The Catholic position concerning belief or unbelief in evolution may never be finally settled, but there are definite parameters to what is acceptable Catholic belief.
Concerning cosmological evolution, the Church has infallibly defined that the universe was specially created out of nothing. Vatican I solemnly defined that everyone must "confess the world and all things which are contained in it, both spiritual and material, as regards their whole substance, have been produced by God from nothing" (Canons on God the Creator of All Things, canon 5).
The Church does not have an official position on whether the stars, nebulae, and planets we see today were created at that time or whether they developed over time (for example, in the aftermath of the Big Bang that modern cosmologists discuss). However, the Church would maintain that, if the stars and planets did develop over time, this still ultimately must be attributed to God and his plan, for Scripture records: "By the word of the Lord the heavens were made, and all their host [stars, nebulae, planets] by the breath of his mouth" (Ps. 33:6).
Concerning biological evolution, the Church does not have an official position on whether various life forms developed over the course of time. However, it says that, if they did develop, then they did so under the impetus and guidance of God, and their ultimate creation must be ascribed to him.
Concerning human evolution, the Church has a more definite teaching. It allows for the possibility that man’s body developed from previous biological forms, under God’s guidance, but it insists on the special creation of his soul. Pope Pius XII declared that "the teaching authority of the Church does not forbid that, in conformity with the present state of human sciences and sacred theology, research and discussions . . . take place with regard to the doctrine of evolution, in as far as it inquires into the origin of the human body as coming from pre-existent and living matter—[but] the Catholic faith obliges us to hold that souls are immediately created by God" (Pius XII, Humani Generis 36). So whether the human body was specially created or developed, we are required to hold as a matter of Catholic faith that the human soul is specially created; it did not evolve, and it is not inherited from our parents, as our bodies are.
While the Church permits belief in either special creation or developmental creation on certain questions, it in no circumstances permits belief in atheistic evolution.