How taming fire made us human

For some 430 million years, fire has been a persistent if fluctuating feature of planet Earth. This is, nevertheless, a surprisingly tiny percentage of Earth history. For the first 90% of Earth history, the planet’s face was entirely untouched by flame. This is because it takes three things to make a fire.

The spark. For all of Earth history, lightning has been zapping the surface, today at the rate of 100 strikes a second. And for the entire history of fire on planet Earth, the vast majority of fires have been set by these bolts from the blue. Astoundingly, though, even at the rate of 100 strikes a second, human pyrophilia today far outpaces lightning, with 84% of the world’s blazes set by people (not counting the controlled fires in every fossil-fueled power plant, furnace, and engine on Earth).

Then you need oxygen. There are no fires without a lot of oxygen in the air, so, as a result, there weren’t fires for most of Earth history. Though oxygen is the by-product of making plant matter from CO₂ through photosynthesis, to build oxygen up to appreciable levels, vast seas of this organic carbon have to be buried in the crust as fossil fuels. Every breath you take is the gift, not of plant life today, as the intuitive cliché goes, but rather the impounding of some small fraction of this life in the crust, leaving behind an oxygen surplus that has taken hundreds of millions of years to accrue. And fire on the surface doesn’t catch until oxygen reaches about 16% of the atmosphere. As discussed, this level of abundant oxygen is a very late feature on the surface of our planet, appearing only in the age of animals, once tectonics and life conspired to bury enough carbon in the crust below.

But even this isn’t enough. The third and final ingredient in fire is the fuel — something to burn. Seeing as rocks generally don’t burn, fuel was available only once life emerged onto land. As a result, the first fires appear in the fossil record only about 430 million years ago, once all three ingredients had arrived on the Earth’s surface. 

The first fires tore across humble, pioneering patches of plant life no taller than your ankle, as well as burning through rather more astonishing, and truly bizarre, 25-foot columns of fungus that briefly held sway on this desolate world before trees. When the tiny plants suddenly gained confidence, though, and shot up toward the sky as trees some 50 million years later in the Devonian Period, the first forests tentatively spread across the world, and fire came into its own. From then on, fire would be a familiar phenomenon on Earth’s surface for hundreds of millions of years, and charcoals in the fossil record testify to its presence, for the most part, ever since.

Fire swept through the high-oxygen rain forests of the Carboniferous and razed the terrestrial biosphere in the End-Permian mass extinction. It also played a crucial role in the rise of flowering plants in the Cretaceous, which remade the living world with more than just the fruit they bore. Flowering plants cycle through more water but are also more flammable, and, according to the paleobotanist Claire Belcher, their incendiary nature helped clear out the less fire-adapted ancient plant world that came before. These plants then established themselves firmly enough that their prodigious water recycling was sufficient to make their own weather — as testified by the clouds wisping off modern rain forests — counteracting this flammability and giving rise to the closed-canopy jungles familiar to the age of mammals. 

As the world lost its CO₂ blanket over this age of mammals, though, and the world chilled, grasslands, having invented a new form of photosynthesis adapted to this drier, lower-CO₂ world, haltingly but inexorably took over much of the Earth. As they did so, these prairies and savannas were now given to explosive grass fires. This was the context of our evolution.

“I think climate change is the fundamental piece of the puzzle,” said Nicole Herzog, an archeologist at the University of Denver, about the continual innovation in our lineage and the rise of this strange fire creature.

“What really prompts people to innovate — or any organism to innovate — is finding yourself on the tail end of a fitness curve. You find yourself out on the edge, in other words. And at some point, there’s no other direction to go in order to survive. So that sort of pushes you into a new space where you have to develop a new technology. And it’s costly to do that. And if you don’t have to, you probably wouldn’t. So all of this ecological change is happening and up to that point, there are lots of different types of hominins, and lots of apes, and we’re all sort of living in this place together. But the ecological change makes this habitat super fragmented. And for some reason, humans end up out on the edge, where all of this aridity is happening. So our ancestors are in contact with fire-altered landscapes, probably much more frequently than chimps who seem to have maintained a foothold in the forested areas.”

Even today, though, chimps show a surprising comfort around fire. In her fieldwork in the Sahel of southeast Senegal, Herzog marvels at the animals’ lack of concern as they patiently wait at the edge of grassland flames that yearly consume up to 75% of their habitat — the majority of them set by local villagers to prevent more devastating fires, promote local millet production and livestock agriculture, and reduce the threat of hiking through shoulder-high grasses filled with snakes and other unseen hazards to life and limb. Once the fires have swept through, the chimpanzees have their pick of cooked critters on a charred landscape cleared of predators, just like the baboons in the forests of Uganda who have been seen foraging for cooked grasshoppers and shoots after fire passes by. 

