What is the origin of mimics in Edge of Tomorrow?

It is explained in the novel.

• Eclipse Magazine:

  ... although the invasion originates from another star system, the actual aliens are terraforming creatures who have emerged from the sea after coming in contact with starfish and frogs.

  Their very nature can change their environment to match that of their creators’ planet.

• Staffer's Book Review:

  ... an alien race has invaded via nanobot proxies to terraform the Earth for their environmental needs.

  Known as mimics, the alien nanobots have perverted starfish to create the perfect front line troops.

• SF Signal:

  ... battle against aliens called the Gitai determined to terraform Earth for themselves, even if that means turning it into a wasteland for its current inhabitants.

  Nicknamed “Mimics” because they mimic the appearance of the first creature they came in contact with — starfish...

• That Was Not In The Book:

  Mimics are ecoforming bots from a star system 40 light years away designed to re-form the Earth for alien use.

  The bots mimic starfish from the ocean and look like a bloated frog with four limbs and a tail.

In short:

• Aliens, who live 40 light years away, have sent nanobots to Earth to ecoform/terraform the planet for them. The nanobots "evolved" into the creatures known as 'Mimics' (their purpose isn't just to kill us, but to make the planet inhabitable for the alien race who created them)

I have a weird theory about this.

I think humans of the future (Who possibly don't live on Earth anymore) found a way to time travel and also create true AI.Thus they created the Omega(Ambition to create God).Learning the true implications of this they tried to destroy Omega but Omega then decided to travel back in time to 2025 and destroy humanity instead. So now Omega comes to 2025, locates Humanity on Earth by receiving the signals humans of today send to reach out aliens.Thus the asteroid crashes on Earth and the events that follow....

My logic for the above is this:

1. In the movie, The scientist says that humanity is Omega's vulnerability. This makes much more sense if humans were responsible for creating Omega, they must have put some fail-safes to give humanity an advantage (to be on the safe side). The strange mechanism of transfer of time-powers from omega to human could be one of these fail-safes

2. Omega comes by asteroid because when Omega travels back in time it is still in coordinates of future humanity

EDIT: I haven't read the book,The above is only in the context of the movie.