Biodiversity, or biological diversity, refers to the number and variety of species present on Earth. Think of biodiversity as a big party, where species are the guests. Today, we know of around 2 million guests (species) at this party, but there might be many more (between 2 and 10 million) who we haven't met yet. It's like there are rooms in the party venue we haven't even explored yet!

For our tiny, single-celled friends, the prokaryotes (bacteria and archaea), it's even more challenging to guess their numbers due to their complexity and diversity.

The story of life on Earth has been dramatic! Fossil evidence is our "time machine" that helps us understand past biodiversity levels. It shows a roller-coaster ride of species numbers, with peaks and troughs corresponding to different times in history.

The past has seen five major "blackout periods" or mass extinctions when a considerable number of species disappeared. Think of these as unexpected and disastrous party crashers who forced many guests to leave.

The last one happened 66 million years ago, caused by a massive asteroid. This catastrophic event caused the departure of all non-avian (non-bird-like) dinosaurs, amongst others.

The previous four extinctions were due to large-scale volcanic activity and substantial changes in the atmosphere and climate – imagine the party venue catching fire or freezing over!