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An oxidizer, fuel, and heat are needed for a fire. When considering oxygen as the oxidizer, an oxygen concentration of approximately 16% is required for combustion. Plant material is the fuel considered when thinking of a wildfire. A couple examples of natural heat sources could be lightning or lava.

According to the diagram below, plants as a fuel source would not be available until at least 1 billion years ago. Atmospheric oxygen concentrations of at least 16% did not occur until approximately 500 million years ago. Therefore, I would guess that there is no evidence of wildfires until within the last 500 million years. What I'm wondering is what is the earliest known evidence of wildfires?

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Source: Oxygen Atmosphere

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  • $\begingroup$ Note that the 16% figure is related to a certain air temperature - the Earth used to be warmer in the past, though I'm not sure if the difference is significant enough to be worth considering. $\endgroup$
    – Luaan
    Commented Jun 6, 2018 at 10:32
  • $\begingroup$ The answer's graph at this post is for a much more little scale, and not logaritmic, but shows some sigmoidal pattern. considere too coal fires, not only plants fires. earthscience.stackexchange.com/questions/14187/… $\endgroup$
    – user12525
    Commented Jun 6, 2018 at 13:20

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A definitive statement comes from the abstract of Scott and Glasspool1, 2006:

Charcoal, a proxy for fire, occurs in the fossil record from the Late Silurian (≈420 Myr) to the present.

One of the tired old truisms you learn is that fire needs three things: Oxygen, fuel, and a source of ignition.

There is little doubt that there has been lightning since Earth's formation. (Wikipedia:Paleolightning). There appears to be evidence of the fossil fulgurite from the Proterozoic Eon.

Oxygen levels were probably sufficient around 470 Myr. (Wikipedia:Fossil record of fire)

So as soon as plant-based fuel appears on land, there is evidence of fire.

Although there is evidence for land plants for as far back as 850 Myr, and some evidence for land-based bryophyta (moss) appears in the Ordovician Period, evidence of widespread plant colonization of the land, and plants with vascular tissue that can dry out into fuel, do not appear until the Silurian period with the appearance of Cooksonia and other early tracheophytes. (Wikipedia: Evolutionary history of plants)

The level of oxygen in the atmosphere does appear to play a role, as there is a gap in the Mid-Devonian period, apparently coincident with a dip in oxygen levels (to about 13%), but there doesn't appear to be a consensus on that. At any rate, fossil charcoal picks up again in the late Devonian as oxygen levels rise again.

1 Andrew C. Scott and Ian J. Glasspool, The diversification of Paleozoic fire systems and fluctuations in atmospheric oxygen concentration,Proc Natl Acad Sci U S A. 2006 Jul 18;103(29):10861-5. Epub 2006 Jul 10. (Abstract at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1544139/#!po=0.694444)

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  • $\begingroup$ Hi Spencer, you mix Mya and myr in this answer; could you adjust to using all of one or the other to avoid confusion? Or possibly using Ma (Mega annum)? $\endgroup$
    – TylerH
    Commented Jun 6, 2018 at 13:55
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    $\begingroup$ @TylerH I don't think the distinction is meaningful, but I went with Myr to match the quote. $\endgroup$
    – Spencer
    Commented Jun 6, 2018 at 14:30
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    $\begingroup$ Myr is a span of time it is not the same as Mya, Myr just means million years not million years ago, for instance" silurian (~420 Myr)" means the silurian is 420 million years long (which is wrong) not it occurred 420 million years ago. Mya is only equivalent to "Myr ago" you need the extra word. in current practice Mya, Maa, MyrBP, and "Myr ago" are all equivalent Myr by itself is not. $\endgroup$
    – John
    Commented Jan 7, 2020 at 13:25
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Fire has been apart of natural ecology for as long as land plants existed. The Silurian period(420 million years ago) high light the most examples of fossil charcoal indicating wild fire Fire’s role in dry land ecology (especially of the US West) is well understood. It's fire suppression that led to the disasters we have now. Fire fills an important niche in warmer ecosystems like scrublands, grassland, chapparrals and dryer forests; It substitutes the role normally reserved for soil decomposers (Fungi/bacteria), but burning the woody debris and vegetation it frees up material and and soluble soil carbon that's essential to plant growth. – it sweeps away weeds that choke the forest floor, allowing successor trees to continue to proliferate. Giant sequoia breeds this way – eliminates competitive weeds/invasives – reintroduces nutrients (ash) back into the soil, with dry ecology; wood doesn't decompose at a fast rate; so fire converts wood/plants into ash which is water soluable. – produces ethylene gas (a natural plant growth booster) – heat/smoke of the fire heats up cones in conifer trees to open and release their seeds – triggers plants lying dormant in the soil

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    $\begingroup$ fire suppression doesn't make that much of a difference, relatively. cali/mediterranean forests burn depending on the severity of dry seasons, naturally it's about every 80 years, but when people have fireworks parties, cigarettes, BBQ's, leave litter,and pyromaniacs, and dryer hotter weather, that average becomes every 50 years, and bigger fires. $\endgroup$
    – bandybabboon
    Commented Sep 29, 2020 at 5:42

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