From a letter to the president of the Council for Polish-Jewish relations:

The creation of the human being differed from the creation of other living species. Among other things, the human being was created as a single individual, unlike other living creatures created in pairs.

One of the reasons, our sages declare, is that it was G‑d’s design that humans everywhere and in all times should know that each and all descend from the one and the same single progenitor—a fully developed human being created in the image of G‑d—so that no human being could claim superior ancestral origin, and this would make it easier to cultivate a true sense of kinship in all interhuman relationships.

Letter to the president of the Council for Polish-Jewish Relations, Office of the President of the Republic of Poland, October 23, 1991. The source of the concept expressed is in Sanhedrin 37a and Tosefta Sanhedrin 8:3.