleton of mammals. The skeletal structure becomes much stronger and more robust relative to the size of the body as the body size increases.
• If, after statistical analyses, a property was found to be smaller than the predicted values this would be called "negative allometry” or hypoallometry, as the values are smaller than predicted by isometry.
• Conversely, the values are higher than predicted by isometry and the organism is said to show "positive allometry" or hyperallometry.

In lecture, I showed different examples of allometric plots of brain weight vs. body weight. The figures showed two linear (i.e., straight-line) relationships — one for mammals and one for lower vertebrates (fishes, amphibians, and reptiles). One difference between mammals and fishes and reptiles is that the latter don’t have 6-layered cortex as mammals do. So this changes the intercept of the two different lines (the point on the Y-axis representing brain weight).

Encephalization quotient (EQ)

In 1973, UCLA psychologist Harry Jerison published the influential book Evolution of the Brain and Intelligence which develops the concept of the Encephalization Quotient (EQ).

A simple way to think of the EQ is that it represents how much bigger or smaller a particular specie's brain is from its predicted size The predicted size is given by the brain sizes of other animals of the same size. So, for example, the EQ of a human brain is the deviation of the size of the average human brain from the average brain size for animals of about the same body size. This latter value is obtained from the regression line of body size vs brain size for a reference group containing a large number of species (e.g., all vertebrates, or all mammals, or all primates). A more technical description can be found here. Regardless of the reference group, humans have the largest EQ.

EQ is a way of systematizing brain/body size relationships across myriad species. But what does it represent in terms of function — for example, does a larger EQ translate into more intelligence? For example, Robert Deaner and his colleagues have argued that in a comparison among primates, absolute brain size (i.e., brain size not scaled by body size) is the best predictor of cognitive ability and not EQ.

Some researchers have opined that EQ was devised to make the human brain appear special. This perspective is captured in this quote from Ralph Holloway (2015):

Just as the human animal is curious, it is also vainglorious, always trying to find a measure that places it at the top. Thus we can fabricate a device, the Encephalization Coefficient or EQ, which shows that relative to any database, the human animal is the most encephalized animal living.

EQs do not evolve, only brain weight/body weight relationships do, and EQs are simply a heuristic device enabling comparisons between taxa; they have no reality outside of the database chosen, or species within a taxa, and are not designed to discuss within-species variation. For example, female humans are “more” encephalized than males, given their smaller body sizes, more body fat which is not innervated, and smaller brains, but the relationship might be simply a statistical artifact with no known gross behavioral manifestation given the sexes equal overall intelligence.

It is notable that humans do not have the largest brain in absolute weight, volume, or percentage of body mass. A blue whale’s brain weighs about 15 pounds compared to 3 pounds in human. But a blue whale can weigh 330,000 pounds compared to our 150 pounds. Thus, a blue whale is 2000 times a human’s body weight and 5 times its brain weight. Put another way, the 3 pound human brain is about 2% of a human’s lean body mass, while the blue whale’s brain is about 0.5% of its body weight. An elephant weighs about 12,000 pounds and its brain weighs about 12 pounds. This is 80 times a human’s body size and 4 times its brain size. However, the mass of the mouse brain is ~10% of its body mass. Among insects, the ant’s brain accounts for ~17% of its body weight.

How exactly, then, to compare brains among such widely differing species is problematic. A more reasonable approach might be to compare brain sizes among more closely related species, where differences in brain function and behavioral repertoire can also be more easily compared.

Evolution of the hominin brain

This is a course about the human brain, so the evolution of the brains of hominids and hominins are of particular interest. Let’s define some terms and establish a rough timeline. Hominids are members of the family Hominidae and are also known as the Great Apes. There are four living genera (the plural of genus) of this family: Pongo (orangutans), Gorilla (gorillas), Pan (chimpanzees and bonobos) and Homo (humans). The great apes diverged from the Lesser Apes (e.g., the Gibbons) about 12-18 million years ago (mya) which split off from old world monkeys about 25-30 mya. Primates diverged from other mammals about 55-85 mya.

Within the Hominids, the chimpanzees (Pan troglodytes) and modern humans (Homo sapiens) diverged from the lineage resulting in modern Gorillas about 12 mya. The last common ancestor of humans and chimps lived some 6-8 mya.

