The Path to Human Intelligence
A Full-Path Causal Reconstruction
from Wildfire-Cooked Meat to Upright Walking

This paper proposes a unified causal reconstruction of the full path of human evolution. The conventional narrative places upright walking at the origin of human evolution, but this narrative contains unbridgeable logical and physical discontinuities. Based on existing data from archaeology, primate behavioral ecology, sleep science, and gut-brain axis research, this paper proposes an entirely new causal chain: human ancestors, as established meat-eaters, opportunistically obtained cooked meat from large animals killed in annual savanna wildfires — an event that, through the gut-brain axis, first activated the cognitive system in trace amounts (“the first intelligence switch”). Over hundreds of thousands of years, opportunistic cooked meat consumption gradually remodeled the bodies of human ancestors — Australopithecus afarensis (“Lucy,” ~3.7 million years ago) is the fossil evidence of this process: the lower body was already adapted to ground locomotion (daytime pursuit of fire sites and cooked meat), while the upper body still retained climbing capabilities (at night, without fire’s protection, they had to sleep in trees). This “split morphology” is the precise physical expression of two different selection pressures acting separately on daytime and nighttime behaviors. Subsequently, the physical constraints of large, partially cooked animal carcasses forced human ancestors to carry both fire and meat chunks back to their camp simultaneously. Fire at the campsite eliminated the sole reason for sleeping in trees, and humans thus began sleeping flat on the ground. Ground sleeping produced a surge in REM proportion from approximately 5% to 25%, generating a systemic leap in focused attention — operationally defined as sustained attention and working memory supported by the prefrontal cortex. Focused attention converted episodic behaviors into sustained behaviors, driving active fire maintenance and the regularization of cooked food consumption, initiating a dual-loop positive feedback system — Loop A (gut-brain axis pathway) and Loop B (lifespan-social pathway). Upper-body climbing adaptations rapidly degenerated once the need for nighttime tree-sleeping vanished, and the body unified from Lucy’s “split morphology” into fully upright form — the emergence of Homo erectus was not a sudden evolutionary event, but the explosive release of long-accumulated change potential once the final constraint (nighttime climbing) was removed. After the body’s structural remodeling was complete, elongated legs and upright posture conferred persistence chase hunting capability; following the migration routes of large prey herds directly drove humans out of Africa, while fire as a “portable civilization kit” enabled humans to instantly establish complete survival infrastructure at each new habitat, ultimately achieving global dispersal. This paper includes a timeline verification aligning key archaeological dates with causal chain steps. All arguments use data already available in the academic literature; by reversing causal directions and filling in missing links, a completely new explanatory framework emerges — internally fully consistent, with logic and physics aligned.

From the First Piece of Cooked Meat to Upright Walking

Upright Walking First — A Hypothesis That Aligns with Neither Logic nor Physics

The mainstream evolutionary narrative arranges human evolution as a linear chain: upright walking → hands freed → tool-making → fire control → cooked food → bigger brain → language → civilization. The first step — upright walking — is treated as the origin of everything. But this starting point itself contains inescapable logical and physical discontinuities.

Logical Discontinuity: If upright walking is the first step, what is the driving force? The answers offered by academia — “savanna expansion required seeing distant predators” or “needed to free hands to carry food” — do not withstand scrutiny. Chimpanzees also live on savanna woodlands (the Fongoli population in Senegal), yet they have not evolved upright walking. Meerkats stand up to scan the horizon, yet they have not evolved bipedalism. As for “freeing hands to carry food” — carry what food? Chimpanzees already carry raw meat and raw fruit, using their mouths or one hand; full bipedalism is not required.

Physical Discontinuity: The transition from quadrupedal to bipedal locomotion, without a clear survival advantage, is an extremely disadvantageous intermediate state. A semi-upright primate can neither run fast (slower than quadrupeds), nor maintain stability (prone to falling), and exposes its abdomen (more vulnerable to predator attack). Natural selection would not tolerate a transitional state that puts individuals at a continuous disadvantage for hundreds of thousands of years, unless an extremely strong selection pressure is driving it. The conventional narrative cannot explain this driving force.

