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An intriguing unanswered question about the evolution of bilateral animalswith internal skeletons is how an internal skeleton evolved in the first place.Computational modeling of the development of bilateral symmetric organismssuggests an answer to this question. Our hypothesis is that an internalskeleton may have evolved from a bilaterally symmetric ancestor with anexternal skeleton. By growing the organism inside-out an external skeletonbecomes an internal skeleton. Our hypothesis is supported by a computationaltheory of bilateral symmetry that allows us to model and simulate this process.Inside-out development is achieved by an orientation switch. Given thedevelopment of two bilateral founder cells that generate a bilateral organism,a mutation that reverses the internal mirror orientation of those bilateralfounder cells leads to inside-out development. The new orientation isepigenetically inherited by all progeny. A key insight is that each cellcontained in the newly evolved organism with the internal skeleton developsaccording to the very same downstream developmental control network thatdirects the development of its exoskeletal ancestor. The networks and theirgenomes are are identical, but the interpretation is different because of thecell's inverted orientation. The result is inside-out bilateral symmetricdevelopment generating an inside-out organism with an internal skeleton.

How to Grow an Organism Inside-Out: Evolution of an internal skeleton from an external skeleton in bilateral organisms