A few days ago, my family was wandering the ruins of the Roman Forum. I explained to my daughters that the fragments of pillars around us were very old. Veronica, who is four, wanted to know how old.

They were made before she was born, I explained. Before her sister Charlotte was born.

Before Charlotte was born? she asked.

Actually, before I was born, I said. They were built before I was born, and fell down before I was born.

That last part was a bit too much for her.

Trying to comprehend deep time was actually the reason we were in Rome in the first place. I was invited to give a lecture about mass extinctions, as part of the Rome Festival of