here's my question: in the pre-history of Islam, Ethiopian/Axumite civilisation looms quite large: there's definitely a lot of interchange between these groups. Arabic then solidifies in part through large conquests of other places (most notably Persia)... is there any way we could trace the existence of a group speaking a common language (or having the dreaded 'mutual intelligibility') between Ethiopic and Arabic in the 6th century (AD/CE)? What would this do to Islamic studies?

I know it's a historian's question (and a rather essentialist one, at that) rather than a linguist's; I'm more interested in whether there are types of evidence other than archival documents, trade records, scriptures, that a linguist would use.