Regarding Proto Celtic, Colin McEvedy's new "Penguin Atlas of Ancient
History" has been recently updated and republished.
He suggests that Proto-Indo-European is to be associated with the
Eastern Tardenosian mesolithic cultures of Central and Eastern
Europe. Thus he argues that Starecevo/Karanovo culture of the
Balkans, the first culture to introduce a Neolithic culture into
continental Europe, would have also been PIE. From this it would
appear that Proto-Celto-Italic is to be associated with Danubian 1
(LBK) and the cultures (eg Michelsberg and Vinca) that developed from
it. This he would argue becomes the "Western" Branch of Indo-
European, from which Celtic, Italic and Illyrian languages
developed. Anatolian languages then develop out of the Gumelnitsa
cultural base, which developed out of Karanovo, while the Eastern
Group of Indo-European develops from Tripolyte and the Srendy Stog,
and North Caspian eoneolithic of the steppes. Eastern PIE then he
argues developes from the Pit Grave and Battle Axe cultures.