Its explained very clearly and concisely in Ott which I
recommend all Catholics get a copy of (Ott, L. (1957). Fundamentals of Catholic
dogma.)
"The whole human race stems from one single human pair.
(Sent. certa.)
Against the Pre-Adamite Theory (first expounded by the Calvinist
Isaac de la Peyrère, 1655), and the view of certain modern scientists,
according to which the various races are derived from several separated stems
(polygenism), the Church teaches that the first human beings, Adam and Eve, are
the progenitors of the whole human race (monogenism). The teaching of the unity
of the human race is not, indeed, a dogma, but it is a necessary
pre-supposition of the dogma of Original Sin and Redemption. According to a
decision of the Bible Commission, the unity of the human race is to be reckoned
among those facts which affect the foundations of the Christian religion, and
which, on this account, are to be understood in their literal, historical sense
(D 2123). The Encyclical “Humani Generis” of Pius XII (1950) rejects polygenism
on account of its incompatibility with the revealed doctrine of original sin.
(D 3028)."
image from the 'Public domain' via wikimedia commons. Adam and Eve by Cranach, 1533
image from the 'Public domain' via wikimedia commons. Adam and Eve by Cranach, 1533
