well if you want to get anal about it
we can say there are no races on earth because if the out of africa theory is to be believed then we all emigrated from Africa. where do you draw the line of a different race? you can't honestly believe that in all of East Asia (China, Mongolia, Korea, Japan, Taiwan, etc) that they are the same race.
30,000 years ago is a long time, those original Jōmon people would have been on the island isolated (due to the land bridge melting) long enough to have developed their own features and into their own race. Hell even the Ainu who are in the northern part of Japan managed to stay isolated enough from the inhabitants of the rest of Japan to develop their own race.
i was just providing an example, most people would see how that would lead to a different race but it seems you won't unless you see scientific evidence. such evidence i am sure you can find yourself if you do a bit of research.
i have no doubt in my mind if you do some research you will find that anthropologists agree that the japanese are a seperace race from the chinese. then as both the han chinese and the japanese races are the majority of the two countries in population you can possibly understand how we are able to mostly, not perfectly, identify which country people come from due to their appearance.
all the best ^^