Rereading Beresheet: אדם ואדמה - Adam & Adama

The court tells the witnesses: Therefore, Adam the first man was created alone, to teach you that with regard to anyone who destroys one soul from the Jewish people, i.e., kills one Jew, the verse ascribes him blame as if he destroyed an entire world, as Adam was one person, from whom the population of an entire world came forth. And conversely, anyone who sustains one soul from the Jewish people, the verse ascribes him credit as if he sustained an entire world.

The mishna cites another reason Adam the first man was created alone: And this was done due to the importance of maintaining peace among people, so that one person will not say to another: My father, i.e., progenitor, is greater than your father. And it was also so that the heretics who believe in multiple gods will not say: There are many authorities in Heaven, and each created a different person. And this serves to tell of the greatness of the Holy One, Blessed be He, as when a person stamps several coins with one seal, they are all similar to each other. But the supreme King of kings, the Holy One, Blessed be He, stamped all people with the seal of Adam the first man, as all of them are his offspring, and not one of them is similar to another. Therefore, since all humanity descends from one person, each and every person is obligated to say: The world was created for me, as one person can be the source of all humanity, and recognize the significance of his actions.

The baraita continues: Alternatively, he was created alone due to the families, so that the families will not quarrel with each other, each one boasting of the heritage of their progenitor. And if now that Adam was created alone, families still quarrel and each family claims superiority, if there were two people created initially, all the more so would they do this. Alternatively, he was created alone due to the robbers and due to those who take by force that which is not theirs, as the feeling of fraternity among all people, having descended from the same forefather, will limit crime. And if now that Adam was created alone, criminals still rob and take by force that which is not theirs, if there were two people created initially, all the more so would this be the case. The mishna teaches: And this serves to tell of the greatness of the Holy One, Blessed be He, as when a person stamps several coins with one seal, they are all similar to each other. But the supreme King of kings, the Holy One, Blessed be He, stamped all people with the seal of Adam the first man, as all are his offspring, and not one of them is similar to another. The Sages taught in a baraita (Tosefta 8:5): The fact that Adam the first man was created alone serves to declare the greatness of the supreme King of kings, the Holy One, Blessed be He, as a person stamps several coins with one seal, and they are all similar to each other. But the Holy One, Blessed be He, stamps all people with the seal of Adam the first man, and not one of them is similar to another....

The Sages taught in a baraita (Tosefta 8:7): Adam the first man was created on Shabbat eve at the close of the six days of Creation. And for what reason was this so? So that the heretics will not be able to say that the Holy One, Blessed be He, had a partner, i.e., Adam, in the acts of Creation. Alternatively, he was created on Shabbat eve so that if a person becomes haughty, God can say to him: The mosquito preceded you in the acts of Creation, as you were created last. Alternatively, he was created on Shabbat eve in order that he enter into the mitzva of observing Shabbat immediately.