Meaning: "a race of one"

This phrase is found in "Stranger From a Strange Land" (Robert Heinlein).

ONCE UPON A TIME when the world was young there was a Martian named Smith.
Valentine Michael Smith was as real as taxes but he was a race of one.

It is found in a different context here.

We are all welcome in Your fold.
We are all a flock of humble lambs
Every women, kid, and man.
Dear God, please bless your children all,
A race of one, both big and small.

The following dictionaries remain mute: Cambridge, Wiktionary, MacMillan, Lexico.

What is the meaning of "race of one", or more generaly of "of one"?