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Basically, you could do what they did in the Avatar movie. Their floating mountain has unobtanium in them, which is a room temperature superconductor. They float because of the Meissner effect with the magnetic field below them.
A superconductor is a material that when you cool it down to a certain temperature, it can conduct electricity effortlessly without much energy loss. This allows them to float in the presence of a magnetic field. You can look up the details, but basically, a magnetic field's flux cannot penetrate a superconductor once it becomes superconducting, so it bends and pushes around them instead, allowing the material to float in midair. So, if I add electricity into those superconductors, they'd be able to run in those materials forever. Unless the earth somehow loses its magnetic field, those islands could stay floating forever.
Now unobtanium is a room temperature superconductor, which takes away the requirement for them to be cool to their critical temperature. And since the gravity is lower than that of earth, the magnetic field can similarly be lower than that of earth, which would allow the use of metal tools and such without the tools propelling upwards.
I'm not sure about the evolution part, so I'll just take a guess. If you want humans on those islands, I imagine life would evolve normally similarly to that of earth, up until about sailing and such.
It probably depends largely on the environment of your islands. How big are your islands? Are they massive continents with diverse biomes, or just small floating island with one single environment? After all, the reason human evolved in the first place is due to the Africa savanna. Without a floating Africa savanna, humanity might not evolve at all and you'd have monkeys eating fruits in their floating island forests happily forever.
If humanity did evolve and formed societies, then depending on how close the islands are to one another, they could build bridges between the islands. If the islands are far apart, perhaps they will never know another island until the invention of the telescope. And they'd still have no means of travel across until the invention of the hot air balloons.
As for if they'd belong to a kingdom or not, it also depends on how big of a population each island can support. Humans naturally form societies and work together. If your islands are small, perhaps they'd form tribes and villages and band together. If you islands are big enough, then they could certainly form kingdoms, given enough time.