Bias of the Fossil Record: Academia places upright walking first because in the fossil record, skeletal morphology changes are the easiest to preserve and observe — pelvic shape, femoral angle, and spinal curvature can all be directly measured. But behavioral changes — cooked meat consumption, changes in sleeping posture, fire embers collection — are completely invisible in the fossil record. Attribution methods can only work with visible evidence, hence the conclusion that “upright walking came first.” But visible does not mean earliest. Invisible does not mean nonexistent.

Meat-Eating Habits and Fire-Scene Cognitive Ability: Two Foundations Already in Place Before Evolution Launched

The preconditions of this paper’s hypothesis are only two, and both were already well-established on the evolutionary timeline.

Precondition One: Meat-eating habits. Meat-eating behavior in human ancestors far predates fire control. The earliest definitive evidence of stone tools co-occurring with butchered animal remains comes from the Gona and Bouri sites in Ethiopia, approximately 2.6 million years ago. By 2 million years ago, the Kanjera South site in Kenya shows evidence of sustained camp-based meat processing. By 1.85 million years ago, the DS site at Olduvai Gorge provides objective evidence that hominins had primary access to large animal meat — human ancestors were primary consumers, not scavengers. This means that before wildfires killed animals, human ancestors were already established meat-eaters; their attention at fire scenes was naturally directed at animals rather than seeds.

Precondition Two: Fire-scene cognitive ability. Long-term research by Iowa State University primatologist Jill Pruetz at Fongoli, Senegal, demonstrates that savanna chimpanzees behave predictively rather than reactively toward wildfires — they show no stress or fear and can calmly predict the direction of fire movement. Pruetz herself had to closely follow the chimpanzees during fires because they were better than she was at judging fire behavior. Researchers note that mastering fire involves three stages: conceptualizing fire’s behavior, controlling fire, and making fire. Chimpanzees have already mastered the first stage. This means that human ancestors — with brain volumes larger than chimpanzees — faced an extremely low cognitive threshold for collecting wildfire embers; no intellectual leap was required.

Wildfire and Large Animals: Nature’s Cooking Classroom

Dry-season wildfires on the African savanna are annual, predictable, and recurrent natural events. Fongoli research data shows that over 75% of chimpanzee habitat is burned by wildfire annually. For human ancestors, wildfire was not a rare catastrophe — it was a routine environmental event encountered every dry season.

When fire swept across the savanna, large animals — antelope, buffalo, hippos, even elephants — some could not escape and were killed by the flames. The fire burned over the carcasses, charring the outer layer of skin and fur, and the meat began to cook. The air filled with the aroma of roasting meat. This process was not instantaneous — a large animal burning in fire could continue for tens of minutes or even hours.

Human ancestors, as established meat-eaters, had their attention naturally directed at animals. They stood at the fire’s edge — as calm as today’s Fongoli chimpanzees — and witnessed the complete process of “fire + animal → cooked meat.” A living animal → chased by fire → killed by fire → fur charred → meat color and texture changed → a distinctive aroma released. This was a complete “cooking lesson,” taught by nature itself.

They did not need to “understand” the chemistry of fire. They only needed to form the most basic sensory association — fire = the smell of roasting meat = something delicious. This association does not even require abstract thought; pure conditioned response is sufficient. And this lesson was repeated every dry season, for hundreds of thousands of years.

Large Partially-Cooked Animals: The Only Physical Solution for Fire Transport

Large animals killed by wildfire — hippos (1.5–3 tons), elephants (several tons), buffalo (several hundred kilograms) — created a unique physical scenario. After the fire passed, the outer layer was cooked while the interior remained raw or partially raw. This physical fact produced three inescapable constraints:

Why chimpanzees would never reach this step: Because what they seek at fire scenes are seeds and insects — small, fully cooked, consumed on the spot. There is no need to “bring something back for further processing.” Therefore chimpanzees never need to transport fire. Large partially-cooked animals are the only natural trigger for fire transport — seeds will not trigger it, small prey will not trigger it; only large partially-cooked meat creates the physical pressure of “must bring fire back.”

