First of all, thank you for posing such interesting questions! I am a sucker for conversations like this.
It is difficult - and objective at some degree – to define civilization. I had this exact discussion a couple of weeks ago with Martin Sweatman (while exchanging emails) and his answer was the size of the society, that at least 1.000 individuals would be required in order to call it a civilization. My own answer would be that a civilization creates and leaves behind something that requires mental activity: art, building houses, making tools, etc. In our case, when we say civilization we mean something a bit more advanced. The civilization I am talking about in my book would correspond, on average, to that of the 18th century. I believe that those people could navigate the sea, knew relatively advanced mathematics, could work stone, were able to produce megalithic monuments, etc. In short, I believe it was a relatively advanced civilization, but could no way compare to our modern civilization. I do not believe they had electricity, for example, much less spaceships and nuke.
I only mention the example with the shoes to make a couple of points. Firstly, that primitive people were no fools; they were as clever as we are, lacking only in technology. Secondly, that civilization is inevitable for humans. If humans were already making shoes ways back then, a technologically advanced civilization could have popped up much earlier than it did. And more importantly, many times throughout human history.
For me, big advances in civilization require a couple of things. Actually, there are many things but two are the most important ones: luck and a charismatic individual. The role of luck is clear: one big comet impact and begone civilization. However, can one individual make a difference? Taking a look at history, I say it can. The vision of one man, Alexander the Great, changed the world and created the ground in which science flourished. The Hellenistic Age gear technology I describe in my book is no doubt an indirect consequence of his campaign. This technology is believed to have passed in Europe, through the Arabs, making the industrial revolution possible.
40.000 years ago some amazing things had already happened, if you take a look at this article on my blog: [ourunknownancientpast.blogspot.com] Humans had some common written symbols and they had a zodiac, almost identical to the one we use today. At the same time, we have the lion-man of the Hohlenstein-Stadel [en.wikipedia.org] , an amazing work of art. This was a primitive civilization, but a civilization no less and I am convinced that it set the ground for the later known civilizations of antiquity.
It is possible to set an upper limit to human civilization, but theoretically, that limit is the time Homo Sapiens first appeared on Earth. Assuming of course that we are only talking about modern man, as other human species like the Neanderthals also had a primitive civilization (i.e. they controlled fire). And make no mistake, modern man did inherit some know-how from the older human species.
Sorry for the long post, I hope at least I clarified how I perceive some things! I would be very interested in hearing your thought on all of this.