How did human language become so structurally complex? This dissertation presents evidence that complex syntactic rules in modern human language emerged via a pre-syntactic stage that was governed by semantic principles. This dissertation investigates the influence of meaning in evolutionarily early language by looking at situations in which people cannot use or learn a language normally. The systems that arise in those situations are called restricted linguistic systems, and examples of such systems are the language of unsupervised adult second language learners and home sign. This dissertation relates observations from restricted linguistic systems to a novel approach taken up in the laboratory, in which participants are asked to communicate about simple events using only gesture and no speech: improvised communication. Together the two branches of evidence constitute a picture of evolutionarily early language in which semantic principles take a central position: they precede and ultimately drive syntactic rules. Because of its interdisciplinary approach, this book targets a wide audience, and it will be relevant to linguists (and non-linguists) interested in meaning, language and evolution.