I've been thinking that they didn't really need to make the planet in Miri identical to Earth.
It could have easily been an Earth colony or an alien planet without changing the story too much.
However I think they wanted to make a message about science going out-of-control on Earth. If it were an alien culture then the message would have less impact. We could think we'd never do anything as stupid as that.
Of course in hindsight its ridiculous to have an 'identical' Earth.