New genetic study on the origin of the Slavs:

The Proto-Balto-Slavs originated from a mix of Bell Beaker, Corded Ware, Baltic Hunter-Gatherer, & Ukrainian Neolithic populations.

The more westerly PBS populations of eastern central Europe were replaced by Scandinavians via the Gothic expansion into Poland, Ukraine, & Hungary, which began during the Iron Age.

The Goths were later replaced by Slavic populations, presumably from the forest-steppe zone of southern Belarus or northern Ukraine. This likely occurred during the Migration Era, when many Goths fled westward to escape the Huns.

These Slavs descended primarily from the more easterly PBS populations, but carried increased EEF & CHG ancestry, likely due to mixing with the Dacians/Thracians of southwestern Ukraine. The Bronze Age & Roman-era populations of Poland did not contribute substantial ancestry.

Note that the paper specifies "Slavic-period genepool"; it's entirely possible that the earliest Slavs were genetically indistinguishable from Balts.