He spoke _________ such assurance that we couldn't but believe him.

Human and primates, the family of apes, gorillas, and chimpanzees, among others, divide many common traits.
While primates are deemed the most intelligent of animals, most researchers believed they lack the capacity to produce language. However, a research project in the 1970s at University of Georgia showed promise that chimpanzees have the ability to learn a certain language, just as human children do.
The project used several chimpanzees as test subjects in which Lana, a female chimp was the study focus.
Though the primates lack the vocal constructions to make human speech patterns, the researchers created a language called Yerkish, using lexigram made up of symbols that represent sounds and words. 125 symbols were placed on a keyboard, which Lana was taught how to use the board to communicate with the researchers. She successfully expressed her thoughts by pressing different keys in succession. In some cases, she used up to seven at times.