In early stages of this shift, the adults will start out poorly adapted to the new food. But there’s so much for them to eat that they still get a decent meal.

“I like the concept — I like that they tried to look for the ultimate cause,” said Joanna Wolfe, a postdoctoral researcher at Harvard. But she wondered if food is the only reward that can help drive the evolution of metamorphosis.

Some species might benefit in other ways. Adults might take on bodies that allowed them to find mates more successfully, for instance. Larvae in the ocean might change their forms in order to be carried far away by the currents, expanding their range.

“I would like to see some things added to their model,” she said.

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Dr. ten Brink agreed that the new study is a foundation for more detailed ones. “This paper is really the start of something,” she said.

If animals so rarely evolve metamorphosis, why is it so common? One reason may be that once metamorphosis arises, it’s very hard for a species to lose it.

It’s easy enough to imagine a situation where giving up metamorphosis would be a benefit. Imagine an outbreak wiping out the food that adults eat. For the species, it would be advantageous for individuals to remain larval and survive on what food remains.