"Zero evidence" is not correct, whether there was massive civilation prior to 8000 years ago, no one can be sure, it might have been massive, it certainly was global, depends on how you define "massive". The thing you have to see is that there was a transition period from 12,000 years ago to 8000 years ago. During that period there wasn't much more than farming and hunting.
You can't discount someone for speculation, everyone has some degree of speculation, this thread does a lot of speculating, when you go back a long time, you are forced to speculate. ETR posted about a period 2.6 million years ago, geologists speculate that there was enough volcanic activity at that time for the global temp to be 10 degrees Celsius hotter than it is today... it is speculation, really, just a wild guess, there's no data on global weather 2.6 million years ago.
However, we are very confident of climate change that occured 10,000 - 12,000 years ago... and it is global. Siberia went from tropical to sub-arctic. North America went from Arctic to moderate. Europe went from Arctic to moderate. North Africa went from Mediterranean to desert. Antarctica went from forested to polar. And there is that long narrow desert in South America that went from at least moderate, possibly rain forest, to desert. So, the whole earth changed latitudes, and it all reflects a meteoric strike in the North Atlantic that drove the impact region southward with the spin of the earth.
Even the location of the polar ice caps changed and everything changed about the same distance and at the same time... 12,000 years ago. Coincidently, Atlantis was said to have gone under water around the same time. Also at the same time, mean sea level rose 350-400 feet, so all the earth's coastal cities went under water, and we find them; off the coast of India, Japan, the UK, Indonesia, in the Bahamas, there's no telling how many disappeared into the sands of time on the ocean floor. So there was civilization running back long before 8000 years ago. Giza pyramids and the Sphinx are around 30,000 years old, and there are much older sites and evidence of plentiful ancient mining and strip mining that was high technology, as well as the megalithic stone work that is also global. Did you know there are over a 100o pyramids in one region alone... in China? There are over 75,000 ancients mines in South Africa; all high tech. There are hundreds of thousands of giant magnetrons that powered the laser mining in those ancient mines. So there is massive evidence of civilization, but no telling how large the populations were... but there is high tech ruins all over the globe.
So, was the earth mostly farmers and hunter/gatherers 8000 - 12,000 years ago? yes. But civilization goes back a lot further, and there's no reason not to speculate that man may have lived on this planet for tens of millions of years, if not hundreds of millions of years. The point being, the earth was prepared for life and for man to live on it... and man was created intelligent. It takes just as much intelligence, if not more, to survive a global apocalypse like what happened 12,000 years ago as it takes to do linear Algebra. Man was never stupid... until Winston came along and trolled this thread.