Some key temporal relationships are shown in the figure below taken from Encyclopedia Britannica’s article on hominins. The dating of divergences in the evolution of species is often uncertain and occasionally controversial. Dates are determined by different methods including fossils, radioactive decay rates, and molecular dating. Thus, the range of dates for the divergence of a species can be considerable, and that variability is expressed in the following figure.

When considering the evolution of species such as chimpanzees and modern humans, it is important to remember that humans did not evolve from chimpanzees. Rather, humans and chimpanzees diverged from a common ancestor. Thus, modern chimps and humans have evolved independently for 6-8 millions of years. However, despite that long period of independent evolution, modern humans and modern chimps have about 98.8% of their DNA in common. This surprising fact underscores the power of regulatory gene networks. That is, both chimps and humans have very similar genes that make similar protein products. However, the sequence in which different genes are turned on and off and the durations over which particular genes are expressed makes a huge difference in the resulting phenotype.

The relationship of body and brain size in primates (prosimians, monkeys, apes, and humans) compared to other mammals has a power law relationship with a higher intercept than observed in mammals other than primates. This is shown in the figure below from Schoenemann (2013). It is notable that species of Homo (both modern and extinct) shows a hyperallometric relationship between body and brain size.

Hominins

The members of the lineage the led to modern humans are called hominins. All species of the hominin lineage are now extinct save for Homo sapiens. This wasn’t always true and there were long periods in which different species of Homo coexisted. Indeed, Homo neanderthalis coexisted in areas of Europe with Homo sapiens for ~5-8 thousand years until the Neanderthals became extinct ~40,000 years ago. There is clear DNA evidence of interactions between these different hominin species. Indeed, modern humans typically have ~1-4% of Neanderthal DNA in their genomes (although that percentage varies regionally). There are also hypotheses about cultural transmission between these two species.

Note: Some researchers refer to Neanderthals as Homo neanderthalis and others use the designation Homo sapiens neanderthalis. This difference in terminology reflects a lack of consensus as to whether Neanderthals are a distinct species of Homo, or a subspecies of Homo sapiens.

Note: Dating of fossils can be controversial and are revised as new techniques for dating emerge. Similarly, determination of lineages and whether a fossil is a member of a known species or a new species can also be controversial. The ability to extract DNA from some fossils has been a revolutionary development that has altered earlier models of hominin lineage. However, DNA recovery from fossils has been most successful in cold climates such as Siberia. Recently, DNA was successfully extracted from a 1 million year old fossil tooth of an extinct mammoth. DNA extraction from fossils has been relatively unsuccessful in warmer climes and in equatorial regions of Africa where the hominin lineage began. Thus, there is an unfortunate regional bias in these important DNA data sources.

Brain changes over four million years of hominin evolution

For our specific purposes, the most interesting aspect of hominin evolution is the growth in brain size. The brain quadrupled in size from the earliest hominin species that diverged from the lineage leading to chimpanzees. Some of this brain growth was allometric — i.e.; it scaled with increases in body size. However, much of the brain growth was hyperallometric — i.e.; It greatly exceeded what would be expected on body size alone.

The figure below is from Schoenemann (2013) and shows evolutionary time in log values on the X-axis and cranial capacity on a linear scale on the Y-axis. Different hominin species are shown with different symbols with the legend on the far right. Vertical lines at the far right show the range of values for Homo sapiens, Great Apes, and other primates. What is particularly interesting is the obvious acceleration in the growth of the brain at about 2.5 mya.

Did hominin brains get smaller?

A close examination of the plot above showing hominin brain size by evolutionary time shows a decrease in brain size for Homo sapiens in the recent past. Although overlapping in distribution, Neanderthal brains are somewhat larger relative to that of H. sapiens. There are also shape differences with Neanderthal brains being more elongated than the more globular H. sapiens brain. These shape differences may reflect changes in the size of association cortices such as the parietal and temporal lobes. However, Neanderthals were also larger in body size than H. sapiens.

