[TougeSil80]

Originally Posted by aohige

hint: amnesia.

He lost memory when he was awakened as Iceman.
He lost memory again when he was saved by Eruru.
He didn't regain these memories back until the last episode.

Spoiler:

It doesn't say, but it's mentioned that the surface world was contaminated, and it took long time (hundreds? thousands? of years) to clense itself.
Humans were living underground, and lost immunity to many virus and germs, and was too weakened to live on the surface.
They found iceman, and researched him.. and from from studying his DNA, they created those humanoid specimen with immune system strong enough to live on the surface.

Time periods between these periods are not mentioned.
Here's the general idea:

1. Present, or close to present time: Archaeologist was killed near the petrified god.
2. Not too distant future: Earth surface contaminated. Most likely a world-wide apocalypse of some sort.
3. Far in the future: humans living in shelter, found iceman.
created beings that can live in the surface world, and work for them.
Researched iceman to create mask that can keep them alive on surface also.
Humans pissed off iceman, and were turned into slimes.
4. Even further in future (dozens, hundreds of generations later): Utawarerumono present time.

Now, there's a bit more story in the game that wasn't shown in the anime.

Spoiler for The fate of human kind:

The shelter iceman was at was in Japan.
There were other shelters around the globe.
However, when the scientists in Japan shelter were turned into slimes, The other shelters around the world decided to abandon them, and destroy them with satelite laser system.
The slimes cursed the world, and wished death to them all.
The god, Uytsuarunemitea (the angered Dii side) heared their call, and demanded Mutusmi to kill all humans around the world.
Mutsumi hacked the Amateras laser system, and used it to bring death to all shelters around the world.

Hence, the total genocide of human race was complete, and only the humanoids that fled to the surface world were left on earth.

Wow, that explained a lot, thanks for the explanations. The series makes more sense now.