Humani Generis is a Papal Encyclical that was promulgated by Pope Pius XII on August 12, 1950. The full text is posted @ http://www.ewtn.com/library/encyc/p12human.htm.
A Papal Encyclical is a letter written by a Pope to either all of the Bishops in a specific country or to all of the Bishops in the world which is intended to guide the Bishops in their relations with their flocks. Encyclicals generally deal with some issue that is causing problems for the faithful. In the case of Humani Generis, the issue was evolution.
Recently, our friend Frankly the Nut gifted me with two books about Young Earth Creationism (YEC), which is:
the religious belief that the Universe, Earth, and all life on Earth were created by direct acts of the Abrahamic God during a relatively short period, sometime between 5,700 and 10,000 years ago. Its primary adherents are Christians and Jews who believe that God created the Earth in six 24-hour days, taking a literal interpretation of the Genesis creation narrative as a basis for their beliefs. http://en.wikipedia.org/wiki/Young_Earth_creationism
I have had virtually no contact with this set of beliefs, having been raised Catholic in a very liberal Ivy League community. I was taught the Catholic position long ago, but undertook today to digest the original source (Humani Generis). If you’re interested, my version of the meaty bits is below.
The theory of evolution deals with much that neither directly nor indirectly touches faith or morals, and which consequently the Church leaves to the free discussion of experts.
However, whatever new truth the sincere human mind is able to find cannot be opposed to truth already acquired, since God, the highest Truth, has created and guides the human intellect.
So while the Church does not forbid research and discussion of evolution, it requires that opinions favorable and unfavorable to the theory be weighed and judged with seriousness, moderation and measure.
The following issues are not up for debate:
Souls are immediately created by God.
Adam is the father of all true men, through natural generation as our first parent.
As to the interpretation of the first eleven chapters of Genesis:
They state truths which are fundamental to our salvation.
They were written with the help of divine inspiration, through which they were rendered immune from any error, so must never be considered on a par with other ancient myths or folk tales which have survived through the ages.
They were written in a metaphorical language adapted to the mentality of a simple people.
Although they conform neither to the historical method used by the best of the ancient Greek and Latin writers nor to that used by by competent authors of our time, nevertheless they do pertain to history in a true sense whose exact nature must be further studied and determined by exegetes.