Pick up a handful of soil and you are holding one of the strangest substances on Earth — part ground-up rock, part the dead bodies of everything that lived in it, laced through with living threads. It feels like the most ordinary thing in the world, the plain brown floor of everywhere. But soil is not a given. There was a time, for most of our planet's history, when the continents had none of it: just bare, wet stone under an empty sky. The story of how that changed is, at its heart, a story about fungi — and about the single most consequential partnership in the history of life on land. Established
This is the second dispatch of Engines of the Living World, our series about fungi not as curiosities but as the machinery that keeps the planet running. Last time we met the fungi that take the world apart. This time, we meet the ones that helped build it.
A green scum on a dead world
Rewind to somewhere between 500 and 470 million years ago. Life had been busy in the oceans for billions of years, but the land was, biologically speaking, almost empty — a bare mineral surface, no forests, no meadows, no soil. The first plants trying to make the leap out of the water were nothing like the plants you know. They were small, simple, green things, more like today's mosses and liverworts than anything with a trunk. And crucially, they had no roots. Roots are a later invention. Established
That is a serious problem, because land is a hostile place for something newly arrived from the sea. There is the constant threat of drying out. And there is a subtler, deadlier issue: the nutrients a plant needs to live — phosphorus, nitrogen, and the rest — were locked up in the rock, not floating freely the way they are in water. A rootless green blob sitting on bare stone is starving on top of a pantry it cannot open. As one team of scientists put it, the earliest plants faced "the twin problems of desiccation and low nutrient-status substrates." So how did they ever get established? Established
The deal that unlocked the land
They didn't do it alone. They did it by going into partnership with a fungus — and the terms of that ancient deal are still being honored, right now, under almost every plant alive. Here is the exchange. A fungus is nothing but thread: fantastically fine filaments that push through soil and rock far more thinly and cheaply than any root could, mining the substrate for scarce minerals. What a fungus is bad at is making food; it has no way to capture the sun. A plant is exactly the reverse — brilliant at photosynthesis, turning sunlight and air into sugar, but hopeless at prospecting a mineral surface. Established
So they traded. The fungus threads out into the ground, gathers up phosphorus and nitrogen, and delivers them into the plant's tissues; in return, the plant hands over a cut of the sugar it makes from sunlight. In the flat words of a recent study, plants' move onto land "was facilitated by the formation of mutualistic symbioses with fungi, through which the earliest plants gained access to mineral nutrients in exchange for photosynthetically fixed carbon." Established For a plant with no roots, the fungus was the root system — an outsourced, living supply line reaching into ground the plant itself couldn't touch. This kind of partnership has a name: mycorrhiza, literally "fungus-root." Some version of it still feeds the overwhelming majority of plant species on Earth. Established
The idea that this deal was the thing that got plants onto land in the first place is an old and powerful one — famously argued back in 1975, when two biologists proposed that "the very evolution of plants was possible only through such mutualistic partnerships." That strong version — that plants simply could not have made it without fungi — is a hypothesis, and a bold one. But the softer, load-bearing claim beneath it — that fungi made the conquest of land dramatically easier, and were there from very early — is on much firmer ground, because we can actually go and look. Established
The fossil that proves it's this old
In the Scottish village of Rhynie there is a bed of rock — the Rhynie chert — that did something almost no fossil site does: it preserved an entire early land ecosystem in three dimensions, cell by cell, roughly 407 million years ago, by flooding it in silica-rich water from a hot spring. Slice it thin, put it under a microscope, and you are looking directly into the tissues of some of the oldest land plants we have. Established
And inside those tissues, in 1994, researchers found the fungi. Not just fungal threads, but arbuscules — the tiny, branching, tree-shaped structures that a mycorrhizal fungus grows inside a plant's cells to trade nutrients across. The fossil arbuscules were, in the researchers' words, "morphologically identical to those of living arbuscular mycorrhizae." The trading structure that fungi and plants use today was already fully formed 400 million years ago, doing the same job in the same way. It was, they wrote, "unequivocal evidence that mycorrhizae were established >400 million years ago." Established
It's worth being honest about exactly what that fossil shows. It proves the partnership was up and running by the Devonian; it does not roll a camera on the very first plant to crawl out of the water tens of millions of years earlier. The scientists who found it said so plainly — that "interpretations of the evolution of mycorrhizal mutualisms continue to be speculative," even as the fossil made clear that "nutrient transfer mutualism may have been in existence when plants invaded the land." The deep past is like that: the wonder is real, and so are the edges of what a single rock can tell you. Established
And then they started building soil
Here is where the partnership stops being a private arrangement between two organisms and starts reshaping the whole planet. A plant and its fungus don't just passively sip minerals from the ground — they actively attack the rock to get them. Roots and fungal threads pump out acids and other compounds that chemically weather stone, prising loose the elements they need. Multiply that by a continent slowly greening over millions of years, and you have a planet-scale rock-grinding machine switching on for the first time. Established
That weathering is, quite literally, the front half of making soil. Soil is what you get when rock is broken down — physically and chemically — and then mixed with the dead remains of the life living on it (the back half of the story, run by the decomposers we met last issue). Plants and their fungi accelerated both halves at once: cracking minerals out of stone, and pouring a steady rain of dead tissue onto the ground for fungi to recycle. The bare mineral continents began, slowly, to grow a living skin. Established
And the side effects were enormous. Weathering rock doesn't just release nutrients; it pulls carbon dioxide out of the air, locking it away in dissolved minerals that eventually wash to the sea. When a team put modern moss on rock in the lab, they watched it measurably "enhance the weathering" of a whole suite of elements out of the stone. Scaled across deep time, their argument runs, the spread of those first land plants "accelerated chemical weathering and may have drawn down enough atmospheric carbon dioxide to trigger the growth of ice sheets" — that the greening of the land may have helped tip the planet into an ice age. Contested Notice the honest "may have" in there; the size of that climate punch is still argued over. But the direction of the effect — plants plus fungi speeding up the weathering of rock and the building of soil — is not in doubt.
Where we get honest
There's a lovely tidy version of this story: the first plants teamed up with the arbuscular mycorrhizal fungi — the same group, the Glomeromycotina, that partner with most plants today — and that one ancient handshake never really changed. For a long time that was the textbook account. It might even be right. But it's currently contested, and the wobble is instructive.
Over the last decade or so, biologists poking at the earliest-branching land plants alive today — humble liverworts and their relatives — kept finding a different group of fungi living inside them: the Mucoromycotina, an ancient lineage separate from the classic arbuscular mycorrhizal fungi. Often both kinds of fungus turn up in the same plant at once. In one striking case, a little clubmoss called Lycopodiella was found partnering with the Mucoromycotina alone, and researchers showed the exchange was a genuine nutritional trade — carbon out, phosphorus and nitrogen in. As they summed up the shift, the evidence "supports the idea that the colonization of Earth's land masses by plants was facilitated not only by Glomeromycotina AM, but also by Mucoromycotina fungal symbionts."
So which fungus struck the very first bargain — or did several strike it at once? That part is genuinely unsettled, and we're not going to flatten it into a clean answer it doesn't have yet. Contested But look at what isn't in dispute, and it's the good part anyway: that plants got onto land arm-in-arm with fungi, trading sugar for minerals, and that the partnership was there near the very beginning. The identity of the first partner is a live question. The existence of the partnership is written into the rock — and into the ground under your feet.