1) I have been somewhat removed from the scientific community in the recent past. I apologize for knowledge deficits that may arise from this.
2) This post makes many distinctions, which may be tedious for some readers, but should be helpful to genuine inquirers. The distinctions are necessary to give adequate treatment to the subject and attempt to be fair to all parties involved.
Evolution refers to a process by which more complex systems arise from less complex parts or systems over a period of time. With regard to origins - What do I come from? - it means that all complex or advanced species came from less advanced species. Any particular theory of evolution must answer further questions: How does change come about? What drives these changes? Is it guided or blind chance? Different answers to these fundamental questions give rise to different theories of evolution. The Catholic Church does not outright object to all theories of evolution; although, the Catholic faith would necessarily reject some particular theories of evolution based on the answers to the further questions noted above.
In a letter to the Pontifical Academy of Sciences in 1996, Pope John Paul II went so far as to say: "Today, more than a half-century after the appearance of that encyclical, some new
findings lead us toward the recognition of evolution as more than an
I'd mentioned that there are different theories of evolution. The most common theory of evolution holds that change is introduced by random genetic mutations, which are then selected based upon the organism's ability to adapt to its environment. Some scientists assert that such a process is blind and has no need of a Designer or Creator. Given that many genetic mutations result in loss of function of the coded protein, some scientists have proposed that successful genetic mutations are guided by an act of intelligent design. I do not have sufficient scientific knowledge to weigh the merits of each position. For now note that both theories accept genetic change as what drives evolution, but differ on how such change comes about. The first position holds that such change is random and blind; whereas, the second proposes an outside process that guides the genetic change. The former rejects the possibility of a Creator, the latter admits it. The former is irreconcilable with the Christian faith; the latter is not. The Catholic Church necessarily rejects the theory of evolution that insists upon blind chance. It is unlikely that the Catholic Church will ever pronounce definitively between the latter theory of evolution or a literal six day Creation seeing it as the proper province of science to answer between them.
That leads to a further difficulty: If science suggests that the period of time was longer than six days, which contradicts a literal reading of the first chapter of Genesis, does that invalidate the authority of the entire Bible as a witness to truth? This difficulty fails to consider two points about Catholic Christianity. Catholics have never considered the Scriptures to be the only authority. Catholic Biblical hermeneutics does not require a fundamentalist, literal interpretation of Genesis 1. I take each in turn in part 2.