Evolution of the Human Brain

Hominin Evolution and Modern Humans

The hominin evolutionary tree spans seven million years. This has relevance to questions about human evolution, intelligence, and behavior.
Typically, multiple hominin species coexisted at any given time in the past.
Currently, only one hominin species (Homo sapiens) exists.
A key question is how Homo sapiens, a species that evolved in the tropics, survived and replaced other hominin populations (e.g., Neanderthals in Europe) adapted to colder environments during ice ages.
The explanation likely involves intelligence or behavioral complexity linked to brain characteristics.

Brain Structure

Comparing primate brains (tarsiers, New World monkeys, lemurs, Old World monkeys, chimpanzees, gorillas, and humans) reveals variations in size and structure.
The human brain consists of the cerebellum (coordination of voluntary movements) and the cerebrum (neocortex and allocortex).
The neocortex is associated with higher cognitive functions.

Overall Brain Size

The lecture will focus on overall brain size and the structure of the brain, the relative contribution of different parts of the brain
Modern humans have an average brain volume of 1200-1300 cubic centimeters.
Blue whales have much larger brains, averaging 6000 cc.
However, larger body size necessitates a larger brain to manage bodily functions.

Brain Size vs. Body Size

A plot of brain mass (grams) versus body mass (kilograms) on a logarithmic scale is used to explore the relationship between brain size and body size.
A line on the plot represents the average expectation for brain size in mammals and birds of a given body size.
Points above the line indicate larger-than-expected brain size, while points below the line indicate smaller-than-expected brain size.
Humans have a brain size that is larger than expected for their body size, whereas blue whales have a brain size that is smaller than expected for their body size.

Encephalization Quotient (EQ)

Encephalization quotient (EQ) = $\frac{\text{observed brain size}}{\text{expected brain size}}$ (relative to body size).
EQ may correlate with intelligence (social, mechanical, etc.), although intelligence is difficult to define, and EQ is a coarse measure of cognitive complexity.
Higher EQ indicates larger relative brain size.
Brain size relative to body size is key, with modern humans having the largest relative brain size.

Brain Size Evolution in Hominins

Australopiths showed little change in brain size over a million years, remaining in the African ape brain size range.
Early Homo (Homo habilis, Homo rudolfensis) showed a slight increase in brain size.
Homo erectus exhibited a continuous increase in brain size over time.
Heidelbergensis, Neanderthalensis, and Sapiens showed a significant increase in brain size.
Body size increased with early Homo and then decreased around 150,000 years ago, possibly related to changes in food acquisition behavior.
Neanderthals were large-bodied and robust, hunting big game up close.

Encephalization Quotient in Hominins (cont.)

Neanderthals had the same size brain or even slightly larger than modern humans, but they were more heavily built and had larger body sizes. Therefore Modern Humans have a higher encephalization quotient than Neanderthals
Neanderthals required robust bodies for close-range hunting of big game.
Modern humans, with changes in food acquisition, experienced reduced body size.
Modern humans have a higher EQ than Neanderthals, even with similar brain sizes, due to differences in body size.
Estimated EQ for various hominin species shows relatively little difference for early australopiths, a slight increase with Homo habilis, and an increasing trend through time.
Homo floresiensis and Homo naledi are exceptions, being late-surviving small-brained hominins.

Models for Increasing Brain Size

Social intelligence model

Many primates, including hominins, live in large social groups.
Larger groups lead to more complex societies, increasing cognitive demands.
Additional members greatly increase social relationships and complexity.
Selection favors an enlarged neocortex for managing complex social relationships.
A feedback loop may exist: larger group size → more complex societies → enlarged neocortex → increased reproductive output → larger groups.

Ecological intelligence model

Brain tissue is metabolically expensive.
Selection for larger brains requires benefits, such as a high-quality diet.
Focusing on resources like fruit requires tracking availability (time of year, location).
High-quality diet selects for complex feeding behaviors.
Complex feeding selects for an enlarged neocortex.
Enlarged neocortex requires a high-quality diet for maintenance.
Complex feeding includes tool use for hunting, digging tubers, etc.
Again, there are feedback loops

Neocortex

Primates have large brains relative to body size and a large neocortex relative to the rest of the brain.
Plots of neocortex volume versus the rest of the brain volume show that for a given brain size, primates have proportionally larger neocortices compared to insectivores.
Haplorrhines (Old World monkeys, New World monkeys, apes, tarsiers) have larger neocortices than strepsirrhines (lemurs, lorises).
The neocortex is composed of subparts with different functions.
The neocortical regions include the motor cortex (voluntary movements) and the visual cortex (visual processing).
Humans have a restructured or reorganized brain compared to close relatives.

Frontal Lobe

The frontal lobe, located at the front of the brain, is a large section of the neocortex in humans, especially compared to other species.
The frontal lobe is expanded in apes as well compared to other primates.
The frontal lobe is associated with planning and higher intelligence.
Functional MRIs show neuron activity in the frontal lobe during planning.

