Three of the most recent popes: John Paul II, Benedict XVI, and Francis, made statements about how they support and agree with the Theory of Evolution.
Yet, there are still Catholics who would support the literal story of creation as told in the Genesis. So there are issues that it’s okay for a Catholic to believe what is contrary to the teachings of Church fathers and elders?
Now, the church also repeatedly stress that human life begins at conception.
As a Catholic, is it okay to believe that human life begins at some other stage? e.g., when twinning is no longer possible, quickening, or when brain waves are first detected?
The Church has no teaching on evolution. It is a matter of science, not theology.

Evolution and Creation are not mutually exclusive terms.
Is it okay to believe that life begins at some point after conception? The answer is no. There is no question, from both a scientific and a religious point of view that human life begins at conception.
I hope that you are not referring to the permissibility of the deliberate destruction of innocent human life. There has been some theological debate in the distant past about when human life began but the Church has always forbidden abortion as murder. See the Didache, from apostolic times. Modern science, however, shows without a doubt that human life begins at conception.

The Doctrine of the Church teaches that “every
spiritual soul is created immediately by God” (CCC 366, in the context of the procreative action of parents).
It also teaches (as Doctrine) “From its conception, the child has the right to life. Direct abortion, that is, abortion willed as an end or as a means, is a “criminal” practice (GS 27 § 3), gravely contrary to the moral law. The Church imposes the canonical penalty of excommunication for this crime against human life” (CCC 2322).
CCC 2271-2272 state (as Doctrine), "Since the first century the Church has affirmed the moral evil of every procured abortion. This teaching has not changed and remains unchangeable. Direct abortion, that is to say, abortion willed either as an end or a means, is gravely contrary to the moral law:
You shall not kill the embryo by abortion and shall not cause the newborn to perish.
God, the Lord of life, has entrusted to men the noble mission of safeguarding life, and men must carry it out in a manner worthy of themselves. Life must be protected with the utmost care from the moment of conception: abortion and infanticide are abominable crimes.
Formal cooperation in an abortion constitutes a grave offense. The Church attaches the canonical penalty of excommunication to this crime against human life. “A person who procures a completed abortion incurs excommunication latae sententiae,” “by the very commission of the offense,” and subject to the conditions provided by Canon Law. The Church does not thereby intend to restrict the scope of mercy. Rather, she makes clear the gravity of the crime committed, the irreparable harm done to the innocent who is put to death, as well as to the parents and the whole of society."
scborromeo.org/ccc.htm
Abortion is a matter of Doctrine. The beginning of life is a matter of both Doctrine and Science. Comparing evolution and abortion is like comparing the theory of relativity with a mathematical equation.