impossible isn't required. it ISN'T impossible for Bigfoot to exist. (it's not like the description is 300 feet tall, 500 tons, made of cheese).
All that's required is that there is no verifiable evidence for the existence of bigfoot in the fossil record, nor at the present date, bringing the probability of it existing down to 0%.
All that's required is that there is no verifiable evidence for the existence of bigfoot in the fossil record, nor at the present date, bringing the probability of it existing down to 0%.