The arrival of the different races in the first era has fascinated many scholars but without direct knowledge from the event it is hard to piece together the truth.
However with the advance of magic recently and the common use of planar travel some groups of scholars have drawn together a possible explanation with some of the origins being more plausible and others more tentative.
Seem to come from Pale I(the Chaoswild) as so many of their traits are so similar to the beings from there. Thus they seem to have their origin most clearly defined.
Likely to come from few near earth semi elemental planes as Ashen deep seems to have inhabitants very similar to dwarves and few other planes in the deep earth planar cluster have beings close to gnomes.
The dragonkind clearly come from near semi elemental planes. The strange thing being mostly how so many different planes have produced so similar creatures. In any case the actual home planes of most of the types of dragons and dragonkind have been identified with fairly high certainty.
Are most likely from pale II originally as many creatures there are very similar and the Grey orcs are a recent arrival with practically no differences to a common creature on pale II and are still colorblind.
The Original homes of most can be seen on the low gravity near elemental planesthat have adapted to the higher gravity by growing more muscles.
Pose a large problem in placing the home, as Humans or near humans live on huge number of primes, in way more planes than any other race, but none of the so far surveyed ones seem to be the orginal home.
Most of the intelligent undead seem to have counterparts in the Graveyard planar cluster, thus indicating likely source there.
Present a problem to the classiers as nothing similar to them has been found as native population anywhere and they exhibit traits similar to Humans, fey and dwarves. Currently there is no theory that is widely accepted about their origin.