The fossil record reveals that fire first appeared on Earth during the Middle Ordovician period, approximately 420 million years ago, coinciding with the establishment of land-based flora. These early plants released vast amounts of oxygen into the atmosphere, and when concentrations rose above 13 percent, the conditions for wildfire became possible. The earliest evidence of this phenomenon appears in the Late Silurian fossil record, dated to 419 million years ago, where fossils of charred plants provide the first physical proof of fire. This geological timeline demonstrates that fire is not merely a human invention but a fundamental planetary process that has shaped the evolution of life itself. The presence of charcoal in the fossil record correlates closely with atmospheric oxygen levels, indicating that oxygen was the key factor in the prevalence of wildfire long before humans walked the Earth. Fire became even more abundant when grasses emerged as the dominant component of many ecosystems around 300 million years ago, providing excellent tinder for rapid fire spread. This widespread emergence of wildfire may have initiated a positive feedback process, producing a warmer, drier climate more conducive to fire, which in turn allowed fire to spread further.
The Dawn of Human Control
The ability to control fire marked a dramatic shift in the habits of early humans, transforming survival strategies and social structures. Evidence of occasional cooked food dates back to 1.5 million years ago, suggesting that fire was used in a controlled fashion during this period. Other sources place the date of regular use at 400,000 years ago, with widespread evidence appearing between 50,000 and 100,000 years ago. This timeline coincides with the evolution of resistance to air pollution in human populations, suggesting a co-evolutionary relationship between fire and human biology. The heat produced by fire allowed people to stay warm in cold weather, enabling them to live in cooler climates and dark caves. It also kept nocturnal predators at bay, providing a sense of security that allowed early human communities to gather and socialize. The use of fire became progressively more sophisticated, as it was used to create charcoal and to control wildlife from tens of thousands of years ago. By the Neolithic Revolution, during the introduction of grain-based agriculture, people all over the world used fire as a tool in landscape management. These fires were typically controlled burns or cool fires, as opposed to uncontrolled hot fires, which damage the soil. Hot fires destroy plants and animals and endanger communities, which is especially problematic in the forests of today where traditional burning is prevented to encourage the growth of timber crops. Cool fires are generally conducted in the spring and autumn, clearing undergrowth and burning up biomass that could trigger a hot fire should it get too dense. They provide a greater variety of environments, which encourages game and plant diversity, and make dense, impassable forests traversable for humans.
Fire is a chemical process in which a fuel and an oxidizing agent react, yielding carbon dioxide and water through a combustion reaction. This process does not proceed directly and involves intermediates, forming what is known as the fire tetrahedron. Fire cannot exist without all of these elements in place and in the right proportions: fuel, oxygen, heat, and a chemical chain reaction. For example, a flammable liquid will start burning only if the fuel and oxygen are in the right proportions. Some fuel-oxygen mixes may require a catalyst, a substance that is not consumed when added in any chemical reaction during combustion, but which enables the reactants to combust more readily. Once ignited, a chain reaction must take place whereby fires can sustain their own heat by the further release of heat energy in the process of combustion and may propagate, provided there is a continuous supply of an oxidizer and fuel. If the oxidizer is oxygen from the surrounding air, the presence of a force of gravity, or of some similar force caused by acceleration, is necessary to produce convection, which removes combustion products and brings a supply of oxygen to the fire. Without gravity, a fire rapidly surrounds itself with its own combustion products and non-oxidizing gases from the air, which exclude oxygen and extinguish the fire. Because of this, the risk of fire in a spacecraft is small when it is coasting in inertial flight, though this does not apply if oxygen is supplied to the fire by some process other than thermal convection. Fire can be extinguished by removing any one of the elements of the fire tetrahedron, such as turning off the gas supply, covering the flame completely, applying an inert gas, applying water, or applying a retardant chemical such as Halon.
The Color of Flames
The glow of a flame is complex, involving black-body radiation emitted from soot, gas, and fuel particles, though the soot particles are too small to behave like perfect blackbodies. There is also photon emission by de-excited atoms and molecules in the gases, with much of the radiation emitted in the visible and infrared bands. The color depends on temperature for the black-body radiation and on chemical makeup for the emission spectra. During the burning of hydrocarbons, for example wood, or the incomplete combustion of gas, incandescent solid particles called soot produce the familiar red-orange glow of fire. This light has a continuous spectrum. Complete combustion of gas has a dim blue color due to the emission of single-wavelength radiation from various electron transitions in the excited molecules formed in the flame. Usually oxygen is involved, but hydrogen burning in chlorine also produces a flame, producing hydrogen chloride. Other possible combinations producing flames, amongst many, are fluorine with hydrogen, and hydrazine with dinitrogen tetroxide. Hydrogen and hydrazine/UDMH flames are similarly pale blue, while burning boron and its compounds, evaluated in mid-20th century as a high energy fuel for jet and rocket engines, emits intense green flame, leading to its informal nickname of Green Dragon. The common distribution of a flame under normal gravity conditions depends on convection, as soot tends to rise to the top of a general flame, as in a candle in normal gravity conditions, making it yellow. In microgravity or zero gravity, such as an environment in outer space, convection no longer occurs, and the flame becomes spherical, with a tendency to become more blue and more efficient, although it may go out if not moved steadily, as the carbon dioxide from combustion does not disperse as readily in microgravity and tends to smother the flame.
