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This whole area of speculation is fascinating. It's probably all the more fascinating because it is so speculative. We're free to invent theories and hypotheses all day long because there's no real way of knowing which are going to be right and which are going to be wrong.

Now what would you say - in your opinion - is the earliest date that civilization-builders could have arisen on our planet?





It would be easy to prove that there WERE another civilisation out there, if we happen to find it, or it finds us. :-) Of course, we cannot PROVE there is NOT one until we have visited every planet of every star and checked under every stone.

Now, as to the earliest date when civilisation could have emerged. I suspect the dinosaurs could easily have done it, if evolution had happened to swing that way. Various dinosaurs had bipedal gait, relatively active metabolism, if not full warm-bloodedness, and a challenging environment where intelligence could have helped.

But they didn't.

Life could have emerged from the seas 10% or 20% earlier than it did. Maybe as much as 50%, but I buy a lot of Brownlee & Ward's comments about how much covert molecular evolution was required before overt physical evolution became possible.

So, in theory, civilisation could have emerged on our Earth some 2 Ga previous, or 500 Ma, or 65 Ma, or maybe it might have missed the chance we now are using.

In short, it's almost impossible to say, but fun to speculate, and sometimes that speculation leads to learning.