The tenants of the theory are this:
In the early 1st era, the Dwemer were able to craft machines that have not been replicated since, including a whole new Brass God. Minor gods like Morihaus and Pelinal fought alongside men in their war against an Umaril, who was blessed by a Daedra and whose father was a "god from a previous kalpa." Later, Reman was born out of the earth itself and launched a program to explore space.
While the second era no longer had gods roaming the earth, it did have a ton of intervention by Daedric beings, a plethora of communicative ghosts, and multiple wherein one sees, or even travels, into the past and into afterlives.
By the time of the third era, the people of Tamriel were furthest removed from divinity. The Gods, when they showed up, manifested as humans and required human aid, rather than the other way around, and many artifacts tying Nirn to its divine past (the Towers, the Amulet of Kings) had been destroyed or profaned. One assumes, then, that Alduin's return in the 4th era places us on the ascent once again, since Alduin's purpose is to end the world and begin a new one, thus returning us to the divine.
This new graph accounts for the theory that each Dawn Era is merely the end of one cycle of time and the beginning of another.
(Of course, this is just a theory, and, because the games and the lore were not created with it in mind, it does not fit everything perfectly. Still, I think it's an interesting way of examining the overall structure of Tamriel's history)