I extracted these lines from the Elder Scrolls' wiki:

The peoples we call "Imperials" are a mix of indigenous populations and of Nordic settlers.

Late Merethic Era - Ysgramor's fleet lands at Hsaarik Head at the extreme northern tip of Skyrim's Broken Cape.
1E 68 — The last Nedic ship from Atmora arrives in Tamriel.
Alessia was a human slave.
1E 242 — Alessia rebels against Cyrodiil.

I've got some innocent questions:

So, between the "Late Merethic Era" and 242 the Nords mixed with other indigenous population conceiving the Imperials (Nebenese and Colovians)?
I don't understand very well how these semi-human races developed in such a short time. Who were these human slaves if not the Nords?
Is the answer hidden in the misterious history of the Nedic people? http://www.uesp.net/wiki/Lore:Nede

Humans were already living in Cyrodiil at the time the Nords came to Tamriel. The Nords mixed with the native populations mostly after the Rebellion, since prior to it they stuck mainly to their northern lands.

As I understand it, the Nords originated on Tamriel as well in the Dawn Era and only later left for Atmora; hence the Nordic myths about High Hrothgar being the seat of the creation of man, and the fact that Ysgramor's expedition to Tamriel is recorded in the "Song of the Return".