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Book Summary: The Book of Humans: A Brief History of Culture, Sex, War and the Evolution of Us

Author: Adam Rutherford

Substory: Fire

Fire is a tool that has utterly transformed our existence, not just in the industrial age, but from well before our particular type of human being had settled into the form we currently enjoy. We have good evidence that Homo erectus, that highly successful human who walked all over the Earth from 1.9 million years ago until around 140,000 years ago, was a fire user in some capacity. The dates when they first utilised fire remain disputed. Sifting through the dirt of ancient human sites is a fiddly business, and while there is molecular evidence of burnt bones and flora as long ago as 1.5 or 1.7 million years ago (depending on where you look), these are open-air sites, and it’s not clear that these are not the result of wildfires triggered by lightning strikes or local volcanoes, rather than deliberate use by early humans. Some have suggested that based on the shapes of their teeth and other morphological dimensions, Homo erectus was cooking food as long ago as 1.9 million years. The earliest secure date for fire in an archaeological context is probably around one million years ago in the Wonderwerk Cave in South Africa.

However and whenever the transition took place, humans moved from an opportunistic use of fire to habitual use, and eventually to obligate pyrophiles. This transition, as with all the stories in human evolution, almost certainly occurred slowly and incrementally over time – there was not one single spark, but many. Archaeologists argue over the earliest evidence of controlled use of fire. Then again, archaeologists argue about a lot of things.

By 100,000 years ago, we had it largely under control. As a source of heat and light, man’s red fire is of obvious benefit, as is the ability not just to control it, but to generate fire from a spark. The Jungle Book’s orangutan-in-chief King Louie expresses his desire to be just like you, specifically by owning this uniquely human ability, and he is wise to sing so. The impact of fire on the development of humankind is incomparable. We expanded north with fire as a source of heat beyond the temperate and tropical zones whence we evolved. This gave us access to a whole new range of beasts both large and small to hunt, cook and feast upon and make tools and clothes and art from their bodies. As is the case today, the social significance of congregating around a hearth or a fire should not be underestimated. Social bonds are forged and consolidated around a fire, stories told, skills passed on and food prepared and shared.

We are the only animal that cooks. Energy and nutrients are sometimes held deep inside the vegetation and flesh that we consume, and digestion is the process whereby they are released. This can be chemical, and mechanical. Teeth can be for grinding, tearing and chewing, but all are for some form of maceration, the process by which food is broken down to make it more accessible to the enzymes that will chew with molecular precision. Plenty of animals use artificial mechanical means to aid digestion. Birds don’t have teeth to macerate, but they do have gizzards – muscular pouches in their digestive tracts that some fill with grit which grinds up food, making it easier to chemically digest. We call these ‘gastroliths’ – stomach stones – and this is an ancient practice. The fossilised remains of many dinosaurs from the Cretaceous and Jurassic periods have been found with smoothed stones inside their body cavities, where once the soft tissue of gizzards would have been.

We outsourced some of our digestive abilities by externalising them. By cooking foods, we break the bonds of complex molecules, and make them easier to digest in our stomachs. Meat is tenderised by heating. Softer foods are quicker to eat too, in that we spend less time chewing a boiled cabbage than a raw one, which means we get access to the essential nutrients more efficiently. Dining is a period of vulnerability: when your face is occupied with ingesting a meal, it is less alert to danger from predation. Spending less time eating means less time being potentially eaten.

All of these things make cooking a desirable and essential part of our evolution. Some researchers have suggested that we became a pyrophilic primate by living among iterant burning ecology and adapting to the benefits it brings. Some have suggested that the origins of cooking, or at least an understanding of how heat changes food, might have begun by apes foraging in burnt landscapes. It’s difficult enough to roast a turkey to perfection in a twenty-first-century oven, so it’s not unreasonable to suppose that animals roasted in wildfires are most likely to be burnt or undercooked. But it may be that these first hot meals were the spark of the idea of using heat to change food for the better.

The other obvious benefit of safely standing to the side of a raging furnace is that you can be presented with an exodus of other animals fleeing from the danger. If these animals are of interest to you as food, then fire provides a free all-you-can-eat buffet. We think that South African vervet monkeys do this, and enjoy an unprecedented access to invertebrates scuttling out of the fire, and into their mouths. We also think that the monkeys know this well, and so increase their normal foraging range into wildfire regions, especially after a recent blaze. There’s another set of benefits from this behaviour too. Vervet monkeys stand up on their hind legs to look out for predators, and thus can see over the grasses and plants. When they are cleared by being burned to stubble, the monkeys can see further. Vervet monkeys in scorched plains spend more time feeding, and feeding their young, and less time standing erect on the lookout for something that will eat them.

Even closer relatives to us, savannah chimpanzees in Fongoli, Senegal, live among fire as part of their natural ecology too. It is hot in the grasslands anyway, but since 2010, the onset of the rainy season has become increasingly erratic. From October, fires start which encroach upon three-quarters of the chimps’ thirty-five square-mile range. These most often ignite at the beginning of the rainy season, when rolling thunder and lightning meets arid bush.

Scientists have been watching these chimps for decades, and in 2017 reported on their relationship with fire. There are several things worth noting. The first is that they are untroubled by wildfires. Mostly they ignore the burning brush, but sometimes wander into and explore areas that were on fire just minutes before. They appear to navigate in burnt areas frequently, which may be the same trick the vervet monkeys are using to increase their lookout range to avoid predators. We know that in the Mara-Serengeti in Kenya, other large herbivores congregate in scorched areas at higher densities than in healthy grasslands, including zebra, warthogs, gazelles and topi. It may also be easier and quicker to traverse land flattened by plants being burned to ash.

The fact that these chimps behave in a specific predictable fashion when their world burns suggests that while they cannot control fire, they certainly can conceptualise it, and crucially predict its behaviour. This is a cognitive benchmark, in which the animal is capable of rationalising and approaching something dangerous, rather than simply taking the safest course of action, which is to flee. It’s a sophisticated reaction too: the way a fire burns is a complex and capricious process, which depends on what is burning, the wind, and a host of other factors, and can change in a flash. Within seconds, fires reach temperatures incompatible with life, and can release smoke and noxious gases that are also threatening to apes.

The vervets and the savannah chimps are potential clues for us when we think about the genesis of our own relationship with fire. We look to nature today to draw comparisons and speculate that what we see now might have been similar to what happened way back when. This may be egocentric. All data is useful in some way, but there is a whiff of presumption in the notion that behaviours in our fellow apes reflect our own journey to the present.