After that, somewhere in the course of mashing the Hundred-Million-Year Button, a change occurred.
"Look at this. Incredible. Photosynthesis!"
"Photosynthesis, you say! Now there's a bit of romance."
A world overrun with bacteria.
And within it, a certain altered strain had begun to multiply.
Among them appeared a kind that took in carbon dioxide and expelled oxygen—photosynthesizers.
"Cyanobacteria, 'tis."
"Right. Which is to say, the future chloroplasts, yeah?"
"Indeed. On Earth this happened but once—the evolution of oxygen-producing photosynthesis."
"Huh."
The carbon dioxide had dissolved into Eishir's seas and waters as well.
The cyanobacteria that broke it down and released oxygen bred explosively.
"Amazing, amazing. The cyanobacteria are having a population boom."
"Nay, this is rather a problem, actually."
"Is it? Prosperity's a fine thing, isn't it?"
"Listen here—oxygen is poison, you know."
"O-oh. It is, huh."
The truth is, the older sort of bacteria are weak to oxygen.
On contact with it, they oxidize and break down.
"Anaerobes, that is. Still found in your gut today."
"Huh, so they are."
And so the oxygen concentration in the atmosphere climbs.
Then something was about to happen that never once occurred on Earth.
"We've got a strain of photosynthesizer that turns mana into mana crystals!"
"Ooh, that's a fantasy world for you."
So this, presumably, is the reason monsters would later come to have mana crystals.
"A mana crystal—a mana crystal, that is."
"And so the crystals these magic bacteria synthesize pile up, and you get mana-crystal strata forming."
"That's it."
With every press of the Hundred-Million-Year Button, from this era on, mana-crystal strata start to form.
These get buried underground, and later on people take to mining the crystals.
"Rather romantic, isn't it. I like it."
"'Tis good. I don't quite follow this 'mana' business, but it can't be stored as it is. You crystallize it to keep it, aye?"
"Hmm, right. It's fantasy, folks."
Along with oxygen, mana crystals spread across the world.
And so the world's balance began to shift.
"Hundred-Million-Year Button—click click click click."
"Oi, it's about time."
"Got it."
The two of them watch.
"Hee hee. At last—the ozone layer has formed."
"Oh, we did it!"
High up in the atmosphere, strong ultraviolet light comes streaming in.
When oxygen—O2—takes in this ultraviolet light, it changes into ozone, O3.
That ozone layer has the power to reflect and absorb the harsh ultraviolet rays, and it shields the surface below.
"Now we can rest easy about our skincare."
"Rough being a girl, huh."
"Even an old man burns black in the sun, you know."
"Oops—fair point."
One thing often noted about this cyanobacteria-and-oxygen business is a rock called a stromatolite, said to form where cyanobacteria are active on its surface.
You can still see them today along the coastlines of Australia and such.
And they turn up excavated from ancient strata too, so the story goes.
Amid the button-mashing, tragedy struck.
"Aaah, a meteor's fallen—!"
"Nojaaaa—!"
The two of them fall speechless before the image of Eishir floating in the hologram.
A colossal meteor has struck; a blast of heat and a cloud of dust spread outward.
Thick clouds blanket the sky, and light no longer reaches the surface.
When that happens, the capacity for photosynthesis drops as well.
"The little bacteria are dying."
"Aye, so they are."
A third of the surface's bacteria were all but wiped out.
"Back to raising them again. And multicellular life was nearly here, too."
"A pity, 'tis."
The planet of darkness persists a while.
Even so, life once more spreads out to cover the world and carries on.
Before we knew it, bacteria had settled across the whole world again.
But the trials don't come in only one variety.
"'Snowball Earth,' this—a total global freeze."
"What is that?"
"The temperature drops and you get an ice age."
"This is hard on living things."
The meteor had already brought the average temperature down and made things difficult; now it falls again.
The world froze over in an ice age.
"My planet's on the rocks."
"Have some shaved ice and wait it out."
The spectators haven't a care in the world.
After all, one press of the Hundred-Million-Year Button and it's solved.
"Finally over."
"It's warming up."
And so, one way and another, evolution marches on.
"About time."
"Yeah, that's right. Ta-daa—eukaryotes!"
"Ooooh, amazing!"
Up to now it had been bacteria—single-celled microbes.
Eukaryotes include both single-celled and multicellular organisms.
At this point they're single-celled. Paramecia, for instance.
"First, before the eukaryotes—mitochondria."
"I've heard of those."
Mitochondria, put simply, take in oxygen and nutrients and produce carbon dioxide and energy for activity.
They form the basis of oxygen respiration.
More primitive organisms don't respire with oxygen.
This let them obtain explosive amounts of energy, and so more active creatures came to be born.
"Right, right—and since this world's got magical organisms too, it's a bit different here."
"I see, I see."
"Er, something called a mana organelle forms inside the cell—a magic-power conversion device that takes in mana and releases it as energy. That's born around the same time."
"Ahh, I follow."
It's a magical world, after all, so its organisms too have magic-applied organelles at the cellular level.
From our own world's standpoint it's awfully strange, but best not to think too hard about it.
"And there we go—eukaryotes, having taken in mitochondria and chloroplasts, are born!"
"Ooh—still just one cell, but the machinery's growing more complex, aye."