The fossil record of fire first appears with the establishment of a land-based flora in the Middle Ordovician period, 470 million years ago, which contributed large amounts of oxygen to the atmosphere, as the new hordes of land plants pumped it out as a waste product. When this concentration rose above 13%, it permitted the possibility of wildfire. Wildfire is first recorded in the Late Silurian fossil record, 420 million years ago, by fossils of charred plants. Apart from a controversial gap in the Late Devonian, charcoal is present ever since. The level of atmospheric oxygen is closely correlated with the amount of charcoal in the fossil record, clearly pointing to oxygen as the key factor in the prevalence of wildfire. Fire also became more abundant when grasses became the dominant component of many ecosystems, around 6 to 7 million years ago, providing excellent tinder for more rapid spread of fire. This widespread emergence of wildfire may have initiated a positive feedback process, whereby they produced a warmer, drier climate more conducive to fire.
The ability to control fire was a dramatic change in the habits of early humans. Making fire to generate heat and light made it possible for people to cook food, simultaneously increasing the variety and availability of nutrients and reducing disease by killing pathogenic microorganisms in the food. The heat produced would also help people stay warm in cold weather, enabling them to live in cooler climates. Fire also kept nocturnal predators at bay. Evidence of occasional cooked food is found from 1 million years ago. Although this evidence shows that fire may have been used in a controlled fashion about 1 million years ago, other sources put the date of regular use at 400,000 years ago. Evidence becomes widespread around 50 to 100 thousand years ago, suggesting regular use from this time; resistance to air pollution started to evolve in human populations at a similar point in time. 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.

Fire has also 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 filled with water, oil, or even lead and then heated over an open fire to the agony of the wearer.

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. This is especially a problem in the forests of today where traditional burning is prevented in order to encourage the growth of timber crops. Cool fires are gener…

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Fire is a chemical process in which a fuel and an oxidizing agent react, yielding carbon dioxide and water. This process, known as a combustion reaction, does not proceed directly and involves intermediates. Although the oxidizing agent is typically oxygen, other compounds are able to fulfill the role. For instance, chlorine trifluoride is able to ignite sand.

Fires start when a flammable or a combustible material, in combination with a sufficient quantity of an oxidizer such as oxygen gas or another oxygen-rich compound (though non-oxygen oxidizers exist), is exposed to a source of heat or ambient temperature above the flash point for the fuel/oxidizer mix, and is able to sustain a rate of rapid oxidation that produces a chain reaction. This is commonly called the fire tetrahedron. Fire cannot exist without all of these elements in place and in the right proportions. 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. 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. Consider a natural gas flame, such as from a stove-top burner. The fire can be extinguished by any of the following:
• turning off the gas supply, which removes the fuel source;
• covering the flame completely, which smothers the flame as the combustion both uses the available oxidizer (the oxygen in the air) and displaces it from the area around the flame with CO2;
• application of an inert gas such as carbon dioxide, smothering the flame by displacing the available oxidizer;

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Fire science is a branch of physical science which includes fire behavior, dynamics, and combustion. Applications of fire science include fire protection, fire investigation, and wildfire management.