A recent paper by Zollikofer and colleagues makes an argument that changes in the skull of H. sapiens over the past 200,000 years were driven not by changes in brain size or in the shape of the brain, but due to a change in diet. They argue that the facial shape and musculature of H. sapiens has changed over that period due to a change in diet. The extensive musculature and facial bones required for that musculature were no longer needed as H. sapiens diets changed. The result was a smaller and more child-like face.

However, the decrease in brain size may have occurred for Homo sapiens over the past 30,000 years (since the end of the last ice age) when Neanderthals were already extinct. While body sizes did get slightly smaller over that time range, the change in brain size was disproportionate to the change in body size. There is no consensus about why (and even if) the brains of Homo sapiens have gotten smaller. One intriguing hypothesis by DeSilva and colleagues (2022) is that increases in social living purged the most aggressive individuals of the species and off-loaded decision-making to the group. These factors reflect a form of self-domestication, as it is observed that domesticated animals (such as cow) have brains that are on average 25% smaller than their wild counterparts.

Why did brains get larger?

Data discussed above indicates that brains of hominins became larger with respect to body size with an acceleration of brain growth relative to body size beginning 2.5 mya with Homo habilis and Homo erectus. What are the factors that led to this growth and how does a larger brain lead to increases in biological fitness?

Considering reasons in the context of evolution is complicated, and asking why is not strictly meaningful. Changes in brain size occurred because some animals experienced mutations that resulted in a larger brain. The environmental pressures were such that possessing a larger brain conferred some advantage that improved their biological fitness and thus those larger brained animals reproduced more successfully — eventually dominating the species. So, when I ask why something occurred, I am using why as shorthand for 'what advantage did a larger brain confer that improved a species's biological fitness'.

With that caveat in mind, let’s now consider four hypotheses for what advantage a larger brain conferred that improved the fitness of a species compared to other species, and/or the fitness of individuals within a species.

Foraging and dietary hypotheses

As we have discussed, animals require energy in the form of food and nutrients to survive long enough to reproduce. A larger brain could be an adaptation to find food and/or store, or cache, food. In lecture, I provided examples from birds and monkeys which I briefly recap below.

The howler monkey are generalist leaf eaters (folivorous) while the spider monkeys are specialists who eat fruit ( frugivorous). Despite being about the same physical size, the frugivorous spider monkey has a larger and more folded brain. This is presumed due to the increased neural processing associated with foraging, spatial memory for the location of fruit, improved vision to discriminate food sources from background, and the cognitive/motor effort involved with the extraction of the food (such as cracking nuts).

The bird example shows two different species of corvids (crows) and titmice. The species that hide (or, cache) their food for later have a bigger hippocampus (a brain structure implicated in spatial memory and navigation) than closely related species that do not cache food. Some species of birds (such as mountain chickadees) survive the winter on cached seeds and so their very survival is based on this ability. This suggests that a larger hippocampus in such birds improves biological fitness by improving their chances of survival. We will encounter chickadees below in discussing sexual selection.

Diet and the expensive tissue hypothesis

An improved diet generates more energy. Two factors have been identified in hominin evolution in which more energy is extracted from food sources. The first is the consumption of meat. Meat is energy rich and hominins eat more meat than any other species. The second is cooking. Cooking makes many food sources more palatable and, importantly, digestible and thus higher in calories and nutrients than raw foods.

A diet completely dependent upon the digestion of raw plant foods requires a longer alimentary canal. The alimentary canal is an ‘expensive tissue’; i.e., it requires many calories for its function. A shift to an improved calorie-rich diet reduces the requirement for a long alimentary canal and thus reduces the calorie consumption by the gut itself. This allows more calories to be devoted to the brain. This is the essence of the expensive tissue hypothesis put forth by Aiello and colleagues. While there is support for this hypothesis (and humans have a much shorter large intestine than other animals), there are inconsistencies between the predictions from this hypothesis for other species.

Nevertheless, the general idea that any adaptation that reduces the caloric needs of other body tissues frees those ‘saved calories’ to support a larger brain. For example, humans have a much greater ratio of fat to muscle than other primates. At rest, a pound of muscle consume 3 times the number of calories as does a pound of fat. From a comparative perspective, humans are ‘under-muscled’ and ‘over-brained’.

