This discovery of fire, probably the greatest ever made by man, excepting language, dates from before the dawn of history.
-Charles Darwin, 1888
I haven’t smoked in years, but I still carry a stainless steel Zippo with me. I purchased it a decade ago upon my promotion from barista to barista shift-supervisor. This nominal promotion came with no accompanying pay rise, but I nevertheless felt compelled to commemorate this accomplishment in some way. At $30, it cost nearly a half day’s labor at the time. Why have I carried it since? Partly as a reminder of harder times and early successes. Partly the iconic and eminently satisfying “click”. Partly for the rare occasion that someone will ask for a light and I’ll finally get to suavely withdraw a sleek, if weathered, steel case from my jacket and, with a flick of my wrist, conjure a flame for my new, smoker friend. But mostly, I carry it because fire is cool.
The flame that we casually pass around is perhaps the most significant technology in the history of mankind. Fire powered the steam engines towing trains across the American frontier and the magnificent vessels traversing the world's oceans. Fire ignited the forges that defined the Bronze and Copper ages. Fire kept warm those early humans that ventured out of Africa and into colder climes. And it was fire that distinguished one group of humble primates from all the other creatures of the world and lit within him the spark of man.
The Wise Man
The defining characteristic of mankind is in the (oft-aspirational) name we have chosen for ourselves: Homo sapiens, wise man. It is with our minds that we envision a world that does not yet exist and bring it into being. We uniquely have a sense of history because we can share stories that persist through hundreds of generations. These stories enable the cooperation of millions of people that have never met. Amidst the grand scale of our collective endeavors and grander ambition, our inner lives are rich and self-reflective. Each of us considers our own condition as well as that of all humans.
Part of our condition is loneliness: of all creation we stand apart. Even our closest living relatives, the chimpanzees and bonobos, while clever in their own way, clearly lack our facilities. Assuming that evolution has no “man-ward impulse” and that our existence was not pre-ordained, how did human intelligence emerge, and why did it only emerge once?
Evolution gives rise to many survival strategies. Some species succeed by becoming faster, others by getting stronger or taller or smaller or resilient or more keen-eyed, crafty, cautious, spotty, stripey, or furry. For our soft and squishy species, there was a selective pressure for intelligence and social cooperation. Eventually though, our predecessors reached a biological limit to intelligence and the equilibrium could not be increased without the intervention of a new technology.
The Just, Kinda Smart Man
Though brain size isn’t everything, it is at least a part of the equation. Australopithecus afarensis1 is a chimpanzee-like species of hominin (humans, their direct ancestors, and probable direct ancestors) that lived around three million years ago. They were roughly 1/3 the size of a modern human. Their brains were about 450cc while the average Homo sapiens brain is around 1350cc. While there are debates about the correlation between intelligence and brain size, encephalization quotient, and brain energy consumption, generally a larger brain is necessary for greater intelligence in the Homo genus2.
Human intelligence emerged gradually through a series of incremental changes, each building upon the last. The first step may have been a slight improvement in diet during lean times. In good times, australopithecines gorged on fruits, nuts, seeds and such. During the dry seasons when food is harder to come by, their close relatives likely supplemented their diet with foliage. During these hard times, the australopithecine may have instead looked to the forest floor rather than its canopy. By digging a little, they would have found tubers and roots, which are both easier to digest and more nutritious.
Around three million years ago, australopithecine made a significant innovation: they began using stones to slice meat and break bones which exposed the nutritious marrow. They may have also used stones or wooden clubs to tenderize meat which makes it easier to digest. These diet enhancements increased the average calories consumed per hour. By substituting lower calorie-per-hour methods for higher ones, they also increased the total calories consumed per day. Over the next million or so years, this hardier diet facilitated the evolution of Homo habilis, the first human species. Homo habilis would innovate further, creating the crafted stone axes discussed in the previous issue. Even with their hand axes and increased access to meat, a new technology, fire, would be necessary for further brain growth.
Catch-22 of Intelligence
Brains are expensive to run. Although the brain only accounts for around 2.5% of a Homo sapiens’ mass, it consumes 20% of its energy. In order to sustain a large brain, enough food must be consumed and efficiently digested to fuel it. This requires the intelligence of a larger brain which in turn requires more nutrients which requires a larger brain and so forth.
Step 1: Obtain food
Just getting enough food to support a large brain was no small matter. First, our habiline (individual members of the Homo habilis species) ancestors needed to locate enough digestible food. Then, they needed to eat it before a competitor could eat it (or them). Homo habilis ate a varied diet that included leaves, grass, fruit, other plant matter, and some meat. Some fruit was probably just about as easy and delicious for them to eat as it is for us or our primate contemporaries, though wild fruit tends to be tougher and harder to eat than its domesticated brethren. Most other components of their diet were much more time-consuming to eat.
