Curious kids: how old is fire on Earth?
Please can I ask how old is fire on earth, not tamed by people but since when has there been fire and flames on the planet.
Samuel, 5, London
You ask a very interesting question. For many years, scientists assumed that fire and humans were so connected that few of them gave any thought to what happened to fire before humans evolved.
Even now, after many years of research, you won’t find much information in books about ancient fire. Indeed, I first started to become interested in this question of fire in the geological past more than 50 years ago, but my work was largely ignored until recently.
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Your question is important today as the Earth’s weather is changing quickly and we are seeing deadly wildfires around the world. Humans may have used fire for a long time but they have never been able to tame fire. The challenge for scientists at the moment is to work out which fires are caused by humans and which ones are natural. To do this, we need to understand ancient fire in the first place.
A lot of our knowledge comes from studies of charcoal found in rocks more than 350 million years old, in a period geologists call the Carboniferous Period. As I say in my book Burning Planet: The Story of Fire Through Time, charcoal preserves the detail of different parts of the plant charcoal is made from. If you visit a place where there was a recent fire that burned a lot of plants, or collect some charcoal from the remains of a bonfire and look at it under a magnifying glass you may be able to see some of this amazing detail.
Over many years I, together with my students at Royal Holloway University of London, have been collecting information on ancient charcoal to help us understand fires of the past.
The key to understanding when fire appeared on Earth comes from what we call the fire triangle and I have discussed this in my small book Fire: A Very Short Introduction.
The first side of the triangle is fuel. Fire needs plants to burn. So we would not expect to have fire on the Earth before plants evolved. Plants first lived in the sea and started to spread on to the land around 420 million years ago. So there couldn’t have been fire before then.
The second side is heat – we need heat or a spark to start the fire – and that in the ancient past would be lightning. There has always been lightning and we can see evidence of this from fused sand grains found in some ancient sediments.
Finally, we need oxygen to allow the burning process to happen, the same way we need oxygen to breathe. We know this from simple ways we might put out a fire. You can cover the flames to stop oxygen or use sand, water or other materials to cut off the oxygen from the fire. Today the air we breathe has 21% oxygen. But experiments have shown that if you reduce the level to below 17% fires will not spread.
And above 30% it would be hard to put out a fire as even wet plants can burn with that level of oxygen. That is also why no fire or smoking is allowed in hospitals where there is oxygen used for the patients.
The level of oxygen in the Earth’s air has changed a lot over time. Scientists have shown that around 350-250 million years ago was a time of high levels of oxygen between 23 and 30% in the atmosphere and a lot of fire.
Evidence of the first fires was around 420 million years ago from charcoal in sedimentary rocks. But plants were small and there weren’t many places on Earth where they could grow. That meant there weren’t many places fire could burn. It was not until around 350 million years ago that fires started burning in lots of places and burnt in some of the first forests to grow on Earth.
Another time period of high fire was between 140 and 65 million years ago when many of our famous dinosaurs such as triceratops and tyrannosaurus were living and also when flowers first appeared. Around 40 million years ago oxygen levels in the atmosphere stabilised to modern levels. Proper tropical rain forest spread widely. This probably made fire rarer as wet rain forests don’t catch alight easily.
But around 7 million years ago grasslands spread, and these were easily burned. The grass-fire cycle began. This is where regular fire kills the saplings of trees, stopping grasslands turning into forests.
It is into this fiery world that humans evolved around 1.5 million years ago.
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Andrew Scott has previously received funding from the Natural Environment Research Council and the Leverhulme Trust.