Science Puzzle
Which Comes First, the Tadpole or the Frog?
A tadpole grows into a frog. But every tadpole hatched from an egg, and every egg was laid by a frog. So which one came first?
Think carefully about what the question is really asking before you answer.
The Answer
Neither. The question assumes a starting line that does not exist. A life cycle is a loop: egg, tadpole, froglet, adult, and the adult lays more eggs. Ask where a circle begins and you are asking the wrong question.
What makes the frog's loop remarkable is metamorphosis, the total rebuild of the body partway through. The tadpole is a water animal: it breathes with gills, swims with a tail, and grazes on algae. It then grows legs, absorbs its own tail for the nutrients, replaces its gills with lungs, and switches to hunting insects on land. Very few animals demolish and rebuild themselves so thoroughly.
If you push the question back far enough it becomes evolutionary rather than developmental, and then it does have an answer: creatures laying eggs in water existed long before anything we would recognise as a frog. The egg came first by hundreds of millions of years.
The principle: Life cycles and metamorphosis. A life cycle is a closed loop with no starting stage, and some animals reorganise their entire body plan partway around it.