Wikipedia
Proto-Arabic is the name given to the hypothetical reconstructed ancestor of all the varieties of Arabic attested since the 9th century BC. Evidence of Arabic becomes more frequent in the 2nd century BC with the attestation of Arabic names in the Nabataean script, as well as evidence of an Arabic substratum in the Nabataean language. The Safaitic and Hismaic inscriptions, composed between the 1st century BC and the 4th century AD in the basalt desert of northwest Arabia and the southern Levant, are also crucial to the reconstruction of Proto-Arabic since they exhibit many features in common with epigraphic Old Arabic and Classical Arabic which set them apart from languages attested further south such as Dadanitic and Taymanitic (see Characteristics below). Old Arabic written in the Nabataean script is first attested in the Negev desert in the 1st century BC, but becomes more frequent in the region after the decline of Safaitic and Hismaic. From the 4th century AD onwards, Old Arabic inscriptions are attested from Northern Syria to the Hijaz, in a script intermediate between cursive Nabataean and the Kufic script of Islamic times.
The urheimat of Proto-Arabic can thus be regarded as the frontier between northwest Arabia and the southern Levant.