Australopithecus afarensis: Descendant of Cooked-Meat Eaters, Split Morphology Before Fire Reached Camp

The conventional narrative treats Lucy (~3.2 million years ago, Australopithecus afarensis) as core evidence for “upright walking first.” But the details of the fossil data tell a completely different story.

Lucy’s body exhibits what scholars have long been unable to explain — a “split morphology” — described by one anthropologist as “bipedal from the waist down, arboreal from the waist up.” The lower body has a pelvis, knees, and foot arches adapted for bipedal walking; the upper body retains ape-like long arms, curved fingers, and scapular structures suited for climbing. CT scans show her arm bones are robustly constructed, similar to chimpanzees, indicating the species spent considerable time climbing trees. The Natural History Museum explicitly states: Australopithecus afarensis most likely retreated to trees at night to avoid predators and obtain sleep.

This paper’s repositioning of Lucy: Lucy is not evidence for “upright walking first” — she is the fossil record of an intermediate state where “cooked meat consumption had already begun, but fire had not yet been brought to camp.” Her body was being pulled by two different selection pressures:

Then, when fire finally arrived at camp — the driving force for nighttime tree-sleeping vanished instantly. Upper-body climbing adaptations lost their selection pressure. Arms shortened, fingers straightened, scapulae transitioned from ape morphology to human morphology. The upper body finally “caught up” with the lower body. The body was no longer split. Fully upright. This is Homo erectus.

This also explains a puzzle that has long perplexed scholars — why the transition from australopithecines to Homo erectus appears so “sudden” in the fossil record. Wrangham called it “the most dramatic transition in the hominin record.” In this paper’s framework, it was not “sudden evolution” — it was the explosive release of long-accumulated change potential once the final constraint was removed. The upper body had been suppressed by the nighttime climbing requirement for over a million years; once that suppression disappeared (fire arrived at camp), the change was released like a compressed spring.

Timeline Verification: Aligning Archaeological Key Dates with Causal Chain Steps

Fire Eliminated the Only Reason for Sleeping in Trees

The sole reason human ancestors slept in trees was to avoid nocturnal predators — leopards, hyenas, lions. Tree-sleeping was never a “good choice” — it was unstable, required nest-building, risked falling, and fragmented sleep. It was a forced choice.

When fire was brought to camp, this sole reason disappeared. Fire repelling nocturnal predators is a physical inevitability — human ancestors did not need to “understand” this. Fire burned there, and animals stayed away. The effect was automatic. And any primate under safe conditions will choose the ground — the ground is flatter, more stable, more comfortable, and requires no nest-building. So it was not that humans “decided” to sleep on the ground — rather, the reason for sleeping in trees disappeared.

Ground-level supine sleep produced a cascade of physiologically inevitable sleep quality improvements:

Why was REM suppressed in trees? The core feature of REM sleep is complete skeletal muscle paralysis (muscle atonia). On an unstable tree branch, muscle paralysis means falling and death. Therefore the brains of arboreal primates actively suppress the depth and duration of REM to stay alive. On the ground, muscle paralysis is no longer a death threat, and the brain can safely enter deep REM.

Additional protection from the campfire: Temperature stability, light repelling snakes and insects, enhanced sense of security. Sleeping in groups around the fire also produced a natural “sentinel effect” — data from Hadza hunter-gatherers shows that in mixed-age groups, asynchronous wakefulness among individuals of different ages meant that at least one person was awake 99.8% of nighttime hours (Samson et al., 2017). Having someone on watch meant every individual could more confidently enter deep sleep.

Evolutionary significance of the extra 4 waking hours: These 4 hours are not “a little more time” — they are the first cognitive surplus time in evolutionary history. Chimpanzees’ 13 waking hours are almost entirely filled by survival-necessary behaviors, leaving no surplus for “learning” and “innovation.” In humans’ 17 waking hours, the nighttime social window around the campfire became an entirely new time slot for tool instruction, hunting strategy discussion, and storytelling — things for which there was no time during the day became possible during fireside evenings.

