[Sam Cree]

The idea that it took humans 245,000 years or so to develop agriculture and then in the next 5,000 years we develop technology to the point where we can travel to the moon just seems silly to me.

When you put it that way it does sound ridiculous.

[Bear_in_RoseBear]

My own pet theory (based on nothing, other than an assessment of human intelligence and ability) is that there have probably been several periods of human civilization... some perhaps as advanced as the Middle Ages we are familiar with. As to where they went, I think it likely they were destroyed by various natural calamities, such as plague, climate change, volcanic activity, even asteroid strikes.

We're probably overdue for some such calamity in our own civilization....