So far in this series we've stayed in the present tense — what a fungus is, how it's related to you, how it eats. Today we go backwards. Way back. To a moment around half a billion years ago when the continents were bare rock and the only life was in the water, and something extraordinary happened: life climbed out. And the leading explanation for how it managed that involves fungi.
This is the episode where we have to be most careful with our words, because it's the one with the most tempting overstatement. So let's set the rule up front: some of what follows is established — fossils you can point to. Some of it is the best current hypothesis, strongly supported but not closed. We'll tell you which is which as we go, because the honesty is the whole point.
The problem of bare rock
Picture an early plant — really an early plant-ancestor, more like a clump of green than anything with leaves — trying to establish itself on raw stone. It has no real roots. The rock holds nutrients, but locked up, in forms the plant can't pry loose on its own. By itself, this pioneer is stuck. The land is a desert of mineral wealth it can't reach.
Here's the leading idea, and we want to flag it clearly as a hypothesis — the best-supported explanation scientists currently have, not a settled fact: those first land plants made it by partnering with fungi. Contested The fungi, already masters of reaching enzymes out into their surroundings and dissolving what's there (remember Ep. 3), could extract nutrients from the rock and trade them to the plant in exchange for sugars the plant made from sunlight. A deal. Carbon for minerals. And that deal, the hypothesis goes, is part of what let plants colonize the land at all.
We say "hypothesis" deliberately. It's well-supported and widely held — but "fungi helped green the planet" is exactly the kind of clean, quotable line that hardens into a false certainty if you let it. So we won't. It's the best current explanation. It is not a closed case.
The smoking gun in stone
What is established — what we can put a finger on — is that the partnership is genuinely ancient. Established In Scotland there's a fossil site called the Rhynie chert, about 407 million years old, that preserves early plants in extraordinary cellular detail. And threaded right into the cells of those plants are fungi, in associations that look just like the mycorrhizal partnerships — "mycorrhiza" literally means "fungus-root" — that still tie fungi to plant roots today.
That's not a model or an inference. That's a 407-million-year-old snapshot of plants and fungi already living together, deep in the period called the Devonian. The partnership is real, and it is old almost beyond comprehension.
And it never stopped. This is the part that gives the deep-time story a pulse: the great majority of land plants alive today still form mycorrhizal partnerships with fungi. This isn't ancient history that ended. It's the still-running operating system underneath the forests and fields you know — the same deal, carbon for minerals, struck quietly under the soil right now.
The giant nobody can identify
Now for the set piece — and it's a genuine mystery, not a tidy fact, which is exactly why we love it.
Picture that same ancient landscape, where the tallest plants stood maybe knee-high. Towering over all of it were trunk-like organisms up to roughly eight meters tall — pillars of life in a world that otherwise hugged the ground. They're called Prototaxites, and the fossils are real and genuinely enormous. That much is established. Established
The question — the wonderful, unresolved question — is what they were. For more than a century and a half, Prototaxites has defied classification. For much of that time the leading guess was that it was a giant fungus. But the case has never closed, and it just got more interesting: a recent analysis of Prototaxites taiti (from that same Rhynie chert) — released as a 2025 preprint and published in Science Advances in early 2026 — found it structurally and chemically distinct from all known fungi — its internal architecture more complex than fungal threads, and notably missing the chitin you'd expect in a fungus. The authors suggest it may belong to an entirely extinct lineage of life — a branch with no living members and no modern name. Contested
We want to be careful here, too. This is contested, an actively-argued question, and we're reporting the latest finding, not declaring a verdict. But sit with the possibility: the largest living thing on land, around 400 million years ago, may have belonged to a kind of life that no longer exists — that we have no box for at all.
The throughline
Notice that we're back to the pattern that opened this whole series, in Episode 1. Better tools — here, fine chemical and structural analysis of a fossil — keep redrawing the map. The land's first giant was filed as a fungus for generations; sharper instruments now suggest it may not belong to any branch of life still standing. Every time we look harder, the picture gets stranger and less finished, not tidier.
The land you walk on was, in part, a fungal collaboration — that's the best story science currently tells. And the tallest thing that ever stood on that early land may be a mystery we'll never fully solve. Both of those sentences are true. Both of them are honest. And both of them are more wonderful than the textbook's tidy version.
Next in the series: from the deep past to a living giant — the single largest organism on Earth, spread across three and a half square miles of Oregon, mostly invisible underground. See you there.