Sustained Cooked Food Initiates the First Loop: The Digestive System Drives Brain Development

Once the focused attention leap drove human ancestors from opportunistic cooked meat to sustained cooked food behavior, Loop A — established in our lab’s prior paper “The Cooked Food Cognitive Awakening Hypothesis” (V2) — was formally activated.

The core mechanism of Loop A is a reversal of causal direction. The mainstream narrative holds that “brain got bigger → needed more nutrition → therefore ate meat/cooked food.” But this hypothesis proposes the opposite direction: cooked food → digestive system remodeling (second brain evolves first) → gut-brain axis hormonal pathway restructuring → structural brain development in the head → cognitive awakening.

The enteric nervous system contains over 100 million nerve cells, using more than 30 neurotransmitters — most of which are identical to those in the brain. Over 90% of the body’s serotonin and approximately 50% of its dopamine reside in the gut. About 90% of vagus nerve fibers are afferent — the brain receives far more information from the digestive system than it sends. When cooked food caused the digestive tract to shrink and become more efficient, the enteric nervous system was partially “released” from the burden of heavy digestion, and the neuroactive substances it produced shifted their functional role from primarily serving digestion to increasingly influencing brain development through the gut-brain axis.

Puberty is the critical amplification window of this loop. Human puberty spans 10–15 years, during which myelination of the prefrontal cortex — the key process that dramatically increases neural signal conduction speed and efficiency — is concentrated. Puberty is also a period of explosive meat consumption demand. The explosion of meat consumption, peak gut-brain axis hormonal output, and prefrontal myelination are highly coupled on the timeline.

(The full argument for Loop A is found in our lab’s “Cooked Food Cognitive Awakening Hypothesis” V2, Chapters 3–6.)

Sustained Cooked Food Initiates the Second Loop: Tooth Retention → Lifespan Extension → Social Phase Transition

Loop B runs in parallel with Loop A. Cooked food dramatically reduced chewing burden (approximately 2.5 million fewer chews per year), slowed tooth wear rates, and caused the jaw to shrink. Jaw reduction produced a triple effect: (1) freed space for flexible lips — the key hardware for language articulation fell into place; (2) tooth retention extended effective lifespan — in the ancient world without modern dentistry, worn-out teeth equaled a death sentence; (3) extended lifespan provided the evolutionary prerequisite for an extended adolescence.

Extended lifespan created “living library networks” — each long-lived individual was a high-density information node. When “knowledge” outweighed “physical strength” in determining a group’s survival probability, social structure underwent an irreversible phase transition: from a violence-based hierarchy (alpha males ruling by physical force) to a knowledge-based hierarchy (chiefs, elders, and sages leading the group by information density). The knowledge hierarchy catalyzed division of labor, expanded cooperation scale, and institutionalized knowledge transmission.

(The full argument for Loop B is found in our lab’s “Cooked Food Cognitive Awakening Hypothesis” V2, Chapters 7–9.)

Upright Walking: Not the Starting Point, but the Endpoint

The conventional narrative treats upright walking as the first link in the evolutionary chain. But in this paper’s causal reconstruction, upright walking is the terminal morphological expression of all accumulated behavioral changes.

Supine sleep → spinal remodeling. Primates sleeping in trees do not lie flat — they curl up, sit semi-upright, and grip branches, keeping the spine curved. Once the transition to ground-level supine sleep occurred, the spine spent 7–10 hours every night extending and recovering without longitudinal gravitational compression. Over hundreds of thousands of years of accumulation, spinal curvature gradually adapted to the dual demands of both lying flat and standing upright — this is the origin of the uniquely human S-shaped spinal curve. Chimpanzees lack this S-curve because they never sleep supine.

No more tree climbing → limb proportion changes. When fire eliminated the need to climb trees, the selection pressure for maintaining climbing ability vanished. Arms shortening and legs lengthening was not actively evolved “for better walking” — it was the natural structural remodeling of the body under new usage patterns (ground-based activity, carrying fire and food, long-distance walking to fire sites) after “tree climbing was no longer necessary.” The fossil record’s own description of Homo erectus supports this — the academic phrasing is “lost features associated with climbing,” not “gained features for upright walking.” Lost, not gained.

