Ancestral origin

Why was it normal for a cousin to marry a cousin in ancient times, and why did it rarely give birth to mentally handicapped children when close relatives married?
Marriage of close relatives River Ancestral origin
01-08
Among the Three Emperor Saints, it is no wonder that Fu Xi and Shennong enthusiastically support the fengshen gods, and you see who zuyuan surnamed Jiang is
Fuxi Yan Surname Jiang Ancestral origin The Yellow Emperor Western Zhou Lü Yue King Wu of Zhou Yandi Shennong Jiang Ziya
01-07