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Artist’s reconstruction of a forest during the Carboniferous period. From ‘Science for All’ by Robert Brown (London, c1880). Illustration by World History Archive, Alamy

The Fantastically Strange Origin of Most Coal on Earth

This is a story about trees—very, very strange looking trees—and some microbes that failed to show up on time. Their non-appearance happened more than 300 million years ago, and what they didn’t do, or rather what happened because they weren’t there, shapes your life and mine.

All you have to do is walk the streets of Beijing or New Delhi or Mexico City: If there’s a smog-laden sky (and there usually is), all that dust blotting out the sun is there because of this story I’m going to tell.

It begins, appropriately enough, in an ancient forest …

… whose trees “would appear fantastic to us in their strangeness,” write Peter Ward and Joseph Kirschvink in their book A New History of Life.

Some of them were giants: 160 feet tall, with delicate fernlike leaves that sat on top of pencil-thin trunks. This was the age when plants were evolving, climbing higher and higher, using cellulose and a tough fiber called lignin to stay upright. Had you been there, you would have felt mouse-sized.

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Drawing by Robert Krulwich

These trees weren’t just odd looking. “One of their strangest traits was their very shallow root system,” write Ward and Kirschvink. “They grew tall and fell over quite easily.”

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Drawing by Robert Krulwich

So imagine, then, these stands of towering, fernlike plants mostly growing in swamps. The air is warm and moist, and the land (Europe, the Americas, and Africa were at the time one continuous mass) is covered by millions—no, billions—of trees that are sucking carbon from the air, growing, aging, dying, falling, and releasing oxygen. This is a world littered with dead trees piling on top of each other.

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Carboniferous Forest Diorama. Photograph by John Weinstein, Field Museum Library, Getty

But when those trees died, the bacteria, fungi, and other microbes that today would have chewed the dead wood into smaller and smaller bits were missing, or as Ward and Kirschvink put it, they “were not yet present.”

Where Are They?

Bacteria existed, of course, but microbes that could ingest lignin and cellulose—the key wood-eaters—had yet to evolve. It’s a curious mismatch. Food to eat but no eaters to eat it. And so enormous loads of wood stayed whole. “Trees would fall and not decompose back,” write Ward and Kirschvink.

Instead, trunks and branches would fall on top of each other, and the weight of all that heavy wood would eventually compress those trees into peat and then, over time, into coal. Had those bacteria been around devouring wood, they’d have broken carbon bonds, releasing carbon and oxygen into the air, but instead the carbon stayed in the wood.

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Artist’s engraving of a carboniferous forest circa 1754. From The Universe by FA Pouchet (London, 1874). Photograph by UniversalImagesGroup, Getty

We’re talking about a spectacular amount of carbon. Biochemist Nick Lane guesses that the rate of coal formation back then was 600 times the normal rate. Ward and Kirschvink say that 90 percent—yup, 90 percent!—of the coal we burn today (and the coal dust we see flying about Beijing and New Delhi) comes from that single geological period, the Carboniferous period.

That’s why it’s called “carboniferous”—because it produced so much carbon. “The Carboniferous period was the time of forest burial on a spectacular scale,” the writers say.

Take Off Your Helmets and Say Thank You

And therefore, in a just (and biologically aware) world coal miners everywhere would be doffing their helmets to salute the tardy arrival of those teeny earth creatures, the wood-eating bacteria. By not being there 350 million years ago, and by not arriving for another 60 million years, giant seams of black coal now warm us, light us, and muck up our atmosphere. Equal numbers of environmentalists might spend the day throwing darts at these little guys for showing up so late.

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A coal miner plants explosives in a coal mine. Photograph by H. Mark Weidman Photography, Alamy

And Now … in Spectacular Magnification, Let Me Introduce …

But enough of me talking about them. It’s time for you to take a close—and I mean close—look at these amazing wood-eaters. They come in many forms, but I’m choosing microbes called Trichonympha because they’re so tiny, so squirmy, and so, well, crazily busy. They’re single-celled and can be found, yes, inside a termite gut. They look, says photographer Richard Howey (who studies them), like teardrops, or pears “wearing wigs.”

Here they are in this Nikon Small World award-winning video by Danielle Parsons and Wonder Science TV:

When I first saw this video, I was shocked by the commotion. I had thought wood-eaters would be mellow, sluggish, and, well, a little less clumped together. So I had questions. A web search brought me to Richard Howey in Wyoming, who has written about and photographed Trichonymphawritten about and photographed Trichonympha, and I asked him to take a look at the video so I could pepper him with questions. Which is what I did …

Me: Wow! This is crazy. So much motion!
Richard Howey: Yes, it looks almost like a game of bumper cars.
Me: So why are they so squished together?
RH: I’m not sure. I was really stunned [when you showed this to me]. It seems like Macy’s on Christmas Eve. [Pause.] I know they reproduce at an incredible rate.
Me: What do you mean? Are we watching them having sex?
RH: They might be [laughs]. Their reproductive process is incredibly complicated … [goes on to discuss mating types]
Me: But mostly they’re eating, right?
RH: Oh, definitely. You see those little white crystals jiggling around?
Me: Yeah, those shiny, stonelike things? What are those?
RH: Those are little cellulose bits; the termite has chewed and shredded the wood, and now these bits have reached its intestines. The microbes scoop them up …
Me: And once they get them inside?
RH: They produce a dissolving agent that’s going to reduce those bits to starches and sugars that the termite can eat.
Me: I like their little wiggly nose-like tops.
RH: Those aren’t noses.
Me: Well, heads then …
RH: Actually … They’re kind of like legs. They have little locomotive hairs, flagella, attached there, and that’s how they propel.
Me: It’s weird. It looks like they know where they’re going …
RH: That’s an illusion. I think they just … go.
Me: Why don’t they stop? Do they ever rest?
RH: No, those flagella are very motile—they keep moving and moving and eating and eating …
Me: That’s it?
RH: That’s what they do. Always.

And we should be oh-so-thankful they do it. Because of them, dead trees get recycled. Soil gets replenished. Smaller organisms get fed. And miners can mine—which is only to say: Sometimes very little creatures make a very big difference.

Editor’s Note: The image of coal featured in this post was updated for accuracy.