Cooked food → jaw reduction → head center-of-gravity shift. Cooked food reduced chewing burden, causing the jaw and chewing muscles to shrink substantially, changing the overall weight distribution of the head. A lighter, shorter face allowed the head to balance more stably atop the spine — further promoting the stability of the upright posture.

Logical and Physical Alignment: Step-by-Step Input-Output Cross-Check

Comparison with the Conventional Narrative: Same Data, Different Causal Direction

Falsifiable Predictions of the Hypothesis

Currently, P2 already has strong positive validation — Samson & Nunn (2015)’s phylogenetic analysis confirms that human REM proportion and sleep duration are both outliers among primates. P6 has been partially validated by Pruetz’s Fongoli research — savanna chimpanzees’ response to fire is indeed calm and predictive. P7 has indirect support — Homo erectus’s gut and tooth reduction correlate highly with brain volume increase on the timeline. P9 already has strong positive validation — Carotenuto et al. (2016)’s biogeographical model confirms that Homo erectus nearly passively followed large herbivore herds during its dispersal. P11 has partial positive validation — Green & Alemseged (2012)’s analysis of Australopithecus afarensis scapular development confirmed that its development pattern matches apes rather than humans, and that lower-body bipedal adaptations precede the disappearance of upper-body climbing features, consistent with this paper’s predicted temporal pattern.

Fire as the Portable Civilization Kit: How Humans Went from Regional Species to Global Species

The development of upright walking and endurance running capability was not the endpoint of evolution — it opened the door for humans to leap from a regional species to a global species. This chapter argues: the progression from passive scavenging to active persistence hunting is a continuous behavioral gradient, and following the migration routes of large prey herds directly drove humans out of Africa.

The Five-Stage Evolution of Hunting Behavior:

In 1984, American biologist David Carrier first proposed the endurance running hypothesis, noting that the human body has multiple adaptations for endurance running — sweat glands for evaporative cooling, lack of body hair, and bipedal locomotion. Paleoanthropologists Daniel Lieberman and Dennis Bramble further expanded this hypothesis, noting that shortened toes, arch support, and elongated legs first appeared in Homo erectus. Simulation studies show that Homo erectus could sustain persistence hunting of large prey for over 5 hours without needing to carry water (Hora et al., 2020).

Following prey herds → Out of Africa:

The essence of persistence chase hunting is following the prey. Prey does not wait in one place — large herbivore herds migrate with seasons, climate, and vegetation. Humans chasing these herds naturally followed prey migration routes. Humans did not “decide” to leave Africa — prey led humans out of Africa.

A 2016 biogeographical study directly verified this: using geostatistical techniques and probabilistic models, researchers found that Homo erectus nearly passively followed large herbivore herds during its dispersal (Carotenuto et al., 2016). Within Africa, dispersal was statistically associated with the presence of large freshwater bodies (East African Rift lakes). The Smithsonian’s Rick Potts put it most directly: “Homo erectus didn’t have a map. They didn’t know they were leaving Africa. They were just going over the next valley to see what was there.”

Fire as the Portable Civilization Kit:

Fire played another critical role in the Out of Africa journey — it was humanity’s mobile infrastructure. At every new habitat, as long as they had fire, they could instantly establish a complete survival system:

This complete path from passive scavenging to global dispersal — every step is the physically inevitable extension of the preceding step: passive scavenging → active fire-scene seeking → opportunistic fire-scene hunting → active fire-drive hunting → persistence chase hunting → following prey herd migrations → out of Africa → fire replicating the complete survival system at each new habitat → global dispersal → modern human civilization.

A Burning Branch

Return to the original question: How did humans go from an ordinary ape to an intelligent species?

This paper’s answer: Through a causal chain in which every step is driven by physical inevitability. Not a single step in this chain requires a “flash of insight,” a “gene mutation,” or a cognitive leap beyond the primate baseline. Every step is the automatic consequence of the preceding one.

