A lone man was created in the world, lest families contend with one another. There is a Kal v'Chomer - if now that [all] people comes from one progenitor, [families] contend with one another, if they were descended from two progenitors, all the more so [they would quarrel even more]!

Also, it is lest Tzadikim say "we descend from a Tzadik, and you descend from Resha'im."

(Mishnah): To tell the greatness of the King of kings, Ha'Kadosh Baruch Hu...

This teaches the greatness of the King of kings, that through one stamp, he makes all molds, and one does not resemble another, for it says "Tis'hafech k'Chomer Chosam."

Why don't their faces look the same? This is lest a man skip and go to his friend's wife or his friend's field.

(Beraisa): They said in the name of R. Meir, there are three things that Hash-m made different in people - facial appearance, mindset and voice. Facial appearance and mindset are due to thieves (lest one be able to pretend to be someone else, or know where he hid his valuables), and voice is due to Ervah (at night people cannot see, so they rely on voice to know that it is their spouse).

(R. Yitzchak): Even figs and wheat, one does not resemble another.