I'm re-listening to Stephen Fry's Mythos, and I've been thinking about how Greek myths seem to be this huge collection of disparate belief systems, woven together. I've tried to look for some history of the development of Greek myth but it's hard to get beyond retellings of the myths themselves. Is there much information about when certain beliefs emerged, or is it mostly speculative?
For instance, there's two simultaneous goddesses of the moon - Semele (Titan) and Artemis (Olympian). Were they simultaneous? Or were they regional? Or did Artemis supplant Semele? Is there anything to show when the stories of e.g. the birth of Artemis and Apollo or Athena emerged, and do these seem to be much later than than the beginning of them being worshipped? Did their cults grow up after the cults of Zeus, Hera, Poseidon, and the other 'original' Olympians, and is there anything to show how and when a coherent sense of the 12 Olympian gods as a complete pantheon came into play?
I find it so fascinating that e.g. Leto, the Titaness mother of Apollo and Artemis, had temples dedicated to her, as per the myth of Leucippus, even though she wasn't a 'god'. Is this comparable to having a devotion to particular saints in Catholicism/Orthodox Christianity?
I find it tempting to read a history of how beliefs developed in the region into the mythic history - with Gaia, Uranus and the nature spirits being older deities, who were supplanted by worship of the Titans - perhaps originally the gods of a conquering tribe - who in turn lost ground to belief in the Olympians as their adherents gained power in the region. However this seems too neat and obvious! Would love some pointers on where to read more about this.
You are right to be confused! "Myths" are known from documents dating to ancient times. The authors of these documents were inspired by various things and drew from diverse backgrounds - both temporally and geographically. These authors drew inspiration from oral traditions and beliefs, but they had diverse motivations for producing records of these narratives (and what they reflected about contemporary tradition).
From a historian's point of view, the approach is to attempt to understand each document in its context and to attempt to compare similar stories and beliefs in an effort to reconcile the diversity or to define what can't be reconciled. Each of the examples you cite has inspired long discussions over decades of historical scholarship.
Here, from the point of view of a folklorist, I will provide some background for an understanding of what was likely occurring in folk culture to inspire these documents.
Myths are not folklore; rather, they draw from and were inspired by contemporary folklore, but they are an imperfect step removed. For definitions, see the thread attached to this one.
Folklore is by its nature diverse and contradictory. It also changes, so one can find contradictions from a single location - from person to person and from generation to generation, and this is accentuated when adding the geographic dimension. If we could transport dozens of folklorists to ancient Greece - to all the places where Greek was spoken and across several centuries, the full potential of this diversity would become apparent. Certain trends, beliefs, and similar stories would emerge from the mix, but so would the contradictions.
People on this sub frequently ask, "what is known about the original myth of XXXX people?" These questions are not possible to answer because they typically ask us to peer into an unknowable prehistory, but they also seek an answer which is contradicted by the very nature of folklore. Because folklore is not codified by a written document, it is free to mutate - and it does exactly that!
Many early documentarians of myth were no doubt attempting to provide just this sort of dogmatic text. Keep in mind, this was not the motivation of all these writers. Regardless of the inspiration, there is a tendency for these texts to concede the difficulty in the task and to hint at the diversity in traditions that they knew existed.
This is the context with which we must view all attempts to document folk traditions - whether it is Egyptian, Greek, Roman, Celtic, Norse, or even when it comes to the lives of some of the Christian saints. With these exception of the last of these (which often followed a different trajectory) these stories and traditions percolated among the folk for countless generations, resulting in a tangle that was at once cohesive and contradictory.