I believe that the Namekians are a race either created by or evolved from the Kai's in order to effect positive changes/ maintain order in the universe
A. Both Namekians and Kai's have pointed ears.
Now, this could easily be attributed to the fact that there are probably thousands of species in the universe with pointed ears. However, how many are actually portrayed, or pointed out as having them as the Kai's and Namekians are? Especially in the Maajin Buu Saga.
B. Both the lower level Kai's and Namekians have antennae.
Again, another physiological difference that could most likely be explained away. And again, how many bipedal/ humanoid lifeforms are presented in the universe?
C. Both the Kai's and Namekians are able to detect events going on across incredible distances.
Multiple episodes show King Kai using his antennae to discover what is happening across not only vast cosmic distances, but across the planes of life an death. And multiple episodes show the Namekians detecting changes on planets extremely far away. Not just special Namekians, either, but the entire population, can detect what is happening on Earth, despite the vast distance between our planets.
D. Every Guardian in the series has been Namekian.
Both Kame and Dende are Namekian, and Grand Elder Guru is the Guardian of Namek. These beings are entrusted with the security of their planets, and also contain special powers that cannot be taught to anyone else.
E. The Namekians are the only ones capable of creating and sustaining the Dragonballs.
Pretty self explanatory. The Kai's are the only other race capable of sustaining/ unleashing a power like this
F. Both the Kai's and the Namekians are capable of materialization/ energy manipulation
Both the Kai's and Namekians are able to create clothing, food, Crystal Balls, and other objects from nothing. They are also the only races able to unleash unused/ unrealized energy from within an individual to reach a higher potential.
There are probably more reasons that I haven't thought of here, and more that I haven't seen. But I think it is pretty self evident that the Kai's either created the Namekians to serve as their mortal overseers, or that the Namekians evolved from the Kai's somehow to become mortal and part of the Realm of the Living