One day, during the dry season on the African savanna, a wildfire killed a large animal. A group of human ancestors — already skilled meat-eaters, already as unafraid of fire as chimpanzees — observed the entire process of meat being cooked at the fire’s edge. They approached and cut off partially-cooked chunks. There was too much meat to eat on the spot. The partially cooked meat would rot by tomorrow. They needed to take the meat and bring fire to continue heating it.

And so, one individual picked up a burning branch.

That branch was brought back to camp. Fire burned at camp. That night, no leopards came close. Nobody needed to climb a tree. For the first time, they lay flat on level ground and fell into a peaceful sleep. That night’s sleep was deeper, longer, and more restful than any fragmented bout of light sleep in the trees.

The next morning, they awoke, and the previous night’s experience had been consolidated during REM deep sleep. The four-way association — “fire = safety = delicious food = good sleep” — was etched into memory. From that day forward, they began actively maintaining fire and actively seeking cooked meat. Fire was never abandoned again.

Hundreds of thousands of years later, their digestive systems had been remodeled by cooked food. Their brains had been driven larger by the gut-brain axis. Their teeth, no longer subjected to relentless chewing, were retained longer, and they lived longer. The elders became living libraries. The power base of society shifted from “who can fight hardest” to “who knows most.” Their spines, from lying flat every night, assumed an S-shape. Their arms, from no longer climbing trees, shortened. Their legs, from walking on the ground, elongated.

They stood upright. Not because they “decided” to stand upright — but because all the accumulated behavioral changes left their bodies no other shape to become.

Then they began to run. Elongated legs, hairless skin, sweat glands covering the entire body — these by-products of ground-based living conferred an ability no quadrupedal prey could match: ceaseless pursuit under the blazing sun. Prey collapsed from overheating mid-run, while the humans kept going. They became Earth’s most persistent pursuers.

Prey herds do not wait in one place. They migrate with the seasons, crossing valleys, traversing savannas, heading for more distant places. The humans chasing them did not know they were leaving Africa — they were simply going over the next valley. But the fire in their hands made them fearless — at every new place, on the very first night, they could kindle a campfire, establish a safe camp, roast new prey, and sit around the fire discussing tomorrow’s direction.

Fire was humanity’s first portable civilization kit. Carrying it, they walked from East Africa to Georgia, from Georgia to Java, from Java to Beijing. They covered half the globe — not because they knew how big the Earth was, but because ahead there was always another herd, another savanna, another valley.

Upright walking was not the starting point. Upright walking was the result. Leaving Africa was not a plan. Leaving Africa was a by-product of chasing prey.

And the starting point of it all was a burning branch.

References and Data Sources

[1] Wrangham R. Catching Fire: How Cooking Made Us Human. Basic Books, 2009.

[2] Samson DR, Nunn CL. Sleep intensity and the evolution of human cognition. Evolutionary Anthropology, 2015; 24(6): 225-237.

[3] Samson DR et al. Chronotype variation drives night-time sentinel-like behaviour in hunter-gatherers. Proc R Soc B, 2017; 284: 20170967.

[4] Pruetz JD, LaDuke TC. Reaction to fire by savanna chimpanzees at Fongoli, Senegal. Am J Phys Anthropol, 2010; 141(4): 646-650.

[5] Pruetz JD, Herzog NM. Savanna chimpanzees at Fongoli, Senegal, navigate a fire landscape. Current Anthropology, 2017; 58(S16): S337-S350.

[6] Coolidge FL, Wynn T. The effects of the tree-to-ground sleep transition in the evolution of cognition in early Homo. Before Farming, 2006; 4: 1-18.

[7] Aiello LC, Wheeler P. The Expensive-Tissue Hypothesis. Current Anthropology, 1995; 36(2): 199-221.

[8] Smith AR, Carmody RN, Dutton RJ, Wrangham RW. The significance of cooking for early hominin scavenging. J Hum Evol, 2015; 84: 62-70.

[9] Ben-Dor M, Barkai R. A bioenergetic approach favors the preservation and protection of prey, not cooking, as the drivers of early fire. Frontiers in Nutrition, 2025.

