My dad used to make this argument: of all the millions of footprints that humans made, after arriving in North America, what is the likelihood that we found the very first of those footprints?<p>The point is, whatever evidence we find, it's unreasonable to think that evidence we've found is the first evidence that existed, so we have to assume the real arrival was a bit before that. When we had evidence of humans arriving 17,000 years ago, it was reasonable to assume humans really arrived 21,000 years ago. When we have evidence of people arriving 23,000 years ago, you have to assume people really arrived 25,000 years ago.<p>There may come a point, centuries from now, when our evidence feels comprehensive, at which point the error estimate can shrink. But modern archeology is barely 100 years old and most subject areas are still under-studied, so error estimates need to remain large, for now, especially for the prehistoric era. (Obviously error estimates are much smaller for the historic era, where we have a relative abundance of evidence.)