In Sumerian language there are traces of an older languages from which Sumerians took words related to agriculture and civilised society. What do we know about this older people?
The idea of a pre-Sumerian substrate is an outdated and now debunked theory, and most non-Sumerian words have been identified as Semitic or Hurrian loanwords. For more on this, see my response to Mesopotamians and Sumerians.