The "fitness" of humanity is the power of the brain. Humans, however, have used that brainpower to bypass survival of the fitness as best they can within humanity. (Mostly, kind of. Technically, we've modified the "fitness" parameters such that humans, over the past few hundred years, have been genetically modified to grow larger and taller. Though, the size difference is also due to differences in diet over time.)
As for anarchy... it only works until someone becomes a warlord, so it tends to break down once the community size grows beyond the maximum that humans can make full personal social connections, which tends to be somewhere between 100 and 300 people. That is, most humans can only socially connect with ~100-300 people. Even inside that number, cliques form, but outside that number the cliques form to such a level that tribalism takes over. Once tribes are formed, even if they are internally anarchistic tribes, they will naturally begin to compete with each other.
Human psychology is geared towards tribalism. We have not yet figured out how to deal with that, as is obvious when you look at the arguments between nations.
The only way we can properly create a non-conflict-ridden humanity without a single united government would be to defeat resource scarcity such that each tribe will have not only adequate, but so much resources that they never have to worry about resource scarcity again. This is, however, probably impossible. I think even with cheap space travel (such that we can colonize) the fact that we can colonize in three dimensions would be dwarfed by the fact that we can grow in population exponentially.
With instantaneous space travel it's possible that the universe is large enough that our exponential growth wouldn't fill it by the time heat death came about. The premise, both about instantaneous travel and universe size, however, is unlikely.
The other alternative, lacking instantaneous travel and a sufficiently large universe, would be simulated reality. But then whoever chooses /not/ to go into the VR pods would have a real world and would have the power to kill everyone in the pods. So maybe we'd have to trick everyone into VR pods. Of course, if we couldn't solve reproduction-while-in-a-VR-pod we'd have to rely on migrants from the mountainhome to replace those that die of old age.
There is, of course, also the belief that we've already done this before, possibly more than once, and are actually in a virtual reality now. Of course, in order to maintain a population level that's sustainable while allowing us to believe that the population level is at an unsustainable level either the "real" world is larger than the virtual world or, more likely, some of the people around us would have to actually be AI. Of course, that could actually just be the third world population. Oh my, that's actually somewhat depressing to think that some people might actually /be/ lesser. But again, it is impossible to determine if someone is a p-zombie, so it's still "wrong" by most thought experiments to harm them.
Of course there's the alternative that the world could just name me dictator and I would have our population surplus create the megaproject that would allow space colonization then colonize the universe and limit birth rate such that we would survive until heat death without scarcity. Many would die, of course, to initially get to that level, but I would be optimizing the standard of living of all of humanity for the rest of the observable universe's time frame. That's surely a greater good, no? I vote Zero for dictator. He would make your life better in the new world. Assuming you survived to see it.