Cognitive buffer hypothesis

A little more brain helps when species is challenged by a change in the environment, food supply, etc. I provided in lecture an example from bird mortality — bigger brained birds (corrected for body size) have lower mortality in nature. Exactly why this is true is not known with certainty. Many investigators have conjectured that the birds with larger brains are more intelligent and that intelligence is useful when there is an environmental challenge.

Researchers have attempted to measure intelligence across species. The best metrics emphasize mental and behavioral flexibility resulting in novel solutions. Using these metrics, one concludes that intelligence has evolved more than once. There are more and less intelligent species within mammals and within birds. Birds and mammals diverged in lineage over 300 mya.

Sexual selection hypothesis

Sexual selection is a different form of evolution that was recognized by Darwin. It is different in that it operates within species and is associated with mating choice and preference. For this reason, it can operate rapidly

Geoffrey Miller has written a popular account of sexual selection as it applies to human mating choice in his book The Mating Mind.

• Sexual selection operates on mate selection within species.
• Competition among males for the opportunity to mate can lead to changing male morphology.
• Mate selection by females can also quickly lead to differentiation among males as the male traits selected by the females can eventually dominate the species.
• Sexual selection can work quickly within a species.

The peacock and Irish Elk are extreme examples what might occur from sexual selection. In these cases, the trait (large and elaborate tails, large antlers) provide information to the female about the general health of the male and, indirectly, about the quality of the male's genes. For example, Stephen J. Gould showed that the antlers of a healthy Irish Elk could weight 40 kg, while the antlers of a sickly or malnourished Irish Elk would weigh as little as 20 kg. Thus, while the antlers themselves were ornamentation and not useful in fighting or other functions, they did provide information useful to the female about the health of the male.

Other data, however, suggest that sexual selection can operate on less showy traits. For example, there is evidence in the mountain chickadee that females choose males on the basis of their spatial navigation ability. This ability is essential for survival in this species, as the birds cache food that they rely upon in the winter.

Geoffrey Miller (and others) have argued that a larger brain may be the result of sexual selection. In his hypothesis, the brain is showy sexual ornamentation. How so? Artistic, athletic and other high skill behaviors signal ‘good genes’ to potential mates in that these behaviors are complex and require good genes and a healthy brain to perform. Think about a Swiss watch. It is highly complex, and everything about it has to work perfectly to give accurate time. So, accurate time keeping is evidence that the internals of the watch are all good. This line of evolutionary thought has been used to 'explain' social skills in the arts and storytelling. This is interesting conjecture, but remember our caveats about adaptationism and spandrels.

Social relationships

In lecture, I highlighted passages from Alison Jolly and Nicholas Humphreys concerning the role of social relationships as a selection pressure for more capable brains. This perspective is also shared by Holloway (2015) as illustrated in the following quotation.

…the major underlying selectional pressures for the evolution of the human brain were mostly social. It was an extraordinary evolutionary ‘decision’ to go with an animal that would take longer to mature, reach sexual maturity later, and be dependent for its food and safety upon its caretakers (parents?) for a longer period of time. The benefits for the animal were many, including a longer learning period, a more advanced, larger, and longer-growing brain, and an increasing dependence on social cohesion and tool making and tool using to cope with the environments that they encountered. Needless to say, language abilities using arbitrary symbol systems were an important ingredient in this evolution.

Holloway (2015) continues and adds two important points:

The fossil record shows us that there was a feedback between the complexity of stone tools (which must be seen as a part of social behavior) and increasing brain size and the expansion of ecological niches. The ‘initial kick,’ however, the process that got the ball rolling, was a neuroendocrinological change affecting regulatory genes and target tissue hormonal interactions that caused delayed maturation of the brain and a longer growing period, during which learning became one of our most important adaptations.

The first point is that there is an evolutionary feedback loop between social constructions — such as tools — and evolutionary pressures. This idea is referred to as niche construction. Brains evolve in response to environmental pressures to improve biological fitness. However, the animal possessing the brain uses it to alter the environment. These alterations then become part of the environment that pressures further brain evolution. Thus, humans use their brains to create tools and then tools drive further evolution of the brain.

