Twins have played a prominent role in the creation stories we have read. They are either significantly similar, or completely opposite; comparable to the aspects of fraternal and identical. For example, Erebos (darkness) and Nyx (night), Hemera (day) and Aether (brightness), Anshar (sky) and Kishar (earth), Hsi Wang Mu (yin) and Dongwanggong (yang), Hunahpu and Xbalanque, etc.
In mythology, twins are portrayed as a multitude of things: lovers, friends, rivals, and heroes, etc.
But what I am wondering is why were people so fascinated by them? And why were they often associated to traits of similarity and/or opposition?
Was it because of their appearance, or their profound ability to understand and/or relate to each other?
I am not really sure myself; maybe it is because the function of genetics was still a mystery to ancient peoples.
In any case, I believe that the way in which twins have been portrayed in mythology has greatly contributed to the interest researchers have in them today.