Christians (of the fundamentalist stripe that hold to a literal interpretation of the Bible) believe that all of life's diversity came about after the Noachian Flood four thousand years ago
Which is outrageously anti-scientific. It is literally impossible for evolution to happen that fast and contradicts everything we know about the history of life on earth.
The real question is: How did life originate in the first place?
Why is this "the real question"?
Christians who take a literalist view of scripture are a small minority - they tend to think they are the only true christians. The fact is that millions of christians accept biological evolution unequivocally and see no conflict with their faith. Francis Collins and Kenneth Miller are two good examples of christians who are also mainstream evolutionary biologists.