While I'm not an anthropologist by any stretch of the imagination, I do have a broad (one could almost say 'shallow') understanding/interest in the dispersions of peoples and the subsequent formation of tribes and then into civilizations. It's a hobby. I'm a nerd. The existence of five distinct tribes is not at all out of the question. Earth-centric tribes exist in abundance, and often a lot closer to each other than you might think. Some European 'tribes' and/or ethnicities, however you choose to view them, are Gaels, Angles, Jutes, Normans, Thracians, Macedonians, Slavs, Finns, Andalusians, Danes, to name a fraction of them. You could argue they are all part of the Caucasian group (or Norsirai, in terms of this discussion). Then you have Middle-eastern cultures, which are again divided into regional nationalities such as Hebrew, Arabic, Turkic, Palestinian, etc. Many of these share a common heritage (i.e., Semitic or 'Ketyai'), but some don't or are only partially genetically related. The list goes on with the various sub-cultures in Asian, Indian, Oriental and African cultures, to a staggering degree, not to mention the multitude of New World tribes in the Americas.
In reference to the tribes of Earwa, then, five tribes is practically nothing on a world-scale. I see no reason why there couldn't be five tribes. The reason there were 'only' five tribes is more problematic. Cross-culture mixing happened all the time, either where one tribe was subsumed by another, or simply by trade (think the Silk Road from China to Europe). There must be some sort of prohibition, on a broad scale, to discourage interbreeding. That's not surprising, either, though. Many religions prohibit or at least frown upon cross-culturization (is that even a word?). The thing that occurs to me as most troubling is why five disparate tribes, obviously having evolved along similar lines as earth tribes, came together under a leader from one tribe alone, in one specific geographic location. Why would five dominant groups of ethnically different backgrounds (I'll use real-world analogies here) congregate and cooperate on a mass scale? African, Semite, Caucasian, Scythian and Chinese (assumedly) tribes all banded together under one man and said "Yeah, let's do that." The reason for that is the real question in my mind.
If I've made any gross errors in here, it was not my intent to offend anyone. As I said, I'm not an expert. Simply an amateur with no formal training in anthropology, but an intense interest in this subject. If anyone can clarify or debunk any of the crap I just spouted, please do!