This seems like a result of the same simple strategy in different environments. The strategy is: "try to get as much power as you can, and reduce the power others have". The environment includes technology and other people. As the humankind develops (technologically, intelectually, socially), different tactics become possible, sometimes small changes can lead to big results. (For other species, the tactics remain rather constant for millenia.)
How difficult is it for the most powerful individual to retain power? How difficult is it for the powerful groups to keep the most powerful individual in check? How difficult is it for masses to keep the powerful groups in check? Answer these, and you will see whether some variant of "monarchy", "aristocracy" or "democracy" will be the most "natural" form of government.
It depends on technology. Cheap weapons, such as sticks or stones, give more power to large groups (a group of weaker humans can stone one strong individual, distributing the risks of counterattack). Expensive weapons, such as nukes or battle droids, give more power to rich groups = the groups currently in power. Centralized media, such as radio or TV, give more power to their rich owners. Distributed media, such as internet, give more power to masses.
None of this is an absolute advantage; for many forms of power there is a defense that weakens it partially. A physically strong person will be also good at sword-fighting. A large group of people, containing very skilled individuals, could steal nukes or hack battle droids. A large group of people could occupy a TV station. A rich organization can pay people to defend their interests on internet. A government can censor internet communication, and punish those who break the rules. Etc.
It also depends on culture, religion, education, fashion, surrounding countries, economical situation, etc. After including many factors, sometimes the strongest individual wins, sometimes the strongest group wins, and sometimes the power is distributed widely.
If you want, you can try to fit these data into a trend (usually by filtering the evidence), or into a "spiral" (which is just a form of saying "sometimes this, sometimes that", while pretending deeper understanding).
What will happen in the future? It will depend on specific inventions and other changes, which is hard to predict, especially if something "completely new" happens. I think that some individuals will always keep trying to concentrate power in their own hands; other people will keep fighting for power as groups; and those unsuccessful will keep hating them.
If I had to make my bet now, I would bet on aristocracy. The world is too complex to be ruled by an individual, and most people are too ignorant for a real democracy. Of course the ruling class may choose a person to represent them, thus creating a nominal monarchy; or allow free election about unimportant topics, thus creating a nominal democracy. Actually I think this has already happened; except unlike old aristocracies the current one does not have exact definition and strict boundaries, so it can be joined by people who have enough skills and luck.
It depends on technology.
I agree that technology is a key factor here, but it isn't clear that weapons are especially important--not in a context of institutional evolution where national defense is taken for granted and relied upon as part of a smoothly-functioning economy. Ability to coordinate on complex goals is probably more important: at present, large number of folks cannot easily coordinate to affect public policy except in relatively crude ways (such as through ideologically-driven organizations). Hence, our current institutions in the West ...
I often hear people speak of democracy as the next, or the final, inevitable stage of human social development. Its inevitability is usually justified not by describing power relations that result in democracy being a stable attractor, but in terms of morality - democracy is more "enlightened". I don't see any inevitability to it - China and the Soviet Union manage(d) to maintain large, technologically-advanced nations for a long time without it - but suppose, for the sake of argument, that democracy is the inevitable next stage of human progress.
The May 18 2012 issue of Science has an article on p. 844, "Ancestral hierarchy and conflict", by Christopher Boehm, which, among other things, describes the changes over time of equality among male hominids. If we add its timeline to recent human history, then here is the history of democracy over time in the evolutionary line leading to humans:
There are two points to observe in this data:
I do believe "progress" is a meaningful term. But there isn't some cosmic niceness built into the universe that makes everything improve monotonically along every dimension at once.