The Little Fish That Could

Eusthenopteron

I have often been teased for my habit of carrying a science book wherever I go. (“That’s such a Brian thing,” an acquaintance once remarked.) If I am going to be waiting for someone or have a few minutes to spare here or there I like to have something to read to fill up the time. It’s either that or fiddle around with Tetris on my cell phone. Some people have told me that this manifestation of bibliophilia makes me seem antisocial,* but I cannot break the habit. I have been toting around science stuff wherever I go from a very young age.

*[My favorite instance was when I was told to “Make myself comfortable” at someone’s home and was later berated for choosing to read while everyone else was milling about before lunch.]

It will come as no surprise that when I was very young I loved all things prehistoric, and one of my favorite things was a set of “Prehistoric Monsters” cards. Each card had a photograph of a model of some prehistoric animal and provided some vital statistics about the creature. I am not sure where they are now (probably in a box with all the other trading cards I amassed), but I recall them fondly, and one of the cards featured a very curious fish. Called Eusthenopteron, it was depicted as a fish that had hauled itself out of a drying pond, much like a mudskipper. (See illustration to the left.)

For most of my childhood Eusthenopteron was the icon of the evolutionary transition between fish and the earliest terrestrial vertebrates. This poor little fish, its watery home evaporated by the scorching heat, had to walk across the searing sand in the hope of finding another patch of dampness in the burning wasteland. It was this quest that gave rise to our earliest land-dwelling ancestors; if it were not for the bravery of Eusthenopteron we would not have evolved.

Today we know differently. Most of the defining features of terrestrial vertebrates, like arms, legs, wrists, fingers, &c. evolved in the water first. Creatures like Tiktaalik, Panderichthys, and Acanthostega (among many others) have helped us to better understand this transition. Eusthenopteron is still included in the transition, but as a fully-aquatic fish that represents the type of vertebrate that gave rise to “fishapods” like Tiktaalik and in turn the earliest tetrapods (four-legged vertebrates).

Indeed, scientists have learned quite a bit about the evolution of the earliest tetrapods during my lifetime, and while our present understanding is much more complete I still have a soft spot for depictions of poor little Eusthenopteron pulling itself across the drying mudflats. Where had this imagery of Eusthenopteron as an evolutionary underdog come from?

The idea that the first tetrapods evolved from fish trying to find ponds in a drying world is most often associated with the great 20th century paleontologist A.S. Romer. It was Romer’s work that helped popularize this “Drying Pond Hypothesis” from the 1950’s onward, but as historian Peter Bowler has demonstrated this idea had much deeper roots. Richard Swan Lull’s 1917 textbook Organic Evolution provides an earlier example of the same idea.

At the time Lull was writing there were several competing hypotheses for why fish had become adapted to life on land. Perhaps fish evolved into the first tetrapods to escape predators, exploit new food resources on land, take in greater amounts of oxygen from the air, or survive the climate changes that were causing their shallow, freshwater homes to dry up. Lull favored the last of these ideas, likening the conditions the ancestors of early tetrapods faced to the dry seasons of Africa in which fish trapped in isolated polls must either go into a kind of hibernation (like lungfish) or perish.

Devonian track

Lull appealed to geology to make his case. The late Devonian rock in which fish like Eusthenopteron had been found, which was considered to have preceded the evolution of the earliest tetrapods, was often the color of dried blood. This was taken as a clue by some geologists who interpreted the late Devonian strata as representing a dry, highly-seasonal world. If it was not entirely arid then the climate was sharply seasonal with a long dry period each year.

Lull hypothesized that, like living lungfish, the ancestors of tetrapods were able to preserve themselves through dry periods. They would have had lungs to survive the foul, oxygen-deprived mud that ponds dried up into, and when all moisture was gone they could go into a torpor underground until the rains returned. Eventually, however, there came a point where the dry periods were too long to survive. The fish could no longer just wait it out. If they were to survive they would have to find water on their own. Lull wrote;

The most ambitious among the lung-breathers, not content with the limitations imposed upon their lives, emerged from the age-long aquatic home and ventured into new and untried habitat. Many may have essayed the emergence, but it is probable that relentless nature, weeding out the less fit for so valorous an undertaking, destroyed all but a single sort, for there is no evidence that the ancestry of the amphibia [i.e. earliest tetrapods] is to be found in more than one evolutionary lineage.

All Lull had to work with were a probable group of bony-finned fish ancestors, a late Devonian trackway marking the appearance of early tetrapods (see above illustration), and the geological evidence. The fossils recording the transition from the water to the land were entirely missing. Those fossils, the petrified tests of Lull’s hypothesis that would later be further developed by Romer, would not be found until much later.

Lull’s hypothesis, and Romer’s formulation of the same idea, has now been rejected in light of new evidence, yet the same imagery remains. The “fishapods” that crawled out of the shallows are often seen as heroic creatures, facing a wild world full of possibilities. If it were not for their “valorous undertaking” our species would not be here today. How this transition has fit into our expectations of “progress” within evolution, however, is a subject for another time.

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