an advanced team sent out to prepare a planet for habitation-- and the advance team was all decoraters, hair dressers, ad writers, agents, middle managers.. etc? (i didn't actualy read the book--i only saw the tv Series.. but i am sure Max can give more details..)
Yes, basically it. They got rid of a useless third of the population, and started the human race. That's why Arthur didn't actually have the Question in his head, just a very warped version. The human/aliens drove the semi-intelligent cavemen (they aren't cavemen!!!!!!) to extinction, and the people back home died because of an unsanitised telephone.

ha ha ha, Max