Last time, we pulled fungi out of the plant drawer where science had filed them for two centuries. We ended on a strange little promise: that a mushroom has more in common with you than with the plant it's growing on. Today we make good on it — because the evidence is real, it's specific, and it's a lot weirder than "well, they're both not-plants."
Here's the claim, stated plainly: animals and fungi are each other's closest living relatives. Established Picture the deep family tree of complex life. Long ago, one branch split off and became plants. Another branch carried on a while longer and then split — into the lineage that became animals, and the lineage that became fungi. So you and the mushroom are siblings who parted ways relatively late. The strawberry is the distant cousin who left home first.
If that feels backwards, good. It's supposed to. Let's look at why we believe it.
The shared name nobody told you about
Animals and fungi belong to the same supergroup, with a wonderful tongue-twister of a name: Opisthokonta. It means, roughly, "rear pole" — and it's named for a single detail their common ancestor shared. That ancestor's cells swam using one flagellum mounted at the back, pushing the cell forward like a tiny outboard motor.
You still carry this. A human sperm cell swims exactly that way — one tail, pushing from behind. And the early fungi did too: their swimming spores were propelled by a single rear flagellum. Most fungi have since given up swimming entirely, but the signature of that shared ancestor is stamped into the group's very name. Animals and fungi are the two great kingdoms of the opisthokonts. Plants aren't in the club. Established
The fingerprint in the protein
Now, a shared swimming style is suggestive, but it isn't proof. The harder evidence is written much smaller — in the proteins themselves.
In 1993, Sandra Baldauf and Jeffrey Palmer went looking for it across 25 different proteins. They found something striking: a set of tiny, shared quirks that animals and fungi have and plants don't. The most quoted is a 12-amino-acid insertion in a protein called elongation factor 1-alpha — a little extra stretch of sequence present in animals and fungi, absent in plants and the protists they compared. They found three more shared gaps in another protein, enolase. Four independent signatures, all pointing the same way: animals and fungi form one group, with plants on the outside. Established
Why does that count as strong evidence? Because a quirk like a 12-amino-acid insertion is the kind of thing that tends to happen once, in a common ancestor, and then gets passed down to everyone descended from it. When animals and fungi both carry the same insertion and plants lack it entirely, the simplest explanation is the one that turned out to be right: animals and fungi inherited it from an ancestor they share after the plant line had already branched away.
This is established science — well-replicated, and the consensus view of how these kingdoms relate. We're stating it plainly because the evidence has earned it.
The everyday tells
Once you know where to look, the kinship shows up in the kitchen-table details, too.
When your body banks spare energy, it stores it as glycogen. So do fungi. Plants, by contrast, store theirs as starch. And the wall a fungus builds around its cells isn't made of plant cellulose — it's made of chitin, the same tough, flexible material in the shell of a crab and the exoskeleton of a beetle. The next time a beetle clicks past you, consider: it's wearing a version of the same stuff the mushroom is built from. That's not a metaphor. It's a shared chemistry, inherited down the animal-and-fungus side of the family. Established
So why does it feel so wrong?
Because your eyes were fooled, and reasonably so. A mushroom is rooted. It's still. It sprouts from the ground and waits. Everything about how it sits in the world says "plant" — and for most of human history, how a thing sits in the world was the best evidence we had. Intuition filed fungi with the ferns because, from the outside, that's exactly what they look like.
The molecules tell a different story than the silhouette. And that gap — between what a fungus looks like and what it actually is — is the whole reason this kingdom keeps surprising us.
One honest footnote
We can tell you that animals and fungi split from each other after plants branched off. We're much shakier on exactly when. You'll see the divergence dated to "around a billion years ago," and that's a fair shorthand — but the real number is genuinely uncertain. Contested Molecular-clock estimates and the thin early fossil record don't fully agree, and the error bars run hundreds of millions of years wide. So we'll say "roughly a billion years ago, give or take a lot," and leave the false precision to someone else. The kinship is solid. The calendar is fuzzy. Both of those are worth saying out loud.
What's not fuzzy is the headline, and it's a good one to carry around: the mushroom is family. Closer family than the plant it's standing on.
Next in the series: if a fungus isn't eating like an animal or feeding like a plant — then how on earth does it eat? The answer is genuinely alien. See you there.