One of the common challenges thrown at evolutionary theory is the apparent uselessness of intermediate forms. What use is half a wing, for instance, or half an eye? If it doesn’t have any survival benefit, then how could it evolve? Evolution, after all, is blind – there is no plan or purpose. Each organism survives or dies on its own merits (or plain dumb luck).
Most of these challenges have been answered long ago. As Prof David Dutch (University of Wisconsin) points out, half a wing – and less – could make all the difference in the world if it lets you jump an extra centimetre. And half an eye puts you way ahead of blind competitors.
But what about flatfish?
Flatfish make a living lying on the sea floor, peering upwards by virtue of the fact that both eyes are on one side of their head. But what earthly use could it be for some ancestor flatfish to have one eye shifted only part-way round its head? It would, after all, still point into the sand. Just not straight down…
The truth is that no-one knows what use it would be, which has lead some to propose that no such intermediate forms exist or are needed. In this ‘hopeful monster’ argument (first made by the geneticist Robert Goldschmidt in the 1930s), some freak mutation may have generated a fish with two eyes on one side in one step – and this freak managed to put the mutation to good use.
But it turns out that this is not what happened. Today’s Nature carries a report of the discovery of an intermediate flatfish ancestor, with eyes swivelled part way round. Now, flatfish start off symmetrical, and one eye moves round as the fish develops. But these are not immature fish, because they are too big and also the bones are ossified – which only happens after metamorphosis.
Still, what good is a half-formed flatfish? This discovery doesn’t answer that – though Dan Cressey, writing in Nature, suggests
“It’s possible that the asymmetrical eyes may have allowed the creatures to bottom-feed, watching for predators above while lifting themselves up on their fins to look for prey on the sea floor.”
What’s just as remarkable about this new discovery is that it wasn’t made in some fossil bed newly discovered in an exotic location, but rather in the bowels of the Natural History Museum in Vienna (and other museums in England, France and Italy). They were originally dug out of limestone quarries in Northern Italy and underneath modern-day Paris, and have lain in collections ever since – until Matt Friedman, a PhD student studying evolutionary biology at the University of Chicago in Illinois, discovered their significance. According to Matt,
“I suppose there is a general perception that museum collections are dusty, static archives, and that everything in them has been carefully studied and precisely identified. But the truth is that they are much more than just long-term storage, because as our interpretive framework matures, we can begin to make sense of specimens that evaded or baffled earlier generations of researchers, or draw new conclusions about materials we mistakenly thought we had figured out ages ago.”
Makes you wonder how many other missing links are field away in some museum drawer somewhere, just waiting to be discovered!
Friedman M. (2008) The evolutionary origin of flatfish asymmetry. Nature, 454(7201), 209-212. DOI: 10.1038/nature07108
Daniel Cressey. The eyes have it. Published online 9 July 2008 | Nature | doi:10.1038/news.2008.946
University of Chicago Medical Center. “Flatfish Fossils Fill In Evolutionary Missing Link.” ScienceDaily 10 July 2008.
Living the Scientific Life blog:
The mysterious origin of the wandering eye.