Are the Han people who originated in the Central Plains double eyelids or single eyelids?

From the beginning, there were both single eyelids and double eyelids, and the latter mainly depended on heredity. But I really can't think of any evolutionary value of small eyes with single eyelids ... (so much so that I have to force the aesthetics with single eyelids now). At best, swollen eyelids in cold areas can only keep the eyes warm (the question is whether all single eyelids are swollen eyelids, and single eyelids with thin eyelids can't do this). Also, they are high latitude, Nordic and the like, and have no swollen eyes.