The second point is a reminder that the causal events in brain evolution are changes to regulatory genes.

The Social Brain Hypothesis

The idea that social relationships have driven brain evolution has been extended by Robin Dunbar who put forth the Social Brain Hypothesis. The Social Brain Hypothesis follows upon Bryne and Whiten’s earlier concept, the Machiavellian Brain Hypothesis. Their hypothesis emphasized that larger brains were associated with social deception, an adaptation selected by evolution. To be able to deceive another, one has to have an understanding of what the other believes. This is often referred to as a Theory of Mind, which we will discuss later in detail in later lectures.

In his writings, Dunbar considers and rejects a number of other proposed adaptations for human's large brain, some of which we have considered already, such as epiphenomenal (size scaling) and gestational pressures. He also considers and rejects dietary, mental maps, and extractive foraging hypotheses as selective pressures for larger brains. Rather, Dunbar proposes that the number of individuals that form an animal's primary clique is the critical variable. In lecture, I showed a plot of the allometric relationship between clique size and neocortex ratio across primates which revealed a linear relationship.

Dunbar's work was popularized by Malcolm Gladwell in his book “The Tipping Point” and this popularization established the Dunbar Number meme. The Dunbar Number for humans is ~150; meaning that we can't track deep social relationships beyond 150 other individuals (which seems already too high to me). One way the Dunbar Number has been explained is that it represents the number of people with whom you would be comfortable having a drink should you accidentally run into them at the airport.

In Dunbar's Social Brain Hypothesis, the Dunbar Number is species-specific. However, several recent studies (including several by Dunbar and colleagues) have examined individual differences in the size of particular brain structures with those individual's social networks using non-invasive MRI brain imaging. Note that this is a big jump in explanatory level. It is quite possible that something like the Dunbar Number would explain differences in sociability across different species, but that variation in brain size within a species would not.

A 2010 study by Lisa Feldman-Barrett received widespread attention when she reported that the size of a brain structure called the amygdala varies in size with the number of an individual's Facebook friends. Several other studies (many co-authored by Dunbar) reported different brain structures were related to social group size. I have been surprised and skeptical of such results. There are many different brain areas to measure, many different metrics of brain structure size (e.g., raw size of structure, size corrected by total brain size, etc), and many different ways to measure an individual’s social network. Thus, the possibility of a false positive result is high. Nevertheless, to my knowledge, these results have not been refuted by the scientific literature. Your instructor, however, remains skeptical.

So which hypothesis is correct?

The Social Brain hypothesis has led to an explosion of interest in Social Neuroscience and to questions whether there are brain regions specific to social interactions (such as the aforementioned amygdala). However, the ecological argument for a larger brain (e.g., diet, spatial memory, extractive strategies) that Dunbar dismissed has never been convincingly rejected, and explanations of brain size based upon species-specific differences in ecological/foraging are still being successfully argued. For example, a 2017 study by DeCasien and colleagues directly compared social complexity and ecological dietary factors in 140 species of primates — far more than Dunbar examined. These researchers found strong evidence that dietary factors explained the evolutionary pressure for larger brains and found no evidence that social complexity explained additional variability in brain size between species.

The extended conflict between competing hypothesis as new data is marshaled in support of one or the other is common in science. It often takes a long time for enough data to be accumulated for a consensus to be established. Moribund hypotheses are sometimes revived when a new method is developed that provides support. This is sometimes disconcerting for students who want to know the facts right now! In science, one often has to live with uncertainty. But it is from uncertainty that creativity is often born. (Also, don’t worry, there are plenty of facts to learn about which we are certain.)

Food production and social cooperation are interdependent

Does there need to be a winning hypothesis? Remember my caveat, evolution does not have a plan. It is perfectly reasonable to expect that a mutation that leads to a larger brain improves memory that enables better foraging and better score-keeping with regards to social interactions. The bigger brain can be supported by an improved diet with significant meat consumption, calorie savings from a decrease in other expensive tissues, and other factors such as bipedalism (25% fewer calories are required to walk upright than to knuckle-drag).

