The Central-Asian steppe has been the home of nomad tribes for centuries. Being nomads, they roamed across the plains, incidentally attacking the urbanized countries to the south, east and west. Although he concentrates on the tribes living in modern Ukraine, which he calls Scythians, we may extrapolate his description to people in Kazakhstan, Turkmenistan, Uzbekistan, Tajikistan, Kyrgyzstan and possibly Mongolia, even though Herodotus usually calls these eastern nomads 'Sacae'. The Chinese called them, at a later stage in history, Sai .
Tribes are, almost by definition, very loose organizations.
Every now and then, new tribal coalitions came into being, and sometimes, new languages became prominent among the nomads from the Central-Asian steppe. The oldest group we know of, is usually called Indo-Iranian. There are no contemporary reports about their migration, but it can be reconstructed from their language. From an archaeological point of view, their migration is attested in the change from the Yamnaya culture into the Andronovo culture. They invaded the country that was later called Afghanistan, where they separated in an Iranian and an Indian branch.