JRRT once stated that the
Fourth Age may have started about 6000 years ago. So any present-day population distributions would be, when looking at history, a poor guide.
Just think of all of the
peoples who have been on the move just in Asia and Europe in the last 2000 years (which would have been about the Sixth Age by JRRT's reckoning, we'd be in about the Seventh). The Celts had been hovering at the edges of the Graeco-Roman "civilized world" or Mediterranean at least 2500 years ago. Germanic tribes started to move about, out of their non-Roman settlement areas at the latest in the fourth century, which also saw the invasion of the Huns. Beginning in the seventh century under the first four Caliphs, the Arab Islamic expansion into North Africa, then Spain, and at least modern Syria and Iraq and some of the former Soviet "Republics". Then the Mongol expansion, which fairly early on became a mixed Mongol-Turkic affair, which reached Europe in the 13th century. Timur in the 14th century did not actually attack Europe, it seems, certainly never got nearly as far west as the Ghengis-Khan and descendants Mongols (and never mind the Huns), but appears to have contributed to the weakening of
the Golden Horde (overlords over Russia) to a degree as to make it easier for the Russians to throw off that yoke. And roughly while the Russians started to expand eastward, the Ottoman Turks did the same westwards (as you probably know at least as well as I do), being brought to a final halt at the second, again unsuccessful siege of Vienna in 1683.
So JRRT might have had quite a bit to choose from for his "
Easterlings". What of all the above was known to him - archaeology and associated historical sciences have made quite a bit of progress since WW II. Until new results are published can take quite some time, though, and until they become the subject of popular science publications even longer.
So for the nearer "Easterlings" east of
Mordor I would guess that they could be equated with Slavic peoples. Further east would mean Mongol, Turkic or a mixture of the two.
Southrons would probably mean mostly North Africans, say Berbers and the like, while "
Far Harad" is identifiable with Sub-Saharan Africa, But all of this is projecting recent conditions (of perhaps the last 1000 years) into a period, 6000 years ago, which was still clearly Neolithic even in Egypt, Mesepotamia, the Indus Valley and China.