Comparative Cognition and the Evolution of the Human Brain

Tools: Windows into the Brain and Cognitive Abilities

Traditional views of human evolution often emphasize a linear progression, leading to the expectation of a 'missing link' between ancestral species and modern humans. However, comparative cognition examines human abilities within a broader phylogenetic context. Tools serve as critical indicators of highly developed cognitive abilities because they provide insight into the nature of complex problem-solving. Creating and using tools involves several sophisticated mental processes:

• Identification of the Need: Recognizing that a problem exists which cannot be solved by physiology alone.

• Mental Planning: Conceptualizing what is required to solve the problem before enacting a solution.

• Material Selection: Identifying the best materials for tool creation.

• Meta-tool Use: Identifying the best materials to use as tools specifically to manufacture other tools.

• Enactment and Application: Executing the mental plan and applying the finished tool to the specific task.

In Konso, Ethiopia, scientists have unearthed more than $350$ ancient tools spanning roughly $1$ million years of evolution. These artifacts demonstrate a gradual progression toward more refined shaping, suggesting a rise in the cognitive sophistication of ancient human ancestors.

Comparative Tool Use Across Species

Tool use is not unique to humans and is observed across various taxa, providing a window into the cognitive capacities of other species:

• Bonobo-Chimpanzees ($Pan\,paniscus$): Individuals like Kanzi have demonstrated stone tool production and utilization in research settings. This behavior is closely related to the nut-cracking behaviors observed in wild chimpanzees.

• Bottlenose Dolphins ($Tursiops\,sp.$): In wild populations, dolphins have been observed wearing marine basket sponges ($Echinodyctium\,mesenterinum$) on their rostrum. This behavior is documented as a form of cultural transmission, used to protect their noses while foraging in substrates littered with debris, rocks, and shells to find hiding prey like the barred sandperch ($Parapercis\,nebulosa$).

• New Caledonian Crows: These birds exhibit spontaneous metatool use. They have been observed shortening twigs if no twigs of the appropriate length are available and demonstrate a causal understanding of water displacement to reach food.

• African Grey Parrots: Alex the parrot, studied by Irene Pepperberg, provided evidence for conceptual quantitative abilities, including the labeling of cardinal sets and the ability to categorize objects based on multiple criteria.

• Wrasse ($Labridae$): These fish have been observed using rocks as anvils to break open shells, indicating tool use even within marine reef environments.

The Palaeolithic and Evolution of Stone Tool Technology

The evolution of stone tools throughout the Pleistocene reveals a trend toward increasing efficiency. In the Early Pleistocene, stone flakes made by $H.\,habilis$ and $H.\,erectus$ (prior to $1$ million years ago) had the shortest working edges. After this period, flake edges became longer as $H.\,erectus$, $H.\,heidelbergensis$, and Neanderthals learned to control specific variables in stone flaking:

1. Platform Depth: The thickness of the flake at the spot where it starts.

2. Exterior Platform Angle: The angle between the surface being hit and the surface of the stone core. A larger exterior platform angle creates a flake with a longer edge relative to its size.

Controlling these variables requires the ability to direct force at a precise location for a given angle, a skill that Režek and colleagues describe as uniquely human. This technological advancement allows for the tracking of how hominin cultures advanced and passed along skills over the last $2.5$ million years.

Chronology of Palaeolithic Tool Cultures

Lower Palaeolithic ($2$ to $1.3$ million years before present)

• Oldowan Culture: Characterized by chopper cores found at Olduvai Gorge, Tanzania. These were used for pounding, breaking, or bashing. Small flakes (approx. $33\,mm$ long) were used for cutting and scraping.

Middle Palaeolithic ($1.2$ million to $40,001$ years before present)

• Acheulean Handaxes: Found in France, the UK, and Africa, dating from $750$ to $90\,KBP$. These tools range in size from $13.1\,cm$ to $24.1\,cm$ long.

• Aterian Points: Found in Morocco, these are characterized by a tanged base for hafting. Dimensions for certain specimens include $4.4\,cm$ length and $4.8\,cm$ by $3.2\,cm$ with a thickness of $9\,mm$.

Upper Palaeolithic ($40$ to $8\,KBP$)

• Artistic and Technical Innovation: This period saw high-quality painting, barbed points, bone tools, fishing/shellfishing implements, and microliths.

• Representative Artifacts:     

  • Lion figurine carved from mammoth ivory ($Vogelherd\,Cave$, SW Germany), approx. $40\,KBP$, measuring in length.     

  • Handprints in caves, which research suggests were mostly made by women.