[10] Plummer T et al. Expanded geographic distribution and dietary strategies of the earliest Oldowan hominins and Paranthropus. Science, 2023; 379: 561-566.

[11] Domínguez-Rodrigo M et al. Computer vision supports primary access to meat by early Homo 1.84 million years ago. PeerJ, 2022; 10: e14148.

[12] Pobiner B. Evidence for meat-eating by early humans. Nature Education Knowledge, 2013; 4(6): 1.

[13] Nunn CL, Samson DR. Sleep in a comparative context: investigating how human sleep differs from sleep in other primates. Am J Phys Anthropol, 2018; 166: 601-612.

[14] Nedergaard M et al. Norepinephrine-mediated slow vasomotion drives glymphatic clearance during sleep. Cell, 2025; 188(1).

[15] Roebroeks W, Villa P. On the earliest evidence for habitual use of fire in Europe. PNAS, 2011; 108(13): 5209-5214.

[16] Ashton N et al. Earliest evidence of intentional fire-making at Barnham, UK. Nature, 2026.

[17] Hawkes K et al. Grandmothering, menopause, and the evolution of human life histories. PNAS, 1998; 95(3): 1336-1339.

[18] Zink KD, Lieberman DE. Impact of meat and Lower Palaeolithic food processing techniques on chewing in humans. Nature, 2016; 531: 500-503.

[19] Carabotti M et al. The gut-brain axis: interactions between enteric microbiota, central and enteric nervous systems. Annals of Gastroenterology, 2015; 28(2): 203-209.

[20] Siegel JM et al. Natural sleep and its seasonal variations in three pre-industrial societies. Current Biology, 2015; 25(21): 2862-2868.

[21] Wadley L et al. Fire and grass-bedding construction by early humans at Border Cave, South Africa. Science, 2020; 369: 863-866.

[22] Navarrete A, Van Schaik CP, Isler K. Human brain expansion during evolution is independent of fire control and cooking. Front Neurosci, 2016; 10: 167.

[23] LEECHO Global AI Research Lab. The Cooked Food Cognitive Awakening Hypothesis V2. LEECHO Original Thought Paper, 2026.

[24] LEECHO Global AI Research Lab. Information Compression and Decompression. LEECHO Original Thought Paper, 2026.

[25] LEECHO Global AI Research Lab. The Past and Present of Cognition. LEECHO Original Thought Paper, 2026.

[26] Carrier DR. The energetic paradox of human running and hominid evolution. Current Anthropology, 1984; 25(4): 483-495.

[27] Bramble DM, Lieberman DE. Endurance running and the evolution of Homo. Nature, 2004; 432: 345-352.

[28] Hora M, Pontzer H, Wall-Scheffler CM, Sládek V. Dehydration and persistence hunting in Homo erectus. J Hum Evol, 2020; 138: 102682.

[29] Liebenberg L. Persistence hunting by modern hunter-gatherers. Current Anthropology, 2006; 47(6): 1017-1026.

[30] Carotenuto F et al. Venturing out safely: the biogeography of Homo erectus dispersal out of Africa. J Hum Evol, 2016; 95: 1-12.

[31] Lieberman DE, Bramble DM, Raichlen DA, Shea JJ. Brains, brawn, and the evolution of human endurance running capabilities. In: The First Humans. Springer, 2009; 77-92.

[32] Johanson DC, Edey ME. Lucy: The Beginnings of Humankind. Simon & Schuster, 1981.

[33] Green DJ, Alemseged Z. Australopithecus afarensis scapular ontogeny, function, and the role of climbing in human evolution. Science, 2012; 338: 514-517.

[34] Lau EYY et al. Rapid-Eye-Movement-Sleep (REM) associated enhancement of working memory performance after a daytime nap. PLOS ONE, 2015; 10(5): e0125752.

[35] Lim J, Dinges DF. A meta-analysis of the impact of short-term sleep deprivation on cognitive variables. Psychological Bulletin, 2010; 136(3): 375-389.

[36] Kuzawa CW et al. Metabolic costs and evolutionary implications of human brain development. PNAS, 2014; 111(36): 13010-13015.