The bizarre metamorphosis that occurs in halibut and other flatfish had even Charles Darwin floundering for an explanation. At birth, these fish have one eye on each side of the skull, but as adults, both eyes reside on the same side. Certainly, for fish that spend their lives along the sea bottom, having both eyes topside confers a survival advantage. But there seemed to be no evolutionary reason to start down the gradual path toward such lopsidedness—any intermediate steps would not seem to be especially helpful. So some biologists theorized that the fish evolved from a single, sudden mutation.

That does not seem to be the case: Matt Friedman of the Field Museum in Chicago reports finding some missing links. He investigated two roughly 50-million-year-old primitive flatfish fossils hidden in museums in Europe for more than a century. These adult specimens possessed somewhat asymmetrical skulls that nonetheless kept eyes on opposite sides of the head. Even incomplete lopsidedness may have given the carnivorous bottom dwellers a better view of the world above than no asymmetry at all, Friedman conjectures. Eye the study in the July 10 Nature.