Brain Size and the Encephalization Factor

Human cognitive superiority cannot be explained by absolute brain size alone, as species like elephants have brains weighing $4$ to $5\,kg$, while humans have brains weighing $1.2$ to $1.5\,kg$. Instead, focus is placed on relative brain size and brain organization.

Comparative Brain Weights

• Shrew: $0.25\,g$

• Mouse: $0.5\,g$

• Sheep: $100\,g$

• Chimpanzee: $400\,g$

• Human: $1400\,g$

• Elephant: $5000\,g$

Brain Weight as % of Body Weight

• Elephant: $0.20\%$

• Sheep: $0.25\%$

• Human: $2.33\%$

• Mouse: $2.08\%$

• Shrew: $3.33\%$

Encephalization Factor (calculated as $\frac{\text{Brain weight}}{\text{Body weight}^{0.69}}$)

• Shrew: $0.06$

• Mouse: $0.07$

• Sheep: $0.19$

• Chimpanzee: $0.26$

• Human: $0.71$

Michael Gazzaniga ($2008$) notes that humans have a neocortex three times bigger than predicted for their body size, alongside more white matter and specific enlarged areas of the cerebellum.

Cellular Scaling Rules and Neuron Count

Suzana Herculano-Houzel's research suggests that the human brain is a linearly scaled-up primate brain, not an evolutionary outlier in its structure, but unique in its total number of neurons.

• Rodent Scaling: As rodent brains increase in size, neurons get bigger, meaning the number of neurons does not increase as rapidly as the brain volume. A $10$-fold increase in cortical neurons in rodents leads to a $50$-fold increase in cortex size.

• Primate Scaling: Primate neurons do not increase in size as the brain scales up. A $10$-fold increase in neurons only leads to a $10$-fold increase in cortex size, allowing primates to pack more neurons into a smaller space.

• The Human Advantage: The human brain has seven times more neurons than expected for a rodent brain of its size ($1.5\,kg$). It contains approximately $86$ billion neurons. This high neuron count is the primary driver of human cognitive abilities.

Metabolic Constraints and the Fire Hypothesis

Neurons are metabolically expensive. Total brain metabolism scales linearly with the number of neurons. This creates a metabolic constraint: Great apes cannot afford the energy cost of more neurons because they are limited by the time required for foraging and eating raw foods ($8\,h/d$ viability curve).

Human ancestors (starting with $H.\,erectus\,/\,H.\,ergaster$ approx. $1.7$ million years ago) bypassed this constraint through food processing efficiency.

• Meat Eating: Evidence of butchery by $A.\,afarensis$ exists from $3.4$ million years ago, but this predates the rapid expansion of brain size.

• The Role of Fire: Evidence of habitual fire use exists from $790,000$ years ago (Gesher Benot Ya‘aqov) and potentially as far back as $1.7$ million years ago (Wonderwerk Cave).

• Consequences of Cooking:     1. Energy Energy Release: Cooked food is easier to digest and releases more energy.     2. Structural Changes: Led to smaller jaws, teeth, and guts (the "Expensive Tissue Hypothesis").     3. Disease Avoidance: Fire kills bacteria on the outside of meat and parasite eggs on the inside.

Antibacterial Action and Disease Avoidance

Cooking serves as a disease avoidance mechanism. High heat ($\approx 400^{\circ}C$) alters bone mineral and neutralizes pathogens.

Common Food Poisoning Bacteria

• Campylobacter: Found in raw chicken, beef, and unpasteurized milk. Causes fever and abdominal pain.

• Salmonella: Found in raw pork, poultry, and eggs.

• Shigella: Found only in the intestines of humans and primates.

• Escherichia coli: Found in raw beef and pork.

Antibacterial Herbs and Compounds

The antiseptic effects of cooking were likely enhanced by the use of plants. Many herbs contain phenolic compounds that destroy the lipid bilayer of bacterial membranes.

• Garlic, Onion, Shallots ($Allium$ family): Contain Allicin (allyl-2-propene thiosulfinate). These inactivate extracellular enzymes via oxidative cleavage of disulfide bonds.

• Oregano: High in Carvacrol, thymol, and eugenol. Decreases intracellular ATP in bacteria, disrupting the cytoplasmic membrane.

• Basil: Contains Methylchavicol and linalool.

• Sage: Contains Salvial and Rosmarinic acid, affecting the phospholipid bilayer.

Summary of Human Uniqueness

While tool use is common across species and human brains are structurally similar to other primates, the human distinction lies in the total number of neurons. This high neural count requires energy levels that cannot be sustained by standard primate foraging. Therefore, the control of fire and the practice of cooking played the critical role in human evolution by providing the caloric surplus necessary to power a high-neuron brain.