Herzog imagines our earliest ancestors might have evinced a similar curiosity about the novel fire-scarred landscape — seeing fire less as a seasonal catastrophe than as a potential caloric windfall.

But where chimps might merely poke over the ashes, our lineage at some point took the next step in promoting and even domesticating the flames — perhaps picking a smoldering stick from the edges of one grassland, and lighting the grassland in the next valley over. The energetic returns to foraging in this sparse landscape suddenly soared. Slowly and tentatively at the dawn of the Pleistocene, and then suddenly around 1.9 million years ago, with the rise of the even brainier Homo erectus, and as grasslands reached their peak, we had become the fire ape.

Today, the entire industrialized world is held up by the energy released by oxidizing organic carbon back to CO₂. Fire, in other words. We just happen to be burning all the plant matter in Earth history that we can get our hands on, buried in the crust. But it’s a trick we learned early, and it’s essential to our nature. We are what’s known as an “obligate pyrophile.” We need fire to survive. 

In the words of a study led by the archeologist Christopher Parker, we are “wholly dependent on fire for survival and reproduction.”

It is impossible to exaggerate the significance of this cultural innovation. For one thing, fire allowed us to remake the world in our image. There’s nothing new about this. Since the dawn of animal life, over a half-billion years ago, animals have set about remaking the landscape in ways that have benefited their own reproduction and survival. Just before the Cambrian, it was marine worms digging into the muck, aerating the seabed, possibly wiping out the blob creatures that placidly lived there, and completely remaking the ocean landscape. In our own time, a modern exemplar is beavers, who build dams, flood forests, and skillfully construct little island castles of lumber to protect themselves from predators, remaking the world to make it more habitable for more beavers. 

This is called ecosystem engineering, and it has been transforming the surface of the planet for as long as there has been animal life. Humans are merely the ultimate ecosystem engineers, and fire has long been the most powerful tool in our arsenal. We can’t survive in a world without fire, so instead we aided its spread as far and wide as we were able. Just as there is no meaningful distinction between a forest waterlogged with beaver ponds and a supposedly more “natural” landscape, so too did the human landscapes of the Pleistocene become hopelessly entangled with the natural world.

In our case, fire allowed hunter-gatherers to reimagine the landscape by creating mosaics of fire-adapted berry bushes and nut trees, and heathlands of shoots and saplings that lured game for the hunt. It concentrated people around these hearths, increasing the pressures of sociality. An engine of sorts was gathering here, drawing calories closer to human settlements, energy that was dissipated in the flickers of flame and the fires of metabolism. 

But the main reason fire was so important to ancient humans is the same attribute that today sends the planet careening toward a climate catastrophe: turning photosynthesis — whether living or fossil — back to CO₂ releases energy. Fire is hot. You can cook with it.

Perhaps the most telling piece of evidence testifying to the importance of fire for our kind was the attendant suite of changes to the anatomy of the now-varied group of hominins, various species of which spanned not only Africa but also Eurasia as the Pleistocene drew on, from Spain to Indonesia. With small guts but big, energy-hogging brains, these creatures were apparently performing much of their metabolism outside of their bodies in order to subsidize their intelligence. This is because there simply aren’t enough hours in the day to forage and digest raw foods sufficient to support brains as energetically expensive as our own, contra many a modern paleo-diet guru. 

Cooking is the only way. This is because brains are ravenous for glucose, an energy-dense sugar synthesized by photosynthesis that we combust with a perpetual metabolic bonfire of oxygen to keep us conscious at every waking moment. But to get at glucose, we first need to cook our plants — wheat, barley, rice, millet, lentils, peas, chickpeas, bitter vetch, cassava, sago, yam, taro, plantains, breadfruit, sweet potato, etc.—in order to explode little packets of starch within them and unravel the long, dense polymers stitched together from glucose. This unlocks the sugar for a much easier metabolism, allowing us to skip the painstaking, energy-intensive step of breaking down these starches in our guts ourselves, and allows us to extract far more energy from these staple foods at lower cost.

Cooking meat, meanwhile, unfolds the involuted ribbons of protein, and avails them to the digestive onslaught of enzymes. It makes the engine of metabolism more energy-efficient. Where chimps might spend six hours a day chewing, and an anaconda might spend weeks motionless as it digests a capybara, humans have outsourced much of this digestive work to fire. This increase in efficiency also allowed humans to significantly reduce the amount of time spent gathering food.

Indeed, one study estimates that in order for a gorilla to get enough energy in its diet to subsidize a brain like ours, it would have to spend more than two extra hours a day foraging for food, in a twelve-hour day most of which is already spent foraging. The math just doesn’t add up without the boost from fire, and any animal that tried to make it so would starve.