The story of the egg spans millions of years, from the first vertebrates that dared to venture onto land to today’s mammals, including the platypus, and of course birds. Like chickens? We’re here to tell you: The egg came first.

For decades, scientists believed dinosaurs were diurnal and tiny mammals were nocturnal. But as researchers have uncovered more mammalian fossils and studied the biology of different dinosaur species, they’ve found some surprising results.