Was There An Advanced Civilization Before Humans? The Silurian Hypothesis

We are always there talking about extra worlds, aliens, and how civilizations might be sitting safely on extrasolar planets, protected by the enormous distances involved.
And it is as if we take it for granted that nearly five billion years of geological and biological evolution of the Earth could have resulted in only one dominant form of intelligence: that of our species.
We, Homo sapiens, as the apex of evolution, the final, unique, and unrepeatable product of a time course all aimed at bringing us to the dominance of this planet…
But… what if this is not really the case? What if before us … long before … there was a way and time for evolutionary roulette to have birthed another terrestrial civilization?
Would it really be far-fetched to think so?
After all, complex life has existed for more than 500 million years. How sure are we that humans are the first intelligent species to have evolved on this planet?
Might there not have been – I don’t know – a Cambrian civilization of intelligent trilobites, building underwater cities powered by hydrothermal vents? Or big-brained Cretaceous theropods capable of using tools and language?
This is not as absurd as it sounds. On the geological time scale, we inhabited this planet for only an instant, and industrial civilization existed for only a fraction of that instant. And if humans became extinct tomorrow, the relics of our society would not last long.
But why these questions? And who are the Silurians?

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