The issue of signal reliability (‘honesty’) is widely recognised in language evolution research as one of the most fundamental problems concerning the evolutionary emergence of protolanguage, i.e. early language-like communication. We propose that nonverbal communication is likely to have played an important but underestimated role in language evolution: not directly in the transfer of message contents, but rather in stabilising the emerging protolanguage. We single out one subset of nonverbal cues - nonvocal nonverbal paralinguistic adaptors (NNPAs) - based on their role as indicators of reliability in present-day communication of humans. We suggest that the relatively involuntary and therefore reliable NNPAs might have served to stabilise more volitionally controlled, and therefore less reliable, verbal communication at the initial, bootstrapping stages of its phylogenetic development.