Great unknowns Huge questions remain unanswered about life on Earth. We asked leading scientists and conservationists: what is the one thing you would like to know about the planet that remains a mystery?

How many species are there on Earth?

“ How many species of animals do we share the planet with? Estimates range from 3 million to as many as 100 million, and there’s not much sign that we are converging on an answer.”

Prof Andy Purvis is a researcher at the Natural History Museum

I’d go back 540m years to see the ‘biological big bang’

“As an evolutionary biologist, I would love a time machine to go back to the Cambrian explosion [when most major animal