My favorite archetypal myth is that of the storm or thunderer god facing off and defeating a giant serpent or dragon. A variation of this myth occurs in practically all Indo-European religions and is most likely occured in the theorized Proto-Indo-European mythology. Thor fighting Jormunandr in the Germanic mythos, the Hittite storm god Tarhunt slayed the serpent Illuyanka, Zeus and Typhon, Perun and Veles in Slavic mythology, Indra and Vrita in the Vedic religion and many more variations. The gods I just mentioned were all thunder/storm gods in their respective mythologies and they all killed a serpent in their stories.
A similar myth exists in the religions of the ancient Near East. Hadad Ba'al was a storm and rain god in the Canaanite and Mesopotamian religion. Hadad Ba'al fought and defeated Lotan, a sea serpent who was the servant of the sea god Yam. A varation of this myth is als found in the Hebrew religion with Yahweh fighting the Leviathan. Yahweh had some storm god qualities and an association with thunder.
I initially figured that the near East adapted the myth from the Hittites, who entered Anatolia around 2000 BC and took over the lands of the Hatti and the Hurrians. However it seems to be that this myth also occurred amongst Hurrians and Hatti.
I now think it was a myth that originated in the Proto-Indo-European Mythology, which was most likely practiced by nomadic pastoralists of the Pontic-Caspian steppe. These people used horse carts and were really mobile, which lead to an expansion of their culture. The Uruk period in Mesopotamia started in 4500 BC and lasted until 3100 BC. During this same period the Kvalynhsk culture of the Pontic steppe developed into the Yamnaya culture, which around 3300 BC started the initial waves of the Indo-European migrations. Given their regional proximity and the mobility of the steppe pastoralists, there certainly could have been contact between these early cultures around that time, and maybe this is what lead to the existence of a Storm god vs Dragon Serpent story in both Indo-European and ancient Near Eastern religions.
These are my thoughts on this subject, but what I would like to know is what the actual historians think is the origin behind this shared story. What are the main theories for why these two cultures share this myth? Did the Mesopotamian adopted this myth from interactions with Indo-European cultures or vice versa? Or is it actually a case of a myth which independently developed amongst two separate cultures?
I've written on this before, and I think you are pretty much correct. I don't see any other point of origin than in the Pontic-Caspian steppe making any sense, no matter how exactly we account for it spreading.
Do you happen to know what the sources for this myth appearing among the Hatti and the Hurrians are?
I am not a historian, but as I have a similar doubt I would like to add a few more things: myths of snake/dragon like linked to storm deities seem not to be unique to Indo-European groups, the Aztecs have the Quetzalcóatl is the god of the wind, the tupi-guarani have boitatá the fire-snake linked to great fires in the florest, the japanese have orochi, a dragon who fought and died against susano the storm god, the Egyptians have apophis "the great serpent" that fights against Ra.
another matter, I see that many legends about floods of these same peoples, the Greeks have Atlanteans, the Tupi have, if I remember correctly, also something similar, and etc...