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If you’re ever tempted to take the notion of intelligent design seriously, consider the flounder.

Or the halibut, or the sole (Dover, rex, or petrale), or the small but tasty sanddab. Any member of the Order Pleuronectiformes will do. All living species of flatfish start out life as normal, bilaterally symmetrical bony fish, with one eye on each side of their bodies. Then as they mature, one eye migrates around to join its partner. The side with the eyes becomes the top side; the eyeless side, on which the fish rests on the substrate, the bottom. Sometimes, as in the sanddabs, both eyes wind up on the fish’s left side; sometimes, as in typical soles, flounders, turbots, and the Pacific halibut, on the right. California halibuts swing both ways. In some species the skin of the eyeless side loses its pigment, becoming fish-belly white; the eyed side has a camouflaging pattern that can change to match its surroundings. 

Now, if you were the Supreme Being, is this how you would go about creating a flatfish? What a Rube Goldberg solution! It’s even worse than the famous thumb of the giant panda, which is actually a repurposed wrist bone; the pseudodigit doesn’t move into place after the panda is born. 

It’s not as if this were the only design option, anyway. Before flounders and their ilk made their appearance, as indicated by the fossil record, there was already a perfectly functional flatfish model in the form of the skates and rays. As Richard Dawkins wrote in The Blind Watchmaker: “They are like sharks that have passed under a steam roller, but they remain symmetrical and ‘the right way up.’” When a bat ray is born, it already has a clearly defined top side, where both eyes are, and a bottom side, where its mouth is. Granted, the arrangement prevents it from seeing what it’s eating, but it has other senses that compensate for this handicap. And it’s worked well enough to last more than 100 million years. 

None of which prevented some of Charles Darwin’s contemporaries from using flatfish as an argument against natural selection. “…if the transit was gradual, then how such transit of one eye a minute fraction of the journey towards the other side of the head could benefit the individual is far from clear,” wrote St. George Jackson Mivart in 1871. “It seems, even, that such an incipient transformation must rather have been injurious.” 

Fortunately, flatfish have a decent fossil history, and the evolutionary transition from bilateral symmetry to their present state can be reconstructed—just like the transition from primitive hoofed mammals to whales, or from dinosaurs to birds. 

The most recent piece of evidence came to light in a museum collection in Vienna, in a slab of limestone that formed 50 million years ago when a warm sea covered what is now northern Italy. This, 15 million years after the demise of the dinosaurs, was a time when the bony fish were undergoing serious diversification. Oxford researcher Matt Friedman discovered that the slab contained the well-preserved remains of a flatfish in the making. The fish, which he named Heteronectes (“different swimmer”), had a flattened body like a modern pleuronectid, but with one eye on each side like a conventional fish. 

This was not Friedman’s first encounter with a fossil protoflatfish. 

(Much of what follows is borrowed from an article by Carl Zimmer in the journal Evolution Education Outreach.) Working on his dissertation at the University of Chicago in 2008, he was struck by an illustration of an odd-looking fish called Amphistium, also about 50 million years old. Friedman tracked down an actual specimen and used a CT scanner to reveal anatomical details obscured by the rock matrix. Amphistium had an eye on each side, but one was higher on the fish’s head than the other. As he examined still more specimens, he became convinced that these were adult fish, not juvenile flatfish preserve with the wandering eye in transit. Then came the discovery of Heteronectes, a member of a still earlier flatfish lineage. 

Friedman speculates that the ancestors of Heteronectes had a tendency to lie flat on the seafloor, propping themselves up with their downward-facing fins. They could still see with the downward-facing eye, but not well. Mutations that moved the eye even slightly toward the top of the head were favored by natural selection, eventually leading to the modern two-eyes-on-one-side model. 

Dawkins again, writing 25 years before either of those transitional forms turned up: “The whole skull of a bony flatfish contains the twisted and distorted evidence of its origins. Its very imperfection is powerful testimony of its ancient history, a history of step-by-step change rather than of deliberate design…[E]volution never starts from a clean drawing board. It has to start from what is already there.”