There is much confusion about how Christians ought to interpret the book of Genesis. The choices boil down to:
1) literal interpretation.
2) figurative / symbolic / allegorical interpretation.
3) a mixture of both.
Does either the Bible or Catholicism require, for example, the serpent in Genesis to be taken absolutely literally in all respects? The answer is no.
International Standard Bible Encyclopedia (Protestant) states in its article, “Serpent”:
4. Figurative: Most of the Biblical references to serpents are of a figurative nature, and they usually imply poisonous qualities. The wicked (Psalms 58:4), the persecutor (Psalms 140:3), and the enemy (Jeremiah 8:17) are likened to venomous serpents. The effects of wine are compared to the bites of serpents (Proverbs 23:32). Satan is a serpent (Genesis 3; Revelation 12:9; 20:2). The term “offspring of vipers” is applied by John the Baptist to the Pharisees and Sadducees (Matthew 3:7) or to the multitudes (Luke 3:7) who came to hear him; and by Jesus to the scribes and Pharisees (Matthew 12:34; 23:33).
Cardinal Ratzinger (later Pope Benedict XVI), in his book, 'In the Beginning . . .': A Catholic Understanding of the Story of Creation and the Fall (Huntington, Indiana: Our Sunday Visitor, 1990, from the 1986 German), wrote:
[T]he Bible is not a natural science textbook, nor does it intend to be such . . . One must distinguish between the form of portrayal and the content that is portrayed. The form would have been chosen from what was understandable at the time -- from the images that surrounded the people who lived then . . . (pp.13-14)
Hence, when he writes later about the serpent and the fall of man (Genesis 3), he uses words like “image” and “symbol”:
[T]he image of the serpent . . . is taken from the eastern fertility cults. These fertility religions were severe temptations for Israel for centuries, tempting it to abandon the covenant and to enter into the religious milieu of the time. . . .
[T]he serpent was a symbol of that wisdom which rules the world . . . the serpent also serves as a symbol of the attraction that these religions exerted over Israel in contrast to the mystery of the God of the covenant.
It is with Israel’s temptation in mind that Holy Scripture portrays Adam’s temptation and, in general, the nature of temptation and sin in every age. (pp. 82-83)
Many biblical skeptics (I've debated scores of them online) have incorrect ideas of biblical interpretation and only a dim appreciation of biblical genre and the ancient Middle Eastern culture that produced it.
Many atheists were raised as fundamentalists, and retain their rather simplistic, hyper-literalistic readings of the Bible, that invariably miss key and essential aspects of any given passage.
Christians aren't required to believe in “talking snakes” any more than they must believe in a literal tree and fruit in the same passage. They are required to believe in the temptation and original sin / fall of man (i.e., rebellion against God) that the story poetically describes.
The events in early Genesis and how they are presented is not necessarily in a literal fashion (that is, literal original sin can be portrayed symbolically or poetically).
I’m not denying that Adam and Eve were literal. They were. The fall was made possible by free will. God allowed the possibility of human beings rejecting Him. We thought we could make it on our own, hence, the rebellion.
The serpent, of course, represents Satan (and angel/spirit), who had already rebelled against God, and was trying to bring down the human race with him. He probably recognized already how stupidly and idiotically he had acted, and so wanted to blame God. Misery loves company.
In one of my debates online, my opponent stated:
You do believe in Adam and Eve being the first two humans despite the impossibility of fitting the genetic diversity of our species into two people and additionally, that at least one snake at the time was able to speak, despite all snakes (both fossilized and modern) not having any indications of vocal chords or a cerebral cortex that contains a broca’s area.
Because many assume hyper-literalism as the only way to interpret the Bible, they can only imagine a process of looking in the fossils for a snake that had vocal cords. They overlook the many types of literary genre in the Bible.
Venerable Pope Pius XII, in his encyclical Humani Generis (1950), dealt with the issue of literal and symbolic biblical interpretation, with regard to Genesis:
38. . . . there are those who boldly transgress the limits and safeguards established by the Church. . . . the first eleven chapters of Genesis, although properly speaking not conforming to the historical method used by the best Greek and Latin writers or by competent authors of our time, do nevertheless pertain to history in a true sense, which however must be further studied and determined by exegetes; the same chapters, . . . in simple and metaphorical language adapted to the mentality of a people but little cultured, both state the principal truths which are fundamental for our salvation, and also give a popular description of the origin of the human race and the chosen people. If, however, the ancient sacred writers have taken anything from popular narrations (and this may be conceded), it must never be forgotten that they did so with the help of divine inspiration, . . .
For a solid philosophical / scientific defense of monogenism (all human descent from one primal pair), see, “Science, Theology, and Monogenesis,” by orthodox Catholic philosopher Kenneth W. Kemp. See also, “Modern biology and original sin” (+ Part Two), by Dr. Edward Feser (also a Catholic philosopher).
The choices in biblical interpretation of portions of Scripture such as early Genesis are not only hyper-literalism or pure symbolism. A third choice is that real events are presented with some non-literal symbolism.