Cognitive Processes

Tool Use

Tool use is not unique to humans; chimpanzees and other primates use tools.
Chimpanzees use tools such as rocks to crack nuts open.
Humans more sophisticated than chimpanzees, but the process is not uniquely human.

Planning

Planning is often thought to be uniquely human, but chimpanzees also exhibit planning behavior.
Observed in chimpanzees gathering rocks in advance to throw at zoo visitors.
Humans do long term planning, but the concept is not uniquely human.

Language

Language is a better candidate for a uniquely human cognitive process, with no evidence of complex spoken language in nonhuman primates.
Considerations for evaluating language in hominins include physical capacity, cognitive capacity, and communication versus language.

Physical Capacity for Language

Sounds in language are created via the airway, including the trachea and larynx (voice box).
Vocal cords in the larynx modulate air flow and sound production.
Modifications to sound are made by movements of the tongue, mouth, teeth, and lips.

Larynx

The pharynx, located above the larynx, is a key region for sound modulation.
The position of the larynx and pharynx differs between humans and chimpanzees.
In humans, the larynx is lower relative to the cranium, expanding the pharynx.
The expanded pharynx allows for more efficient modification of air and a greater variety of sounds.
However, this configuration also increases the risk of choking.
The capacity to produce complex sounds in language outweighs the risk of choking in terms of natural selection.

Reconstructing Larynx Position

The position of the larynx can be reconstructed by measuring basicranial flexion.
Australopithecus africanus has a chimpanzee-like pattern of basicranial flexion.
Neanderthals also have a chimpanzee-like pattern. Neanderthals likely did not have the range of sounds of anatomically modern humans.

Cognitive Capacity for Language

Brain structures such as Broca's area and Wernicke's area are associated with language.
Damage to these areas can compromise language ability.
Broca's area often exhibits lateralization asymmetry, being larger on one side of the brain.
Language-related lateralization may be linked to the evolution of manual dexterity and handedness.

Motor Cortex

Motor cortex controls movement. The regions that control the mouth are important for language
The motor cortex is associated with motor control of the mouth and lips.
A homunculus illustrates the proportion of the motor cortex dedicated to different body parts.
A large portion of the motor cortex is devoted to face and mouth/tongue control.

Fossil Endocasts and Lateralization

Fossil endocasts (casts of the inside of the brain case) can provide information about brain structure.
Lateralization (asymmetry) of Broca's area is examined in fossil endocasts.
However, fossilization processes can distort specimens, introducing artificial asymmetry.
Homo habilis (or Homo rudolfensis) shows possible lateralization but may be due to distortion.
Homo erectus shows better evidence of asymmetry in Broca's area.

Hypoglossal Nerve and Tongue Motor Control

The hypoglossal nerve innervates the tongue, enabling tongue movement.
The hypoglossal canal at the base of the skull allows passage of the hypoglossal nerve.
An early hypothesis suggested that a larger hypoglossal canal size indicates greater tongue motor variability.
Initial findings seemed to support this hypothesis, but later reevaluation with larger sample sizes showed overlap between humans, chimps, and gibbons.
Therefore, the hypoglossal canal size is not a reliable indicator of language ability.
This illustrates the scientific method: hypotheses are tested, reevaluated, and sometimes rejected.

Communication vs. Language

Forms of Communication

Primates communicate through auditory, visual, olfactory, and tactile means.
Olfactory communication (scent-marking) is common in strepsirrhines.

Social complexity Hypothesis

The complexity of communication may increase with social complexity / social group size.
Species in larger social groups spend more neocortex volume to overall brain size This implies there needs to be more complex communication if you are in large social groups.

Visual Communication

Geladas use lip flips to communicate warnings.
Primate facial mobility, measured through facial expressions, correlates with group size.

Grooming in Primates

Grooming is an important behavior in primate social groups.

Importance of Grooming

It helps with the dispersal of parasites.
It also has a public health benefit in terms of dampening down aggression within the community.
Beyond that, it's simply a social bonding function within different primate social groups.

Grooming vs. Language

Spending time grooming increases with group size. Without language, humans social groups would exceed typical grooming/time ratios.
One hypothesis suggests human language is a replacement for grooming.
Language allows for the management of social relationships without physical touch (gossip hypothesis).
This allows groups to expand beyond the normal limits predicted by grooming requirements.
Language has a side benefit: it enables communication of ideas and rapid transmission of information.
Modern humans may have out-survived other hominin species such as Neanderthals because communication allowed them to adjust behaviors.

Conclusions

Broad overview of the evolution of brain size, the evolution of body size, how they interact with each other to give a measure of relative brain size that we can kind of use as a proxy for different levels of intelligence between species, and also looking at the various different components that associate with behavioral complexity on the one hand, linguistic complexity on the other hand, and how they may have interplayed with each other to produce the landscape that we see today in terms of we're the only hominin left.