The Pope decided it was okay. that's about it.
So here's the reference that I found on the evolution issue from www.catholic.com:
http://www.catholic.com/library/adam_eve_and_evolution.asp
It seems that Pope Pius XII sanctioned it(Pius XII, Humani Generis 36).
Here's the citation:
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)
My undergraduate degree in Cellular and Molecular Biology gives me a peek into the probability of mankind arising from evolutionary processes. If you take the time to do the math, the probability is ZERO. I know that there are plenty of secular AND 'christian' academics and scientists who adhere to the theory of evolution, but not this soldier.