"If mankind is to be ruled, let man be the model." Under this premise, they were created with a mind and body structure similar to humans. However, there was a birth defect. The True Ancestors bore a flaw, the vampiric impulse. Probably, it's because the original that was used as a reference had contained that flaw as well.

Tsukihime Dokuhon PlusPeriod - TSUKIHIME Dictionary: True Ancestor