What's the Name of That Book??? discussion

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SOLVED: Adult Fiction
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SOLVED. Sci-fi book about forced colonists on distant planet. [s]
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It's a space sci-fi novel set in the future, where a guy finds himself on a spaceship bound for a new world with a bunch of other people who've been forced to become colonists. They land and the military people aboard "help" them to set up the basics for their new life before abandoning them. People are on edge and start to act out and go nuts, and a few are killed. One of the military women is abandoned there with them because they wanted more of a gender balance.
From what I remember, before long everyone is dead except for the main guy and the soldier woman, who survive together and eventually have a daughter. When the climate starts to change, they follow these "builder" creatures across the land as they migrate and after many years they run into other survivors who had been settled in different regions.
Many more years after that, they reach the "heart" of the planet where the main military base has been set up to watch them, to document their social evolution. They force their way into the base and then at some point the main guy discovers a hatch that turns out to be a portal and it takes him back to earth (or Venus?). His daughter and wife and some others go through later and are resettled on earth but he's not allowed. I think in the end, as an old man, he agrees to go off to some new world (or back to the other one) to live out his remaining days there.
I can remember so much of the book, almost from start to finish, but can't remember or track down the title or author. Can anyone help?