That's one reason I mentioned the fossil record. It's one thing to say there may be some population of modern organisms unknown to science--after all, wild animals are skittish and don't approach researchers. Bones, though--they tend to stay where they're put. If bigfoot came from somewhere, it had to do so at some time. And life through time is my domain. If bigfoot were real, it would leave a trace--we have too many lagerstatten (and if Chris wishes to dispute this, I invite him to list the lagerstatten; that alone will demonstrate his competance in this field, as most only know the big one) for there to not be ape remains were bigfoot real.
We know what intelligent hominids look like in the fossil record. "Peopling of North America" outlines the lines of evidence that demonstrate it. We do not see it before Homo sapiens sapiens. Therefore not only do bigfoot believers expect us to believe that something exists without physical evidence--they also expect us to believe that something existed outside of time.
Who is John Galt?