In the following discussion I'm drawing primarily on the Hordelands boxed set, Old Empires, and Empires of the Sands as source material, but I use other sources as well.
(I'm looking at Faerun, i.e the western Realms, only. There were obviously various eastern Asian and Central American influxes as well.)
There may or may not have been an "original" race of humans. Regardless, there were numerous waves of immigration of humans from other worlds. Based on the deities worshipped in the Realms, we can conclude that there were cultures that resembled the following:
Sumerian (and Babylonian) (the Untheric pantheons)and possibly others.
Egyptian (the Mulhorandi pantheons)
Norse (Tyr)
Finnish (Mielikki)
Celtic (Oghma)
Greek (Tyche)
According to Old Empires, there are three racial groups: Mulan, Turami and Tethen. The Mulan were the slaves of the Imarski empire and are descibed as sallow-colored, the Turami were the original "barbarian" inhabitants of modern day Mulhorand who interbred with with the Mulan to varying degrees (less so in Unther) or were pushed west. The Tethens were "everyone else." Based on the additional information elsewhere, this is probably a gross generalization by the peoples of the Old Empires who viewed themselves as the center of the world.
According to the Hordelands (one of the cardstocks), both Mulhorand and Raumathar were created by refugees from the Imarski empire. Both countries spoke a derivative of Rauric, as the t mpire language was known.
According to Empires of the Sands, ancient Calimshan was inhabited by a race of people known as the Djen from the Elemental Plane of Air. Their language survives in its descendant Alzhedo. Their culture survives in the quasi-Arabian feel of Calimshan. Perhaps they were migrants from Zakhara, we shall probably never know.
Modern Calimshan is inhabited by the majority Calishite race (lets call them Calim) who are shorter than average, with brown skin. They also include a race with a large amount of Tethyr blood (lets call them Tethen using the Mulhorandi term) who are taller and more far-skinned. Two other races are the ebony-skinned traders of Chult (lets call them Chultan) and the long-faced, slightly yellow-tinted men of the Shaar (lets call them Shaaran).
In addition, unnamed sources have revealed that the country of Turmish is inhabited by mahogany-skinned humans, while the Vilhon Reach (particularly Chondath) is inhabited by an olive-skinned people.
Finally, Daughter of the Drow discusses a Viking people, known as the Rus who were active on the Sword Coast. Some of their descendants made it as far as modern-day Rashemen.
So what are the ancient cultures? From what I've seen, the most ancient known human cultures included the following:
There are probably at least five or six general races of humans in Faerun today. Each is an amalagamation of a variety of quasi-Earth cultures brought by immigrants from other worlds.
The Mulan are a mixture of the original Imarski (source unknown), the quasi-Egyptian immigrants, and the quasi-Sumerian immigrants who seem to have evolved a quasi-Babylonian culture in the Realms. Their first civilizations of note were probably Mulhorand, Unther, and Raumanthar. Surviving civilizations in which they are dominant include Mulhorand, Unther, and Thay. Civilizations in which they are partial contributors include Chessenta and perhaps parts of the Vilhon Reach or the Eastern Shaar.
The Turami, came in a separate wave of quasi-Turkish immigrants but don't seem to have developed a civilization in the Realms until exposed to the Mulan. Their first civilization of note was probably Turmish, which survives today, although they were prime contributors to Mulhorand, and less-so to Unther.
The Tethens are the descendents of several small waves of immigrants from a variety of quasi-northern European cultures. Their first civilizations seems to have been the Rus and the Ffolk, although they may have given birth to Netheril. It seems more likely that the Netherese brought their culture intact from another world, and simply physically resembled the Rus. The Tethens/Netherese seem to have spawned a wide variety of short-lived cultures in the North and their descendants inhabit the North, the western Heartlands, the eastern Heartlands, the Vast, Impiltur, the Bloodstone Lands, Amn and Tethyr today. The Netherese culture in particular survives in distant Halruaa. Possibly the Tethens founded Lantan and Nimbral, or perhaps the other way around, as the inhabitants of those lands are described as extremely pale-skinned, and seem physically similar to the Tethens.
The Chultans are the descendants of one or more waves of quasi-African cultures into Chult, and their descendants founded Mezro after a great war in which the __ culture destroyed the __ culture. Aside from Mezro, it seems likely they colonized much of the lands surrounding the Shining Sea at one time and their descendants survive in Calimshan and interbred with other races in Tharsult and Lapaillya (sp?).
The fifth major racial group is the the Shaarans about which almost nothing is known at this time. It seems likely that they interbred with the Turami and Mulan to make up the population of the Vilhon Reach, but aside from Chondath, it is not known what cultures they have created.
A sixth neo-human race was the Djen, who seem to have interbred with the Tethens and Chultans of southwestern Faerun to form the Calim.
Finally, other lands have had some influence on the racial make-up of the Realms. Loosely these include the Maztican, Kara-Turan, and Zakharan peoples
So we can speculate on the primary racial composition of the prominent lands of Faerun as follows in order of importance:
The Shining Plains | Turami, Shaaran, Tethen |
Turmish | Turami |
The Vilhon Reach | Turami, Mulan, Shaaran |
Chessenta | Mulan, Turami, Tethen |
Unther | Mulan, Turami |
Mulhorand | Mulan, Turami |
Murghom | Mulan |
Semphar | Mulan |
Thay | Mulan, Tethen |
Hordelands | Kara-Turan, Mulan |
Thesk | Mulan, Kara-Turan, Tethen |
Rashemen | Tethen |
Narfell | Tethen? |
Vaasa & Damara | Tethen |
Aglarond & Coastal Cities | Mulan, Tethen |
Impiltur | Tethen |
The Vast | Tethen |
The Moonsea | Tethen |
The Dalelands | Tethen |
Anauroch | Tethen (strong Netherese influence) |
Sembia | Tethen, [Turami, Mulan, Shaaran] |
Cormyr | Tethen |
Dragon Coast | Tethen, Shaaran |
Western Heartlands | Tethen |
The North | Tethen (strong Netherese influence) |
The Islands off the Sword Coast | Tethen |
Mooshaes | Tethen (strong Ffolk influence) |
Amn | Tethen (recent Maztican influx) |
Tethyr | Tethen |
Calimshan | Calim, Chultan, Tethen, Shaaran |
The Lake of Steam | Calim, Chultan, Tethen, Shaaran |
The Border Kingdoms | Shaaran, Tethen, Calim, Chultan |
The Shaar | Shaaran? |
Chult | Chultan (recent Tethen influx) |
Tharsult | Chultan, Shaaran, Calim |
Lapaillya | Shaaran, Chultan, Calim |
Halruaa | Tethen (strong Netherese influence) |
Dambrath | Shaaran?, Drow, Elvish |
Durpar, Estagund, and Ulgarth | Mulan, Zakharan |