Step 2: Digesting
Outside of the garden of Eden, you can’t always count on a steady supply of fruit. What there is usually a bountiful supply of are leaves and grasses. As a child, I had a go at eating a leaf. I don’t know exactly how long I chewed before I gave up, but it must have been at least five minutes. Modern chimps, which have molars better adapted for eating leaves take around 5 minutes to eat a mouthful. Leaves and grasses simply take too long to digest and don’t provide enough energy to support an energy-hungry brain.
Meat is quite energy dense, but raw meat is challenging (and energy-intensive) to digest. Chewing a mouthful of raw meat can take around 20 minutes, yielding about as many calories per hour as fruit. While meat-eating increased the total amount of calories available to habilines, it did so by increasing the average calories-per-hour consumed, not the maximum.
Even with a well-balanced diet including fruits, nuts, and a little bit of raw meat, much of their day would have been spent gathering and chewing their food. Estimates based on sapiens body size and other great ape’s diets suggest that we would spend around five hours a day chewing food if we didn’t process our food at all. Even with unlimited food supply, habilines were faced with a time constraint: they could only digest so many calories per hour.
Step 3: You mean I have to do this every single day?
During bountiful times, habilines would have had no problem scrounging up enough food to satisfy their metabolic needs. On an evolutionary time scale however, lean times are guaranteed. Had the brain-fueling strategy simply been “eat more food” they would have faced a metabolic vulnerability. All animals have a basal metabolic rate: the number of calories consumed when the body is at rest. Even during temporary periods of food instability, a higher basal metabolic rate increases the minimum calories required to continue living. All else equal, larger brains, which require more food would have increased the basal metabolic rate. As useful as intelligence is, evolving a larger brain without the means to reliably fuel it is a vulnerability.
However, Homo sapiens do not have this vulnerability: our basal metabolic rate is in line with what would be expected for a great ape of our size. As we evolved, our overall basal energy consumption remained steady while significantly increasing the absolute energy use of the brain. How so? By diverting energy from another organ: the gut.
Digestive tracts can be energy intensive, but more variably so than brains. While the brain must always be active and its energy consumption varies only slightly, the digestive tract consumes more energy when it is digesting food and less when it is not. The gut’s energy consumption also depends on how challenging the food is to digest: hard leaves and grass require lots of energy to digest while cooked meat and fruit are much easier. Accordingly, the size of an animals’ gut is a function of its diet. Animals for whom leaves and grasses comprise a large share of their diet have large guts, while herbivores that mostly eat fruits, nuts, and tubers have smaller guts, and carnivores have the smallest of all.
The Brain Growth Dynamic In Summary
Intelligence is beneficial and exerts a positive evolutionary pressure. Generally, it requires a larger brain. Brains require lots of energy to run, so a larger brain exerts a negative pressure. Energy supply is dependent on both the quantity and the quality of diet: limitless food supply may still be insufficient to run a large brain if it takes a lot of time and energy to consume. To increase the energy supply available to the brain, the total calories consumed per day must increase. There are limits on the number of hours that can be spent eating, so more efficient diets that increase the average calories consumed per hour is necessary. A larger brain increases the basal metabolic rate. To offset the risk of a larger basal metabolic rate, energy consumption from another organ must decrease.
Fire: The Engine of Intelligence
Up until the 19th century, the fastest, easiest, most efficient way to travel across land was usually by horse. Then a new invention revolutionized land travel: the steam engine3. Businesses, seeing the opportunity, quickly invested massive resources into building trains and tracks to carry them. By the end of the 19th century, railroads criss-crossed America and trains carted people and goods safely and quickly across the nation. This technology, powered by fire, facilitated monumental improvements in trade, manufacturing, transit, and human wellbeing.
As with the steam engine of the 19th century, the control of fire would facilitate monumental changes to the brain, culminating in human intelligence and consciousness.
The origin of fire control
When did hominins begin using fire? Using fire for cooking was critical to developing larger brains and increasing cognitive ability, but evolution doesn’t work quickly. Early habilines weren’t whipping out their Zippos to ignite their evening hearth. The very first habilines to control fire probably came across the smoldering remnants of a forest fire, perhaps around 1.8 million years ago. Surveying the wreckage, they likely noticed some baked bird eggs or the braised carcass of a woodland creature that failed to escape the conflagration. Perhaps they dug around for tubers and noticed that they were much easier and tastier to eat than usual.
