High Concept

When a geostorm strikes earth, it splits earth into two dimensions. All the animals disappear from earth 1, leaving it with only people who have no clue what happened to the animals. The animals are on earth 2, and are now sentient since to new elements in their atmosphere.

The people must learn to adjust to a world with no meat from animals. This means most turn to science and agriculture. Leaving cities more sparsely populated before.

The animals must simultaneously build new civilizations on their earth, and must work out what sentience means when it comes to their diet. Is it ethical to eat another sentient being?

Eventually a portal is found, allowing animals to cross over into the "human" earth--but at the risk of losing their sentience unless a solution can be found--and allowing people to cross over in the "animal" earth--but how will animals perceive these humans? As invaders? As lost friends?