Where did it start? How did it grow complex? Why did it get big? Three moments in Earth’s early history when evolution suddenly, inexplicably turned up the dial.

© Photoillustration by Will Warasila for The New York Times

We asked 100 scientists about unsolved problems in their fields. They sent back provocations — on the origins of life, the possibilities for human progress and much more.

© Photo illustration by Jane Stockdale for The New York Times