In this regard, I was very much impressed by a 2022 paper by Kraft and colleagues concerning human energy needs. In my mind, it makes a nice case for the interdependence of foraging, diet, and social brains. Kraft begins with what we have learned: humans need calories to support a big body and a big brain, and they must produce a caloric surplus to support the caloric needs of immature infants and children. Kraft argues that two factors had major impacts upon hominins ‘capture of energy’: (1) the shift to a hunter-gatherer social organization, and (2) the development of farming.

Kraft’s study compared how much time and energy was spent on subsistence in human hunter-gatherer societies (Hazda and others), slash and burn horticulturists, and great apes. Kraft’s results are interesting and somewhat counter-intuitive. It appears that humans use more energy on subsistence relative to great apes, despite having more efficient locomotion. However, humans receive a greater rate of return on this energy investment. Moreover, horticulture has a higher rate of return compared to hunting and gathering. Kraft's bottom line is that humans solved the energy problem by acquiring more energy and not by becoming more efficient by tool use or upright posture. Humans used more energy to acquire energy, but not more time to acquire energy. This provided humans with more leisure time.

The high cost extractive approach to obtaining energy by humans require cooperation and division of labor. Division of labor depends upon a social arrangements and the skills to navigate those social arrangements.

It is interesting and notable that the surplus energy is used to support pregnant females and immature infants and children. This was particularly true for farming than for hunting and gathering. Farming requires less work investment and provided more calories for reproduction. This led to increases in fertility for farming based societies. Thus, the shift to farming greatly increased human population density.

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What limits brain size?

If a larger brain improves biological fitness, why not a larger brain still? What keeps our brains from growing as large as the outsized brains often depicted for alien species in science fiction movies?

Energy requirements

Follow the energy — brains are expensive organs. Here are some human brain facts:

• Brains are roughly 2% of human lean body weight.
• Brain consumes 20% of adult metabolism.
• Brain consumes >60% of infant metabolism

In lecture, I discussed a simulation showing the brain-brawn tradeoff for a typical non-human primate diet. To maintain a given body size and a given brain size, a certain number of calories have to be consumed each day. Gorillas eat about the maximum numbers of hours per day possible to maintain their brain-brawn. To change this ratio, the diet would need to change and more calories would need to be extracted from food and/or fewer calories spent by that species. Thus, one important limitation in brain size is its enormous energy needs. It is truly an expensive organ.

Several possibilities for increased calories from an improved diet have been discussed already, including more meat and cooking. Recent studies suggest that carbohydrates were also very important for increased brain size. For example, there are more copies of the amylase gene (amylase breaks down carbohydrates/starches into sugars) in humans than in chimps (humans' closest genetic relative). We have also discussed ways in which calories may be saved, including bipedalism, increased body fat to muscle ratio, and a smaller gut.

Connectivity

A larger brain means more brain cells. As we will discuss in upcoming lectures, the principal cell of the brain is the neuron, and communication in the brain is made though connections among neurons. As neurons increase linearly, their possible connections increase as roughly the square of neurons actually, N*(N-1). So even a small increase in the number of neurons can lead to a great increase in the number of connections.

The connections among neurons are made through protoplasmic processes called axons, and so have mass, take up space, and consume energy. Our brains would be enormous in size and weight if the number of neurons increased greatly and the connections per neuron stayed the same. Not all species have the same amount of connectivity among neurons; for example, rodent brains have more connections between neurons than human brains. It has been argued that a normally connected rodent brain with the same number of neurons as the human brain (~89B) would weigh 77 pounds. We will discuss limits on connectivity in later lectures that consider sparse connectivity and modularity of brain function.

The argument made above is valid, but simplified, as there are other factors that influence brain volume and weight beyond number of neurons and their connections. Factors such as the packing density of neurons, the size of neurons, and the number of others cells in the brain, such as glia cells, also influence brain volume and weight. An interesting and recent summary of this literature can be found in this article by Suzana Herculano-Houzel.

Gestational limitations

One often reads (as undisputed fact) about the obstetrical dilemma in which human infants born premature (relative to other species) because a large head at birth is incompatible with a pelvis optimized for bipedalism. I have tried to track down evidence for this 'fact', but found no good data. Indeed, recent simulations suggest it is may not be true. However, there are indisputable limitations upon the energy costs of gestation.

