Dear Quetzal,
Lucky I am, you still are standing on a general (comprehensible) knowledge layer for your answering. I'm seeking for some Margulis's PDF papers.
Your arguments gave me an idea.
Briefly, perhaps the first metazoan was a viable hybridation of two different prokaryote species, formerly symbiotic and each specialized in two complementary inorganic recycling. For an unknown reason, genetic material was mixed (viruses, solar radiance?) and so the genetic discrepancy of this artificial hybrid was both (1) so important that not allowed any more further separate division and (2) rather than disintegrate (should have been expected) the duo multiplied itself with the same genetic alteration.
Of course this is speculation. The main assumption is that both organisms had a very simple genetic material and each of them was together in a lucky period.
However, I think that it will be quite interesting to dig more deeply in the "association" field. These phenomena probably played important (crucial) roles in evolution.
Still much to learn before receiving a "Prix Nobel". Humour.
Denesha