The Pyrocene Epoch
The period of history characterized by the influence of human-caused fire activity on Earth has been dubbed the pyrocene. This epoch includes the burning of fossil fuels, especially for technological uses, and has transformed the planet's ecological systems. Globally today, as much as 5 million square kilometres, an area more than half the size of the United States, burns in a given year. Growing population, fragmentation of forests, and warming climate are making the earth's surface more prone to ever-larger escaped fires. These harm ecosystems and human infrastructure, cause health problems, and send up spirals of carbon and soot that may encourage even more warming of the atmosphere, thus feeding back into more fires. The burning of vegetation releases nitrogen into the atmosphere, unlike other plant nutrients such as potassium and phosphorus which remain in the ash and are quickly recycled into the soil. This loss of nitrogen produces a long-term reduction in the fertility of the soil, though it can be recovered by nitrogen-fixing plants such as clover, peas, and beans, by decomposition of animal waste and corpses, and by natural phenomena such as lightning. When fire removes protective vegetation, heavy rainfall can cause soil erosion. The positive effects of fire include stimulating plant growth and maintaining ecological balance, while its negative effects include hazards to life and property, atmospheric pollution, and water contamination. Fire has been used for centuries as a method of torture and execution, as evidenced by death by burning as well as torture devices such as the iron boot, which could be heated over an open fire to the agony of the wearer.
Fire in the Forge of War
The use of fire in warfare has a long history, serving as the basis of all early thermal weapons, including incendiary devices, heated projectiles, and the use of smoke. This class of weapons was particularly evident during naval battles and siege warfare. The Byzantine fleet used Greek fire to attack ships and men, a substance that could burn even on water. The invention of gunpowder in China led to the fire lance, a flame-thrower weapon dating to around 1000 CE which was a precursor to projectile weapons driven by burning gunpowder. The earliest modern flamethrowers were used by infantry in the First World War, first used by German troops against entrenched French troops near Verdun in February 1915. They were later successfully mounted on armoured vehicles in the Second World War. Hand-thrown incendiary bombs improvised from glass bottles, later known as Molotov cocktails, were deployed during the Spanish Civil War in the 1930s. During that war, incendiary bombs were deployed against Guernica by Fascist Italian and Nazi German air forces that had been created specifically to support Franco's Nationalists. Incendiary bombs were dropped by Axis and Allies during the Second World War, notably on Coventry, Tokyo, Rotterdam, London, Hamburg and Dresden. In the latter two cases, firestorms were deliberately caused in which a ring of fire surrounding each city was drawn inward by an updraft created by a central cluster of fires. The United States Army Air Force extensively used incendiaries against Japanese targets in the latter months of the war, devastating entire cities constructed primarily of wood and paper houses. The incendiary fluid napalm was used in July 1944, towards the end of the Second World War, although its use did not gain public attention until the Vietnam War.
The Sacred and the Profane
Fire has been an important element of human culture since the Lower Paleolithic, with archaeological evidence demonstrating that fire worship has been widely practiced since prehistory, with dedicated structures found dating from at least the Chalcolithic period. The religion of Zoroastrianism is closely linked to this practice, viewing fire as a sacred component. In some societies fire was a deity, while others viewed it as the manifestation of the divine. The fire in a hearth was perceived as symbolic of the Heavenly Fire, and thus is considered a sacred component by fire worshipping cultures. The origin of fire became a subject of mythology, with the Titan-god Prometheus in ancient Greek culture responsible for stealing heavenly fire and gifting it to humanity. The use of a pyre as a funerary practice dates back to at least the Ancient Roman period in the West, and to about 4,000 years ago on the Indian subcontinent. Cremation of corpses is a tradition long practiced in some cultures, including Hindu. After early religious resistance in some countries, in the 19th century this practice became more widespread and is now commonplace. In some nations, suicide by self-immolation remains common. The symbology of fire remains important to the present day, where wood is plentiful, the bonfire can be used for celebration purposes, in many cases as part of a tradition. An example is Guy Fawkes Night in England. The barbecue is a fire-based cultural tradition in the United States. The fiery ignition of fireworks has become a modern tradition to celebrate the New Years arrival. In contrast, book burning has been used as a form of protest, whether for political, religious, or moral reasons. The act of burning in effigy has a similar role, as in the annual burning of Judas ritual. Humans lack an instinctual fascination with fire, yet in modern societies adults can become drawn to it out of curiosity. In societies that are dependent on daily fire use, children lose interest in fire at about age seven due to regular exposure. Arson is the act of intentionally setting fire to a property. A separate but related behavior is pyromania, which is classified as an impulse-control disorder where individuals repeatedly fail to resist impulses to deliberately start fires. In contrast is pyrophobia, an irrational fear of fire, which is a less common phobia.