kengr

Or if they had just gone with the original - not available until years later - pilot.

The Robinsons abandon the _Gemini 12_ because of the increasing cold, move south and explore the planet in the Chariot, uncovering signs of an ancient civilization wiped out by war. This would explain the devastation of the planet - possibly even that eccentric orbit - and why the original inhabitants had to move underground. Also, all those strange lifeforms were from a "salting the earth" ploy by the victors in the war, to try and ensure the inhabitants could never raise an army again.