One hundred and eighty-one years ago, on January 12, 1836, HMS Beagle entered Sydney Harbour with the 26-year old Charles Darwin on board. Sydney was just one of many ports of call for the Beagle on its five-year round-the-world surveying voyage.
While he was in Australia, Charles Darwin saw animals that lead him to think about how animals came to be and natural selection and thus began his thinking and work that lead to his Theory of Evolution.
Talking about this anniversary on Charles Darwin's birthday with students always elicits much discussion and there are always requests for books so they can read more.
These three books are a good start but there are many picture books on evolution that are easy enough for eight year olds to begin an interest. Also see here for two other other books.