You’ll find the schwa in lots and lots of languages, including English (uh…. ; about; and in fact most unstressed vowels of English). You won’t find it in Ancient Greek.
Schwa used to be reconstructed in Proto-Indo-European though, as the phoneme behind the correspondence of i in Indic to a in Greek. For example, pitár ~ patēr was reconstructed as *pəter-. The distribution of the “schwa indogermanicum” was somewhat problematic, and it is now more economically reconstructed as a syllabic laryngeal (*p-h̥₂ter-); it’s plausible that in late Indo-European, the earlier syllabic laryngeal would have been pronounced as a schwa.