History, of course, begins with creation. Sadly, all of the objectivity and solid evidence we require of other events in our records must be dismissed at this early point. Within each province, each culture, each religion, each family there exists a different understanding of how Nirn came to be. It defines us, this belief in where we came from.
That said, one persistent story that is accepted by many cultures is that as the world congealed into reality, the Gods made a great tower to discuss how best to proceed with the making of Mundus. The physical, temporal, spiritual, and magical elements of Nirn were set at this convention, and the tower itself remained behind even as some of the Gods disappeared into Aetherius. Today it is the Adamantine Tower on the Isle of Balfiera between High Rock and Hammerfell in the Iliac Bay. That such a humanoid structure remains the sole footprint of the Aedra speaks perhaps of the essentially mortal nature of Nirn.
It is generally understood that neither proto elves, Aldmer, nor the proto men, or Nedics, lived in Tamriel during the earliest years of creation. The Hist Trees of Black Marsh, most say, were the original life forms on Tamriel, followed by the progenitors of the modern Khajiit, the modern Argonian, the modern Sload, the modern Dreugh, and other beast folk, some now gone from the land, some so shy or rare that their presence is seldom detected.
In what historians call the Mythic Era, the years before formal historical reckoning, the Aldmer came to Tamriel from the legendary mysterious land called Old Ehlnofey or Aldmeris. They settled in the Summerset Isles, and then began to spread out eastward. The Nedic people meanwhile came from the frozen land of Atmora to the north to what is today Skyrim. Where elves and men met, inevitably, there was hostility.
The Aldmer changed over time culturally according to their new environments, being at first temperamentally and then physically very distinct races separate from one another. The ones who stayed in Summerset became known as the Altmer, in Valenwood, Bosmer, in Morrowind, Chimer and Dwemer, in Cyrodiil, Ayleid, a mix between Nedic and Aldmer birthed the Bretons. The Orsimer or Orcs were also created at this time, Altmer warped by the destruction of their leader Trinimac, who it is said became the Daedric Prince Malacath. This disparate chorus may have been crafted consciously by the Daedra or the shifts of the earthbones.