“They thrice aspired to the supreme command in Asia; while they themselves remained always either unmolested or unconquered by any foreign power. Darius, king of the Persians, they forced to quit Scythia in disgraceful flight. They slew Cyrus with his whole army. They cut off in like manner Zopyrion, a general of Alexander the Great, with all his forces. Of the arms of the Romans they have heard, but never felt them. They founded the Parthian and Bactrian powers. They are a nation hardy in toils and warfare; their strength of body is extraordinary; they take possession of nothing which they fear to lose, and covet, when they are conquerors, nothing but glory.” – Historian Justin, “Epitome of Pompeius Trogus”
“This level and fertile area was occupied by the Scythians, who remain today troublesome neighbours, with settlements both in Europe and Asia. Those living across the Bosphorus [Cimmerican Bosphorus, Kerch Strait] are considered Asiatic, the Europeans being those whose territory extends from the west of Thrace to the river Borysthenes [Dnieper] and from there in a straight line to the Tanais [Don]. the river which separates Europe and Asia. There is no doubt that the Scythians, the ancestors of the Parthians, made their incursions from the area of Europe, not from the Bosphorus.” – Quintus Curtius Rufus, “History of Alexander the Great”
The statements above appear to have been confirmed by modern-day methods such as DNA testing. Here is Yamnaya Culture Wikipedia page quote:
“According to Unterländer et al. (2017), all Iron Age Scythian Steppe nomads can best be described as a mixture of Yamnaya-related ancestry and an East Asian-related component, which most closely corresponds to the modern North Siberian Nganasan people of the lower Yenisey River, to varying degrees, but generally higher among Eastern Scythians.”
Yamnaya culture is the focus of a 2018 book Who we are and how we got here by a Professor in the Department of Genetics at the Harvard Medical School David Reich. The latest technological advances in genomics provided invaluable insights into the mysteries and secrets of the past:
[Dr. Reich] has argued that the genetic data supports the likelihood that the people of the Yamnaya culture were a “single, genetically coherent group” who were responsible for spreading many Indo-European languages. Reich also argues that the genetic evidence shows that Yamnaya society was an oligarchy dominated by a small number of elite males.
“Royal Scythia, Greece, Kyiv Rus” has more insights into the topic.
Newly published “Cradle of Civilizations” has even more unique facts on the topic.