I have a few theories of what happened in some of the missing areas, but we really can't be certian, even though a few of mine are plausible.
One theory at hand is Cera's mom and a few others not surviving the journey during LBT 5. I'd imagine that not all of the Great Valley residents survived through LBT 5 for a safe return home. I wouldn't be surprised if a number of them died due to either starvation, dehydration or get eaten by a sharptooth.
A part of this theory also seems to fit in Dinah and Dana strangely enough. I also happen to have the theory that one of Cera's older sisters that we don't know anything about was impregnated during the journey and as such, produced the eggs containing Dinah and Dana probably either when they were staying at that place where they found some fresh water and green food or not long after they return at the end of LBT 5. I'd guess the latter based on the fact that those two would have a better chance of survival and would less than likely be able to make the very long trek. Why we never saw their mother, who is supposedly one of Cera's much older sisters, I have yet to work out. Might take me a little time to finish up the whole theory.
In fact, my theory ties in with my current LBT story when I plan to bring in Dinah and Dana, so I'm hoping for it to be true. I just might have some of the closest possible theories around.

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Here's hoping they appear in the possible season 2.
