Proto Indo European (Ringe 2009).

"As is usual with protolanguages of the distant past we can't say with
certainty where and when PIE was spoken; a reasonable guess will be
the river valleys of Ukraine in the centuries around 4000 B.C, though
one can't absolutely exclude a somewhat earlier date, and nor a place
somewhat further east. The best discussion of the "IE homeland
problem" is still Mallory 1989 (this is pathetic); it is cautious and
not fully conclusive, as is reasonable under the circumstance (Ringe
2009, p.2, parenthesis added).

Ringe, D. (2009). From proto indo European to proto Germanic: a
linguistic history of English volume I. Oxford University Press.
ISBN-13: 978-0199552290 (Paperback).

M. Kelkar