Once you've got eukaryotes, the next step's a foregone conclusion.
"Little multicellular organisms—born! Bada-bing."
"Ooh, splendid. It's finally starting to look like proper life."
In the sea, all manner of multicellular life is born and species multiply explosively.
This is around a billion years before the present.
"You know what came into being? A mouth."
"A mouth?"
"The primitive kind of mouth, like a coral has nowadays. With it, they started to prey on things."
"So they've finally taken to eating other creatures, aye."
"That's the one."
There's a mouth, but no anus.
Until now they'd lived by taking things in through the cell membrane, but this time it became a proper mouth.
"There's another meteor strike, and another Snowball Earth."
"Ahh, again, is it."
"Yep, yep."
Happens over and over, doesn't it. I know, I know.
On a scale of hundreds of millions of years, apparently these things happen.
Meanwhile, the little multicellular creatures develop an anus to go with the mouth.
And then a type is born that grows the anus first and the mouth after.
Organs form, and the interior of the body grows ever more subdivided.
"Starting to look like a real creature."
"Aye, so it is."
By this point, maybe we don't need the Hundred-Million-Year Button anymore.
Let's use the Thousand-Year Button instead.
"The ancient life explosion."
"What exploded?"
"The variety of living things exploded. A sudden surge. Still confined to the sea, mind you."
"Is that so."
"The equivalent of Earth's Cambrian explosion, this."
"The age of trilobites, 'tis."
As it happens, plants are still in the sea, not yet on land.
So the land is bare soil—the whole world like a barren desert.
There were bacteria on land too, it seems, but they're too small to see.
This is an era roughly five hundred million years back.
"And now, at last—plants make landfall!"
"Oooh, finally."
Around four hundred million years ago, perhaps.
Plants—ferns and the like—climb up onto the land.
"Next up, the arthropods start their invasion of the land."
"Now the land grows rich as well, aye."
Forests form on land, and it becomes an arthropod paradise.
Giant centipedes roaming freely across the ground—that sort of world.
"That's a touch frightening, though."
"Well, when you picture it, yeah."
The world's gotten quite fun by now.
"Incidentally, there aren't yet any fungi that can break down wood."
"Huh."
"So fallen trees just stayed as they were, got buried, and formed coal seams."
"Ahh, I see."
"Vertebrates are already around too."
"Ooh."
And of course it all proceeds through repeated meteor strikes and mass extinctions.
"Starting with fish, then amphibians climbing onto land, then reptiles."
"At last!"
"That's right. And reptiles into dinosaurs—"
"Except this is another world, so—"
"True."
"Let me rephrase. Welcome, dragons."
Dragonkin that fly through the sky were born.
These fellows are a fairly ancient lineage, actually.
In a sense, you could call a fantasy world the timeline where the dinosaurs never went extinct.
There are pterosaur types too—which makes them wyverns.
A single one lives a thousand years or so, so their life cycle's a bit of a mystery.
"And on that note, isekai-wise, there's a species that ought to show up about now."
"There is indeed. After you."
"No, after you."
"Ta-da-daaa. Welcome, elves!"
The last of the species that had evolved all the way to mammals kept right on evolving—and became elves.
This one owes a bit to us gods' cheat-powered forced evolution, so it differs from the world of Earth.
"The Forced Evolution Button—this one was a thousand-gold charge as well."
"Charged again, were you. Quite the investor."
"Well, it's a necessary expense."
"Can't be helped, I suppose."
Oh, right—there was one I'd forgotten.
"The tree of creation, Yggdrasil, also sprouted and began to grow before the elves."
"Ah, there was such a tree, wasn't there."
"A huge, huge tree. The pinnacle of plant life. The World Tree."
It grows at the center of the world and is said to stand some three thousand meters tall.
That World Tree, too, was once nothing more than a single tree grown from a tiny sprout.
There must have been many Yggdrasils; the one that escaped all manner of disasters and survived to the present day is the one called the World Tree.
"So around this era, quite a few reasonably sized Yggdrasils were growing, aye."
"Indeed."