Yuzu, Ichang papeda, and Changsha are believed to be closely related, but it's not sure how they are related. I believe it may not be a direct relationship of one being the simple parent of the other.
In general it is usually viewed that Yuzu is a hybrid of Ichang papeda, but I believe it could also be very possible that the two just share the same two ancestors, with differing degrees of heritage. This postulates some original pure papeda species that no longer is found in existence today, with the Ichang papeda coming closest to resembling that original ancestor in morphology. Yuzu appears closer to Ichang papeda than Changsha is, and it's possible Changsha evolved separately, although no one is completely sure.
Yuzu did originate in China in ancient times before it came to Japan. But like many other plants of Chinese origin, it first came to the West by way of Japan. (Another little example, in America we most commonly refer to Tofu by its Japanese name, though on a menu in a Chinese restaurant it's referred to as bean curd). In 1914, the famous U.S. Department of Agriculture researcher Frank Meyer (for whom the "Meyer" lemon is named after) a plant explorer for the USDA, discovered Yuzu growing wild in the southern part of Gansu province in China, among palms, loquats and bamboo. He estimated that the temperatures in that area dipped to 10 F, and no other cultivated citrus grew nearby. Although he originally named it "Kansu orange", he later realized this citrus was identical to the Japanese Yuzu the USDA already had in their collections.
It's just that in China Yuzu appears to have fallen into obscurity, whereas in Japan it became prevalent and valued for bath fragrance and cuisine. There are many possible for reasons for this, all the civil wars in China, and Japan having a more limited variety of different culinary plant species that could grow. Of course now with China rapidly developing and opening up to the rest of the world, the name "Yuzu" is becoming a little problematic since in China the word is now used to refer to pomelo (and even sometimes grapefruit).