It seems that small-bodied primitive organisms could support the development and function of a basic nervous system without glial cells. Longer axons and more complex networks presumably require neuroglia for both development and functioning. I am curious as to which one actually originated first and how long of a timespan passed before the introduction of the other.
Humans don't have 90% glia, that's an old myth - in the forebrain the ratio is actually close to 1:1 (Azevedo FAC, Carvalho LRB, Grinberg LT, Farfel JM, Ferretti REL, Leite REP, et al. Equal numbers of neuronal and nonneuronal cells make the human brain an isometrically scaled-up primate brain. J Comp Neurol. 2009 Apr 10;513(5):532–41).
What's of greater relevance is that the neurons and macroglia derive from the same progenitor population, which shifts from generating neurons to astrocytes to oligodendrocytes under the control of local signalling pathways. Basically when there are sufficient neurons in a region, the progenitors start making astrocytes, then switch again when there are enough astrocytes. On that basis we might argue that neurons are the "basic type" and the other macroglia are derivatives, but the real truth is that there is no sharp divide between neurons and glia, they are just types of complex signalling cells which work together in optimal teams. You could even argue that there is greater developmental difference between interneurons and pyramidal cells in the cortex, than between pyramidal cells and astrocytes.
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