It is fair to say that the discovery of chimpanzees using tools in the wild (van Lawick-Goodall 1968) was one of the most significant discoveries in field primatology and perhaps in animal behavior as a whole. This finding not only questioned the long-held assumption that using and manufacturing tools were the hallmark of humanity. but also reinvigorated the study of cognitive processes in apes. I say reinvigorated because tool use had been used by psychologists to investigate ape cognition long before its discovery in the wild.
Both Köhler's and Yerkes' pioneering work in the 1920s on ape problem solving drew heavily on tool use. Yerkes saw tool use as an example of ideation, thinking in the absence of the actual stimuli, whereas Köhler saw it as an example of finding indirect solutions when the direct route was not possible. Köhler emphasized the insightful nature of several tool-use episodes, and he contrasted such a sudden mode of solution with the gradual trial-and-error learning that Thorndike had described with his puzzle boxes. Historically, this is the forerunner of a debate between association (Le, apes respond to the presence of certain observable cues but without any real causal understanding) and reasoning that continues unresolved today