Etymological dictionaries of Proto-Indo-European (PIE) such as Mallory-Adams1 reconstruct two words for 'head', namely *ɣeβōl- (Tocharian, Greek, Germanic2) and *kapōl-o- (Old English hafola, Sanskrit kapá:la-), which show an alternation between voiced (traditional voiced aspirated) and voiceless stops3.
I consider these words to be loanwords from the languages spoken by Neolithic European farmers, akin to Semitic *gVbVl- 'mountain; boundary, border' (e.g. Arabic ʒabal 'mountain'), with a straightforward semantic shift. The Semitic word is in turn derived from Afrasian *gVbVl- 'bank, side' (Militarev), also reflected in Egyptian and Western Chadic and possibly related to *gab- 'side, bank; beach' (HSED 856).
1 J.P. Mallory & Q.D. Adams (2006): The Oxford Introduction to PIE and the PIE world, p. 174.
2 Although the Germanic meaning is 'gable'.
3 Although pairs like this (e.g. *gab- ~ *kap- 'to take') are by no way uncommon, they aren't explained in the framework of mainstream IE studies.