I wouldn't say Nords or Redguards, Imperials and Bretons are the mongrel races of the man side.
Nords are pure human and so are redguards since they were the first humans to arrive on Tamriel, but other races beat them to it obviously, was it the Beast folk that were native to Tamriel or was it the Alyieds?
To answer about the first races on Tamriel.
The latest archaeological study, using divination and sorcery, of the Direnni Tower on the Isle of Balfiera in High Rock has dated its construction at around ME 2500, making it the oldest known structure in Tamriel. The earliest known date in Aldmeri history.
The beast races did live on the land in the early Merethic Era.