Answer: kurgans (burial mounds) of the Eurasian steppes. The hypothesis suggests that the Indo-Europeans, a nomadic culture of the Pontic-Caspian steppe (now part of Eastern Ukraine and Southern Russia), expanded in several waves during the 3rd millennium BC.
Step-by-step explanation:
The most widely accepted proposal about the location of the Proto-Indo-European homeland is the steppe hypothesis, which puts the archaic, early and late PIE homeland in the Pontic–Caspian steppe around 4000 BC. The leading competitor is the Anatolian hypothesis, which puts it in Anatolia around 8000 BC.