I've been puzzling over how to better answer this question.
it's an infuriating question because once you understand the answer it's obvious and simple but it's a perennial - it just won't go away. The reason it won't is because evolution is not understood by most people and ignorant religionists repeat this supposedly slam dunk question over and over.
I read on a thread on this site what I thought was a really good start to an answer that can be understood by anyone - I just wanted to flesh it out and develop the full picture. (If this has already been done somewhere, just point me at it and I'll go away.)
This is what I have so far:
The first step is to put out of your mind the fact that you are alive but your great-great grandmother is dead. That's not the explanation; it's the cause of the confusion.
Instead have a go at this mind game.
Close your eyes and imagine that you are standing face to face with a chimpanzee (The chimp is one of our closest relatives.)
Now imagine that with your left hand you are holding the hand of your mother and that your mother is holding the hand of her mother and so on for thousands of generations back into the past.
Imagine the chimp is doing the same but with its right hand.
You now have an imaginary parallel line of people and chimps holding hands backwards in time - like a railway track with people and chimps lining each side.
You can now walk down the centre of these lines and look carefully at your mother's family line and the chimp's family line going back millions of years.
So what would our walker between the lines actually see? Can we tell the story back in time to the common ancestor in simple language but in enough detail for the walker to be able to research further?