[saispas]

I'm not sure every era did think it was the apex of human progress, though, because sometimes they had physical or cultural evidence that previous generations could do things that they couldn't.

E.g. if you were a petty kingdom emerging in the centuries after the Romans left Britain, you'd be fairly sure that you no longer had the technology to build aqueducts, baths or villas. And for centuries after that, a large element of learning was trying to recreate / understand the classics - e.g. the influence of Galen and Aristotle.

Doesn't the modern idea of inevitable human progress really come in with the enlightenment?

[bluGill]

Those petty kingdoms did have the technology to build that - they just were too small to have enough people.

[saispas]

Did they? For example, the Roman formula for waterproof concrete was lost and only rediscovered (in the 15th Century) when a manuscript came to light. I think it's more than just a lack of people or wealth (though that comes into it of course): people thought of the Romans as being 'superior', hence the effort they went to to preserve surviving classic manuscripts.

It's not to say that the early medieval period couldn't eventually built magnificent edifices or build on the knowledge, but for many centuries, Rome and Greece was seen as something to aspire to.