Evolution? Shhh, it's a secret

Keep it to yourself, but apparently there's this thing called evolution...

Out of the blue (which is where all good questions come from) Tilly asks:

T: where do baby sharks come from?

H: they come from mummy and daddy sharks.

T: yes, but where do mummy sharks come from?

H: well, you know how millions and millions of years ago there weren't any people on the earth.  There weren't even any dinosaurs.  There was probably something small that lived in the sea.  And after lots and lots of time, one of those things that lived in the sea got a bit cleverer and started to live on the land.  Do you remember us talking about that?

T: yes, and that's how we got monkeys.

H: that's right - some of the things from the sea changed and became animals that live on the land, and some of them changed into whales and dolphins and sharks.

T: I know.

H: all these changes that happened over millions and billions of years, we call it evolution.

T: [pause for thought] I know!  Let's not tell Reuben about evolution.  Let's keep it a secret!