From The Gates of Europe: A History of Ukraine, by Serhii Plokhy (Basic Books, 2017), Kindle pp. 39-41:
Ovid’s contemporary Strabo, author of the acclaimed Geographies, knew more about the Pontic steppe than did the famous Roman exile. From Strabo we learn the names of the Sarmatian tribes and the areas under their control. According to him, the Iazyges and Roxolani were “wagon dwellers,” or nomads, but the famous geographer gives us literally nothing about the sedentary population of the forest-steppe areas around the Dnieper, not to mention the wooded areas farther to the north. Unlike Ovid, however, he did not live among the peoples of the region; nor were his sources as good as those of Herodotus. They knew nothing about the “northerners,” and Strabo complained about the ignorance that prevailed “in regard to the rest of the peoples that come next in order in the north; for I know neither the Bastarnae, nor the Sauromatae, nor, in a word, any of the peoples who dwell above the Pontus, nor how far distant they are from the Atlantic Sea, nor whether their countries border upon it.”
Strabo’s informants came from one of the colonies, but if Herodotus made numerous references to the Dnieper, Strabo seemed more familiar with the Don. His sources likely came from Tanais, a Greek colony at the mouth of the Don that belonged to the Bosporan Kingdom, the most powerful union of Greek colonies revived with the arrival of the Romans. For Strabo, the Don had a special meaning. It served as the easternmost boundary of Europe, the term used in the Aegean homeland to describe the expanse of the Greek presence in the outer world. Europe lay to the west of the Don, while Asia began to the east of it.
Thus, at the beginning of the first millennium AD, when the Romans came to the Pontic colonies, the Ukrainian territories found themselves once again at the very edge of what would become Western civilization. The northern frontier of the Hellenic world had now become the eastern boundary of Europe. There it would remain for almost two thousand years, until the rise of the Russian Empire in the eighteenth century redrew the map of Europe, moving its eastern boundary all the way to the Urals.
The division of the Pontic steppes into European and Asian parts did not mean much in the time of the Romans. Strabo wrote about the Sarmatians on both the left and right banks of the Don, and Ptolemy, one of his successors, wrote in the second century AD about two Sarmatias, one European, the other Asian—a division that would remain constant in the works of European geographers for another millennium and a half. More important than the imagined eastern boundary of Europe was the real civilizational frontier between the Mediterranean colonies on the northern shore of the Black Sea and the nomads of the Pontic steppes. Unlike the Greek colonies with their surrounding fortifications, that frontier was never set in stone, creating instead a broad zone of interaction between colonists and locals in which languages, religions, and cultures intermixed, producing new cultural and social realities.
The all-important boundary between the steppe nomads and the agriculturalists of the forest-steppe areas that was known to Herodotus became invisible for Strabo. Whether it disappeared altogether or Mediterranean writers simply did not know about it is hard to say. Geography and ecology stayed the same, while the population probably did not. It certainly refused to stay put in the middle of the first millennium AD, when we next encounter references to the region in the writings of learned Greeks.