Critics of evolution often point to a perceived lack of transitional forms in the fossil record as evidence that some forms of life don't share a common genetic heritage but rather arose independently. Creationists have been using this line of argument for over a hundred years. But, they do so in spite of the evidence. Intermediate forms are those organisms that have a mosaic of characteristics linking two seemingly unrelated groups or an organism that exhibits a character state in between those found in two or more other organisms. Creationists claim these forms are lacking in the fossil record. However, the transition between theropod dinosaurs and modern birds, the evolution of the mammalian ear bones from a reptilian jaw, the evolution of whales from terrestrial mammals and even the fossil record for our own species are all classic examples of evolutionary transitions complete with several intermediate forms.
Add to the growing list of evolutionary transitions another example from the fossil record. Vertebrate animals have a bilateral body plan, that is they exhibit a front and back and dividing the body down the middle results in two symmetrical sides. There are few exceptions to the symmetrical vertebrate body plan. One striking exception familiar to us all are the flatfishes (Order: Pleuronectiformes). This group includes fish found at your local market or seafood restaurant including sole and halibut. Adult flatfishes are asymmetrical. They start off life with the typical symmetrical body found in other fishes however as they grow their skull undergoes a radical developmental change with the eyes rotating to one side of their head. A flatfish on the sea bed is therefore a fish essentially lying on it's side with an asymmetrical head. Imagine lying on your left side with both your eyes on the right half of your face and you'll get the picture.
The question is how did this unusual body plan evolve? There are clear benefits to exploiting an open ecological niche and becoming specialized to be a bottom feeder but doing so can mean a radical change in an organism's body plan. The flatfish body plan clearly evolved from a typical symmetrical fish plan. Comparing the details of flatfish anatomy and their genes shows that they fit within the large radiation of bony fishes, nearly all of which have a symmetrical body plan. Also, the turbots (Psettodes sp.) are the living representatives of the earliest branch of the flatfish family tree and it, as expected if flatfish evolved from a symmetrical ancestor, they have one eye that doesn't quite make it all the way around the head during development. What's more, the adult asymmetrical flatfish plan develops from a symmetrical larval body plan. Together all these data indicate that the asymmetrical flatfish plan evolved from a symmetrical ancestral body plan. However, as creationists are fond to ask, where are the intermediates?
Along comes University of Chicago evolutionary biologist Matt Friedman. In the July 10th edition of the journal Nature Friedman provides evidence for the evolutionary transition between symmetrical bony fishes and the asymmetrical flatfishes. Friedman provides highly detailed descriptions of fossil fishes in the genus Amphistium and describes a new species, Heteronectes chaneti. These fossils were available to researchers before, however, Friedman applied computed tomography to the specimens to obtain detailed three-dimensional images of their anatomy. Computed tomography involves moving an x-ray source and detector around a specimen and digitally reconstructing a detailed three-dimensional image from the x-ray exposures. Previously it was difficult to tell an asymmetrical flatfish-style skull from a symmetrical skull crushed by the weight of sediment during fossilization. With the new imaging technology Freidman concluded that these fossil fish do indeed show different intermediate stages leading to the fully asymmetrical modern flatfish body plan. Specifically, the part of the skull containing the orbits, the neurocrania, rotates over evolutionary time. A close relative of the flatfishes, Trachinotus, shows the classic symmetrical condition with one eye on either side of the head. The fossil Amphistium and Heteronectes show an intermediate stage with one eye squarely on one side of the head and the other eye on the other side of the head but shifted up towards the top of the head. The earliest branch on the flatfish family tree, the turbots, one eye rests nearly on top of the head and in the rest of the flatfish the eye has migrated fully to the other side of the head.
New findings are popping up all the time from fields ranging from paleontology to developmental genetics that affirm the conclusion that life shares a common ancestry and that these unusual body plans, such as the asymmetrical head of a flatfish, have arisen through evolutionary processes.
Friedman, M. (2008). The evolutionary origin of flatfish asymmetry. Nature, 454(7201), 209-212. DOI: 10.1038/nature07108