“Evolution is change in heritable traits of biological populations over successive generations.” (Evolution - Wikipedia)
These small changes in animals or plants eventually leads to new species. These changes have been taking place since the first complex molecule was able to reproduce itself and therefore be considered by some to be ‘alive’.
"Since the publication of Charles Darwin's On the Origin of Species in 1859, the attitude of the Catholic Church on the theory of evolution has slowly been refined. Early contributions to the development of evolutionary theory were made by Catholic scientists such as Jean-Baptiste Lamarck and the Augustinian monk Gregor Mendel. For nearly a century, the papacy offered no authoritative pronouncement on Darwin's theories. In the 1950 encyclical Humani generis, Pope Pius XII confirmed that there is no intrinsic conflict between Christianity and the theory of evolution, provided that Christians believe that the individual soul is a direct creation by God and not the product of purely material forces." (Catholic Church and evolution - Wikipedia)
The Catholic Church has no authority to declare scientific hypotheses as truth, but some of its teachings do have implications for science. To better understand this let's have a look at what Pope Pius XII actually wrote:
"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, on the part of men experienced in both fields, 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 - for the Catholic faith obliges us to hold that souls are immediately created by God." (Pius XII, Humani Generis 1950: #36)
What this quote makes clear is that anyone is free to believe in evolution since the Church does "not forbid" this provided that Catholics believe that "souls are immediately created by God." The Church can never teach evolution since that is not it's purpose. But it would reject any 'science' that would dispute that 'souls are immediately created by God' since not only is that not in line with its teachings, but it is not science! Science can never teach about the soul since there are no experiments that can be done on souls. You can't use science to prove or disprove the human soul. It is a matter for belief alone. Therefore, there is no contradiction between science and Catholicism since they are complementary to each other. Let us explore the importance of the Catholic teaching that 'souls are immediately created by God'.
You were personally created by God. As pope Benedict teaches:
“We are not some casual and meaningless product of evolution. Each of us is the result of a thought of God. Each of us is willed, each of us is loved, each of us is necessary.”
Pope Benedict XVI
But even the process of bringing about the human body is one planned by God since God set the universe in motion and destined it to bring about humanity.
The Catholic Church officially has no problem in people believing in evolution. The highest level of Catholic teaching states, "The Teaching Authority of the Church does not forbid ... 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 - for the Catholic faith obliges us to hold that souls are immediately created by God." (Pius XII, Humani Generis 1950: #36). Catholics are free to believe in evolution, provided they believe that God personally creates every individual soul. As pope Benedict XVI says, "We are not some casual and meaningless product of evolution. Each of us is the result of a thought of God. Each of us is willed, each of us is loved, each of us is necessary." Science and Catholicism are totally compatible. Science can't explore the reality of the soul because it is beyond science. Catholicism has no expertise on the development of species. The two work together to give us a complete picture of reality.