Thinking about the (relatively) small communities and lack of infrastructure at and before the dawn of cities, it seems a logical consequence that chances were high all available mating partners are related, if the community continued over a long enough time-frame, which seems to be the case. Did this happen? If so, was there a measurable impact on genetic stability or diversity? If not, where is the flaw in my reasoning, or what assumption is incorrect, or what other factor have I overlooked? I am asking in the general case of communities before cities were a regional feature, not in any specific area or time-frame.
I'm going to throw this out there -- incest is a social construct. (Please don't shoot me. As I type that, I realize it sounds creepy. Bear with me) What I mean is that what one culture views as an inappropriately close relationship between relatives, another culture might view as a thoroughly appropriate relationship between two people who are free to marry. You can see that in modern life with the question of cousins. Is cousin-marriage incest? In America, we'd say yes, that's gross. But still there are millions of people around the world who are married to their cousins and don't think of themselves as gross at all.
Most societies have rules that aren't necessarily the same as our own. One common rule of marriage has to do with cross and parallel cousins. In many cultures, you'll have a situation where to marry your male uncle's daughter is incest, to marry your female uncle's daughter is perfect. This is because in some cultures, the cousins who are the descendants of father's brothers and mother's sisters differ in quality (being like brothers and sisters to you) from the children of your father's sisters and your mother's brothers (who are not really related to you.)
Another way that some cultures cut down on incest (as they define it) is by considering all children the sole product of the father, or the sole product of the mother. In the first case, two half-siblings with different fathers could get together, no consequences. (This used to be the case in Ancient Japan.)
I guess my point is -- yeah. There historically has been and continues to be tons and tons and tons of consanguinity in small communities around the world. (I like that term instead of incest, because like I said, incest is a cultural concept.) You get some negative results eventually. Common examples that people use when discussing this are deafness among the Amish, Tay-Sachs among the Azkhenasi (forgive me for my spelling) Jews, and so on. Remember that human beings can suffer from a huge amount of pedigree collapse before there's a guarantee of trouble.
In eleventh century Europe: almost universal. The Church prohibited marriage within the 'seventh degree' which was calculated by counting up to the closest common ancestor from both partners and then adding the results, so a parent would be the 'first degree', a sibling the second, a first cousin the fourth etc. However in the eleventh century they changed this so that you counted seven generations back from each partner, meaning that everyone closer than a sixth cousin was off limits. So if you didn't like a marriage that was about to happen, or if you were tired of your spouse or something, and you were closer than sixth cousins (outside a couple of big cities this would have been almost certainly the case) then you could just tell a priest, and the marriage would become invalid. I got all this from 'The War on Heresy' by R. I. Moore. He thinks that this was almost intended to make most marriages invalid, and then be used as a form of control by parents on their children's marriages, which was important for passing down property.