Life on our planet has changed constantly over millions of years. The odd thing is: extinction is an essential part of the process.
Some new species evolve as others die out, but the turnover is usually small. We can make a fairly good guess at how often a species appears and disappears by
studying the Fossil Record. It seems most species last for around 5 million years before they become extinct.
That spectacularly well-adapted predator, the Shark, may have been around for almost 100 million years. We have found fossilized Great White Shark teeth dated to 65 million years ago.
