Most all contemporary thought about the biological foundations of language is based on the core assumption that language and speech are neurologically privileged from birth. There is, however, a flaw with this assumption: Given that only languages utilizing the speech modality are studied, it is, a priori, impossible to find evidence that would do anything but support this hypothesis. Only when a modality other than speech is analyzed can any generalization about the brain’s predisposition for speech and language be evaluated. Petitto’s research has been directed at understanding the biological foundations of human language, and it has included important comparative analyses of spoken and signed languages. Studies of signed languages offer an especially clear window into the biological foundations of all of human language. On the one hand, signed and spoken languages utilize different modalities (signed languages are perceived through sight and expressed primarily with the hands as well as other non-manual markers, spoken languages are perceived through sound and expressed primarily with the tongue). Yet on the other hand, signed and spoken languages possess identical linguistic structures that are universal to all languages (phonological, morphological, syntactic, semantic, discourse, and pragmatic levels of language organization). This profound similarity lets us tease apart those aspects of human language that are properties of the modality of language expression/reception, and those that form the core parts of language structure and its organization in the human brain. For example, those aspects of natural language that get pushed out onto both the hands and the tongue teach us about the core nature of all of human language in all brains. Those aspects of language structure that differ, teach us how modality (be it sign or speech) can impact certain “surface” aspects of language expression (for example, whether it has a “linear” organization, as can be seen in spoken languages, or a “simultaneous” organization, as can be seen in signed languages). Despite rather stunning modality differences, Petitto and colleagues have found that the identical brain tissue is involved in the processing of syntax, morphology, and, most fascinatingly, phonology, across signed and spoken languages. To be sure, brain tissue assumed to be uni-modal sound processing tissue for the past 125 years and associated with the processing of phonology (i.e., the Superior Temporal Gyrus, STG), has recently been shown by Petitto and colleagues to be also involved in the processing of phonology in natural signed languages. Moreover, despite the modality differences, signed and spoken languages are acquired by young children being exposed to signed languages on the identical developmental / maturational timetable as children acquiring spoken languages. The existence of natural signed languages, then, permits us to test prevailing assumptions about what is important about language in our species. Is it simply sound and speaking that makes human language special? Or, is it the presence of highly specific underlying patterns that is most key (i.e., patterns universal to the structure of natural language)? Signed languages have taught us that sound and speaking are not what makes human language special. Instead, it is the presence of identical underlying patterns unique to natural language structure that render human language special (be they signed or spoken languages). The existence of natural signed languages force us to “decouple” speech from language, and have caused an exciting revolution in our understanding of the essence of human language in our species. See also scientific rationale provided at this site in Brain Imaging Studies of Language Processing In Deaf & Hearing People.