Our research seeks to uncover the psychological and linguistic factors that give rise to the structure of natural human language, within individuals and across a language community. Our primary objective is to discover the abilities all humans apply to acquire and use language, by capturing how those abilities create and shape language over generations, particularly when language exposure is initially incomplete. The recent emergence of a new sign language among Deaf children and adolescents in Nicaragua provides an opportunity to examine how grammatical systems develop without a pre- existing, complete language model. Over the past four decades, deaf Nicaraguans have been creating a new sign language. An initial cohort started with the gestures available in their environment and began to shape them into a new, grammatically complex system. Nicaraguan Sign Language (NSL) has continued to develop and change ever since, as new cohorts of children enter the community and learn the language from older peers. Now with archived video data of NSL going back two decades, and comparable data from other sign languages, we have the opportunity to tease apart certain factors that led to the developments we have observed in NSL. Accordingly, our research aims first, to characterize the influence of learners on the emerging linguistic structure of NSL. We will systematically compare sequential age cohorts, at multiple points in time, from earlier in NSL’s development (from our archives) to present-day, focusing on the word order and spatial devices for indicating who-does-what-to-whom, and the creation of categorical expressions for spatial relations and semantic transparency in the lexicon. We will also test whether NSL changed diachronically as predicted by biases in the motor system and acuity constraints in the visual system, by comparing lexical signs and narratives collected across decades of NSL, and comparing NSL to matching data in our archive from the older British Sign Language (BSL). In the process, we will continue to build and share the archive of video and written documentation of NSL. This growing archive is the first and only recording of a language from its origin. We are making use of it in the present grant, and are sharing it with other research laboratories carrying out other work, such as testing models of community, language growth and change.