In the 1830s a German naturalist named Renous was arrested in San Fernando, Chile for heresy. His claim? He could turn caterpillars into butterflies. A few years later, Renous recounted his tale to Charles Darwin, who noted it in The Voyage of the Beagle.
Imprisoning someone for asserting what today qualifies as common knowledge might seem extreme, but metamorphosis—the process through which some animals abruptly transform their bodies after birth—has long inspired misunderstanding and mysticism. People have known since at least the time of ancient Egypt that worms and grubs develop into adult insects, but the evolution of insect metamorphosis remains a genuine biological mystery even today. Some scientists have proposed outlandish origin tales, such as Donald Williamson's idea that butterfly metamorphosis resulted from an ancient and accidental mating between two different species—one that wriggled along ground and one that flitted through the air.

Metamorphosis is a truly bizarre process, but an explanation of its evolution does not require such unsubstantiated theories (for a critique of Williamson's hypothesis, see this study). By combining evidence from the fossil record with studies on insect anatomy and development, biologists have established a plausible narrative about the origin of insect metamorphosis, which they continue to revise as new information surfaces. The earliest insects in Earth's history did not metamorphose; they hatched from eggs, essentially as miniature adults. Between 280 million and 300 million years ago, however, some insects began to mature a little differently—they hatched in forms that neither looked nor behaved like their adult versions. This shift proved remarkably beneficial: young and old insects were no longer competing for the same resources. Metamorphosis was so successful that, today, as many as 65 percent of all animal species on the planet are metamorphosing insects.