After some research we have found that there are numerous theories on this point O0 and significant controversy as well. Some say that ancient decoded pre-sanscrit texts contained the first mark-up in partially recovered document fragments. Others say it was a solely the result of a much more recent group effort :
xml-dev - Re: The Origins of XML

I think the key members were Tim Bray and James Clark. "XML was developed by a Generic SGML Editorial Review Board formed under the auspices of the W3 Consortium in 1996"
-- Extensible Markup Language (XML)

all the best,