Maternal energy hypothesis

Human infants are born with ~25% of the neurons they will eventually have, as there is considerable brain growth up to age 7. As mentioned earlier, > 60% of infant metabolism supports the brain. There is a limit on the number of calories a mother can provide a developing fetus, and that may be the determinant of when human infants are born. However, after birth, the infant still requires a great number of calories to continue developing its body and brain, and this pressure may select for brains that support socially adept behaviors. Some of those behaviors may include:

• Cooperative breeding may help provide calories required by developing infant.
• 'Grandmothering' whereby non-reproducing members of the group contribute to the development of the infants.
• Specialized food acquisition behaviors (hunter – gatherer, farming)

Large brain summary

There is probably no single reason the brain of humans is so large. A large brain is most likely the result of interactions among a number of important factors such as those listed below:

• Improved diet through foraging and the discovery of cooking.
• Reduced calorie costs of bipedalism compared to knuckle-dragging
• Cognitive buffering
• Maternal subsidies (e.g., grandmothering) and cooperative breeding.
• Managing social relationships.

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Are human brains special?

There has been interest since antiquity about what is different about humans that make them special or unique among the animal species. I provided an example in class about the debate between Huxley and Owen about the uniqueness of the hippocampus (a brain structure known to be important in memory). Although their debate has been misrepresented, Owen argued that the hippocampus was one, of several brain structures, that was uniquely human and not found in the gorilla. This is factually wrong (indeed, the hippocampus is an ancient brain structure found in all mammals and many other species), but debate continues as to the existence of ‘uniquely human’ brain areas. Recently a particular neuron type called the spindle cell was thought to be only present in humans (it is not, it has since been found in elephants and other animals). Because of its relative restricted location in the brain, spindle cells were even proposed to be responsible for consciousness. My advice to the aspiring scientist is to be skeptical of such claims for human uniqueness. Evolution doesn't work that way.

Nevertheless, humans appear to have capabilities unmatched (at least in terms of magnitude) in other animals. We have observed that humans deviate from the linear relationship between body size and brain size and have the largest EQ. Are human capabilities simply reflective of the quantity of brain matter and not some special and unique quality of brain matter? This question is usually asked with reference to other primates. One way to state that question is whether or not the human brain is simply a 'scaled-up' version of a primate brain (monkeys and apes). This issue remains contentious.

As discussed above, there has been a large increase in relative brain size in hominins (current and extinct) compared to great apes. There are also reports that different brain structures in humans show hyperallometry. For example, some have reported that the size of neocortex in humans is hyperallometric compared to other primates.

One 'fact' that had become dogmatic for many years was that the frontal lobes were the part of the brain that made humans human — as they appeared to be hyperallometric compared to other primates, and because (as we will learn later in the semester) appear involved in high-level abstract behavior. However, more recent data suggest that the frontal lobes are actually isoallometric; that is, human frontal lobes are about the size you would expect for a primate of human's body size. However, prefrontal gyrification appears hyperallometric in humans – meaning that the prefrontal cortices are proportionally more folded in humans than in other primates. Whether gyrification is just a way to pack more surface area into a smaller space, or whether it has additional functional consequences is unclear.

The debate about whether the human brain is unique in some way or has proportionally 'more of' something than other primates is still being debated. I again point you to the article by Suzana Herculano-Houzel for a relatively recent review of the evidence. My own view is that a quantitative increase of sufficient magnitude can lead to an apparent qualitative change in function.

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Revisions

These notes were last updated on July 18, 2022

Videos

Prerecorded lectures for Fall 2020

Why have a brain? (7:37)

The size of the brain (26:35)

Why do brains get larger? (21:19)

Limits on brain size (46:17)

The Social Brain hypothesis (18:32)

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Lecture recorded live in Fall 2018

Here is the lecture on this topic as presented in Fall, 2018. I update and sometimes completely rewrite my lectures each year, so this older lecture is somewhat out of date. Note also that I deleted a few sections from the original lecture where I was either making announcements or conducting a Poll Everywhere poll.

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Brief summary of lecture recorded in 2017

I summarized the key points of this lecture in the following video excerpt recorded in 2017:

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