Rodolfo R. Llinas (author of I of the Vortex: From Neurons to Self) claims that language simpliciter, but particularly human language, "arose as an extension of premotor conditions, namely, those of the increasing complexities of intentionality as abstract thinking grew richer" (242). Llinas defines intentionality as "the premotor detail of the desired result of movement through which a particular emotional state is expressed: the choice of what to do before the doing of it" (228). Notice that intentionality, as opposed to being primarily mentalistic is associated with "a motor representation of what is happening inside our heads" (ibid).
Intentionality expressed in premotor activity adumbrates motor patterns (according to Llinas). He insists that language arose because premotor activity increasingly grew more complex while abstract cerebration became richer. Llinas thus provides the following account to explain the origins of abstract thinking: "Abstraction, or the collection of neural processes that generate abstraction, is a fundamental principle of nervous system function." In other words, abstraction is a natural feature of biological organisms that are equipped with neural machinery and nervous systems like we possess.
However we explain abstract thought, I believe it can be accounted for naturalistically. I have read most of Jerome M. Adler's book on the subject--as a colleague suggested I do--but it seems that anytime we're talking about linguistic phenomena, we're dealing with things that possibly yield to naturalistic explanations ex toto.
On the other hand, by "naturalistic explanations," I do not mean to exclude God (Jehovah). It just appears that a soul is unnecesary to explain speech and conceptual thought.