Did humanity evolve on Earth, or did we come from some place else?

I think both.

So here's how they explained it to me...

It is a matter of reincarnation.

There are different types of "soul" or particular vibration wavelength of consciousness. The souls need compatible biological bodies to incarnate into and require a particular biological marker. In that case of humans, and most of life as humans currently understand it, that marker is found in DNA (although this is not always, or even usually in the bigger picture, the case). That marker must be passed through some sort of reproduction, natural or artificial. Humans were part of a project to populate planets with souls with minimum impact on the biological balance to be habitable. Biological markers were added to the most suitable species of the planet to make them able to contain the souls and to boost the evolutionary process on an exponential curve to allow maximum use of consciousness in minimal development time (still over the course of hundreds of millennia).