Cosmologies, like Hesiod's Theogony, are accounts of the beginnings of the universe, and they generally also, through the course of telling the universe’s origins, tell of the universe’s workings: i.e., why the sun rises every day. Cosmology most likely antedates the written language, since non-literate peoples during historical times had cosmologies (like the Amerindian Coyote myths).
The earliest written cosmologies were, like Hesiod's, “theogonies” (stories of the origins of the gods) because the creators of various elements of the universe, and oftentimes the elements themselves, were thought to be gods: super-human rational beings.
The earliest surviving written cosmology is the Babylonian Enuma Elish, which is thought to have descended from Sumerian sources. According to this work, the primal state of the world (which of course to an ancient was the whole universe) was a water mass in which salt water (Tiamat), fresh water (Apsu), and clouds (Mummu) were all intermingled. But these inter-mingled entities were not lifeless masses. They were characters, with personalities and rationality. Neither were they merely gods of the various forms of water: over-grown humans with command over their respective elements. Tiamat had a distinct personality, but she was also literally salt water itself (”tiamat” literally meant “salt water”).
Given that the Sumerians lived near the mouth of two great rivers, on can understand their preoccupation with water. The antiquarian Georges Roux, in his excellent work Ancient Iraq, mused on how the ancient poet might have envisioned such a primeval scene:
“If we stand on a misty morning near the present Iraqi sea-shore…what do we see? Low banks of clouds hang over the horizon; large pools of sweet water…mingle freely with salty waters of the Persian Gulf…all around us sea, sky and earth are mixed in a nebulous, watery chaos.”
“Creation” happened when this watery chaos began to un-mingle. First arose silt islands, like the ones which can be found where the Tigris and Euphrates pour into the Persian Gulf (personified as the gods Lahmu and Lahamu) Then came the horizon (personified as the gods Anshar and Kishar).
This evolution in the story evinces a greater sophistication than myth-makers are often given credit for. Aristotle called myth-makers theologi, because they explained the mysteries of the universe by invoking personal forces (gods, or theoi), in marked contrast to those he called phusici, the later philosopher-cosmologists who explained those same mysteries by theorizing impersonal forces (nature, or phusis). But as I shall attempt to show below, many ancient myth-makers seemed to be trying to make rational cosmic theories within the bounds of the god-myth format.
Myths are also often derided for “just being stories”. But really compelling stories are often compelling for their plausibility. Thus, stories that survive the test of time are often the most plausible ones, and evince an author’s good grasp the plausible.
Let us examine the first two generations of gods in Enuma Elish, ignoring for the time being the proper-noun treatment given to the cosmic entities, and keeping an eye out for plausibility.
Firstly, let us imagine the three water gods as simply dumb masses of water, and that the poet is merely postulating that the earliest state of the world was a single, indiscernible foamy blob of fresh water, salt water, and mist. The notion that all discernible things arose from a single mass is not inherently fanciful. In fact it is quite plausible. There are only two possibilities regarding the primal state of the universe: either (1) things were distinct, various and discernible, as they are now or (2) they were indistinct, single and indiscernible. The Enuma Elish authors simply chose option number 2, as did many of the Greek philosophers of the 6th century1, and chose water as that single thing.
Once the primal state is established, a cosmologist must decide which cosmic entity would arise out of the primal state first. The Enuma Elish authors chose silt islands, which is a very plausible choice for a Mesopotamian, given that silt islands occur right where the waters above (cloud and mist) and the waters below (salt and fresh waters) still mingle (as described in the Roux’s colorful quote above).
Then, if the clouds and the waters continued to separate out, the first discernible things one could see would be the bottom of the sky flush with the top of the earth: in other words, the horizon.
This account, arrived at from a sense of plausibility arrived at via induction, given the lack of scientific instruments at the time, is a perfectly adequate theory for how the world that ancient man saw around him could have come to be. Scientists even today use their sense of plausibility to construct theories. Of course they have much more powerful techniques for testing those theories, but other than that, there is not as much difference between ancient and modern cosmologists as might be supposed.
1In fact, Thales, regarded as the first Greek philosopher, is thought to have traveled to Babylon and may have been directly influenced by Enuma Elish in his assertion that the world was created out of water, and that everything fundamentally is water. He, as Carl Sagan said, told the Babylonian story, but “took the gods out of it.” Sagan used the comparison to exemplify how different the mythmakers and early philosophers were. I think it exemplifies how surprisingly similar they were.