Some smart chap, having remembered the good meal he’d salvaged from the forest fire had the ingenuity to harvest the fire itself. Picking up a still smoldering branch, this rudimentary torch could be carried with the band so long as they took care to keep it lit. Once they got used to caring for the fire, they probably would have made this a regular practice as the benefits are so great and immediate: cooked food is delicious and fire provides warmth through the chilly night. When the fire eventually snuffed out, they would have plenty of opportunities to harvest more. As forest fires are quite common.
At some point, hominins would discover that they already held the key to creating fire in their hands! Two stones, flint and pyrite could be struck against each other to create sparks. With care and attention, those sparks could be directed to ignite dry kindling and the community hearth. Eventually, perhaps half a million years ago, they developed more sophisticated methods such as fire drilling. Over time, hominins would harvest less fire and create more on their own, though they probably used both methods concurrently for hundreds of thousands of years.
Benefits of fire
Aside from the caloric benefits of fire, cooking also detoxifies food and reduces spoilage. It is useful for scaring off predators and staying safe at night. When hominins eventually ventured into northern climes, it was essential for keeping warm. There is evidence that it was used even tens of thousands of years ago to clear forests and make the landscape more productive for human needs. And of course, fire is the basis of many important technologies. An early example is the making of “pitch” or glues by boiling tree barks for hours at a time. This glue was used to adhere Levallois points, or spearheads, to wooden shafts to create a more effective spear.
Beyond directly improving diet and all the associated benefits, fire usage compounded the already social nature of hominins. Cooking freed up a lot of time for other activities. More nutritious food meant that less time had to be spent gathering food in the first place while tenderer food required less time to eat. Fire also enabled night-eating, expanding the number of productive waking hours. This time was likely spent on non-functional socialization: playing, alliance-making, etc. This reinforced the already social nature of hominins and increased the evolutionary value of the ability to manage complex and numerous social relationships, and thus intelligence.
Cooking also changed how habilines procured food. Fire increased the benefit derived from meat, incentivizing hunting as a larger share of time expenditure. Hunting though, is risky, both because it is dangerous, and because it does not provide a consistent supply of food: some days the hunt is successful, others it is not. This led to a sex-based division of labor: men hunted while women gathered and prepared food. This sexually dimorphic calorie-procurement strategy is an early example of diversification: they lowered their overall risk profile by investing half their energy on high-risk, high-reward activities and the other half on lower-risk, more consistent activities. The combined strategy yielded a higher quality diet able to support ever larger brains.
Conclusion
Assuming that nothing is pre-ordained, and that the foundations of the world we inherit are the product of organic processes helps us to understand the world our ancestors were responding to and affirms their agency. Over the course of millions of years, hominins strove to improve their lot, making incremental improvements wherever possible. They did this without designs on the far-reaching future. Their most important advancements in this era: stone tools and fire control, were created simply to get a slightly tastier, heartier meal.
They could not possibly have predicted that these technologies would, over millions of years, lead to decedents that have conversations that go on for millennia. Descendants for whom no single work, nor even a museum of them could capture the depth of their experience. They could not imagine that we would shape the world as much as the world has shaped us.
Next in the Prehistory series
References
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Diamond, Jared M. 2005. Guns, Germs, and Steel : The Fates of Human Society. New York ; London: W.W. Norton & Company.
Gowlett, J. A. “The Discovery of Fire by Humans: A Long and Convoluted Process.” Philosophical Transactions of the Royal Society B: Biological Sciences 371, no. 1696 (June 5, 2016): 20150164. https://doi.org/10.1098/rstb.2015.0164.
Harari, Yuval Noah. 2011. Sapiens: A Brief History of Humankind. New York: Harper Perennial.
McPherron, Shannon P., Zeresenay Alemseged, Curtis W. Marean, Jonathan G. Wynn, Denné Reed, Denis Geraads, René Bobe, and Hamdallah A. Béarat. “Evidence for Stone-Tool-Assisted Consumption of Animal Tissues before 3.39 Million Years Ago at Dikika, Ethiopia.” Nature 466, no. 7308 (August 2010): 857–60. https://doi.org/10.1038/nature09248.
Pausas, Juli G., and Jon E. Keeley. “A Burning Story: The Role of Fire in the History of Life.” BioScience 59, no. 7 (July 2009): 593–601. https://doi.org/10.1525/bio.2009.59.7.10.
Wrangham, Richard W. 2010. Catching Fire : How Cooking Made Us Human. New York: Basic Books.
The link between Australopithecus afarensis and the Homo genus is still unproven as the “missing link” between the two has not yet been found.
The correlation between brain size and intelligence is an imperfect and imprecise one and is used in this context because brain size the only direct method of inferring the intelligence of extinct species.
The steam engine found some commercial applications in the 18th century but it wasn’t until the 19th century that it powered a locomotion revolution.