I'm a little confused. How did the people in the past in Terra Nova know they weren't going to alter the future of human race? Or, is it not their past but a whole new planet they colonized?
The past they went to was not the same "timeline" as the future they came from. The technology they use to travel through time allows them to jump to "parallel time streams" at arbitrary points along that timeline.
So, nothing that happened in the past could affect the colonists future. It's never really explored what they were doing to that timeline's previously-existing future, and if their actions would impact the possible development of human life there.