Written in Stone: Evolution, the Fossil Record, and Our Place in Nature

by Brian Switek

  Owen did not stop at pointing out the common pattern underlying the form of vertebrate arms and legs. Through homology he attempted to work backwards to find the least common denominator of the vertebrate form through which any vertebrate skeleton could be created with only slight modifications. This vertebrate archetype looked like little more than a bony tube, but in it Owen saw the glimmerings of greatness manifest in the human skeleton.

  What Owen needed now was a mechanism to drive the transformations. Considerations of “secondary laws” were already replacing previously held beliefs about creation by divine fiat, but rather than look to external forces, as Darwin had, Owen preferred internal drives. Owen envisioned the production of vertebrate types as the result of a competition between two natural forces pulling against each other. One force caused bodies to repeat certain parts over and over again, like the segments of an earthworm or vertebrae that compose our spinal column. This force dominated in the bodies of organisms traditionally thought of as being simple in construction like insects and worms. The second competing force, which generated more diversity, was considered by Owen to be an adaptive force that modified segments into new shapes. Its action was most commonly seen in “higher” or more complex animals, and the most superior organisms were those that could best overcome the influence of the repetitious force.

  These speculations put Owen at odds with some of his allies. As a social conservative, he depended upon the support of those who feared the concept of evolution. That life could evolve was a revolutionary idea that threatened to throw the created order, and hence the social order, into chaos, a fear made real by the French Revolution some years before. As expressed in the children’s hymn that Cecil Alexander would write a few years later in 1848,All things bright and beautiful,

  All creatures great and small,

  All things wise and wonderful,

  The Lord God made them all.

  Each little flower that opens,

  Each little bird that sings,

  He made their glowing colours,

  He made their tiny wings.

  The rich man in his castle,

  The poor man at his gate,

  God made them high and lowly,

  And ordered their estate.

  As it was in nature, so it was in society, and to destabilize the order of nature was to deny the divine order of the universe. Owen attempted to find a middle way to express the operation of natural laws that would not offend his social allies, and in the conclusion to On the Nature of Limbs he wrote:To what natural laws or secondary causes the orderly succession and progression of such organic phaenomena may have been committed we are as yet ignorant. But if, without derogation of the Divine power, we may conceive the existence of such ministers, and personify them by the term “Nature,” we learn from the past history of our globe that she has advanced with slow and stately steps, guided by the archetypal light, amidst the wreck of worlds, from the first embodiment of the Vertebrate idea under its old Ichthyic vestment, until it became arrayed in the glorious garb of the Human form.

  Evolutionists were perplexed by Owen’s fuzzy mix of theology and science, while the anatomist’s social allies were horrified that Owen had suggested that nature was not immutable. By attempting a compromise Owen pleased no one, though his ideas were influential.

  Contrary to his later opinion of natural selection, Owen’s formulation of the vertebrate archetype gave Darwin an anatomical base from which the fixity of species could be attacked. Through homology, even a species as glorified as our own could be connected to the lowly, mudgrubbing lungfish. Bone for bone, our skeletons formed according to the same basic anatomical framework. Owen had detected the pattern, but Darwin had the mechanism, and in the margin of his copy of On the Nature of Limbs Darwin scribbled:I look at Owen’s Archetypes as more than ideal, as a real representation as far as the most consummate skill and loftiest generalization can represent the parent form of the Vertebrata.

  For Darwin, Owen’s archetype was a hypothetical common ancestor for all vertebrates, and it was natural selection, the interaction between organisms themselves, that could turn the bony tube that was Owen’s archetype into the panoply of vertebrate species. It would be left to the paleontologists to fill out the details of just how these changes had occurred. Embryology and anatomy could help naturalists identify these shared structures, but the record of transformation was to be found among fossil species.

  Once again it was Owen who began to fill the gap. In 1847 the German paleontologist Georg August Goldfuss described the remains of a “primeval reptile” that he called Archegosaurus. It was similar to a crocodile, with a long snout full of sharp teeth fit for snatching small prey from the water, yet it in some ways it more closely resembled amphibians than reptiles. The crenulations inside its teeth, for example, placed it among the labyrinthodonts, extinct amphibians that looked like giant salamanders. When Owen inspected the bones, though, he saw that the skull more closely resembled its counterpart in Lepidosiren than those of reptiles. Much like the lungfish, it seemed to contain a mix of characters, and just two months before the publication of On the Origin of Species, Owen stood before the British Association for the Advancement of Science and said:The Lepidosiren and Archegosaurus are intermediate gradations, one having more of the piscine, the other more of the reptilian, characters. The Archegosaurus conducts the march of development from the ganoid fishes to the labyrinthodont type, the Lepidosiren to the perennibranchiate type.

  This was a bold statement for the time. Here Owen was identifying two transitional types that marked a great divergence in the “march of development” of vertebrates. Archegosaurus, on the one side, was transitional between fish like gars (ganoids) and early amphibians, while Lepidosiren represented the type from which certain salamanders that retained gills throughout life (perrenibranchiates) had been derived. Even though, as late as 1841, Owen had insisted that there was a natural barrier between fish and amphibians that could not be crossed, he

  believed that together these creatures broke down “the line of demarcation between the Fishes and the Reptiles.”

  Despite the weight this evidence could have afforded this theory, however, Darwin was cautious in identi-fying specific species, living or fossil, as the transitional forms that his hypothesis predicted. In On the Origin of Species, Darwin acknowledged that Lepidosiren was a connecting form between living groups, but more importantly it was a powerful example of how natural selection operated. For Darwin, the Lepidosiren showed that if organisms became isolated from the intense competition in the “struggle for existence” they might persist, virtually unchanged, for extended periods of time. Hence Lepidosiren was like a “thin straggling branch springing from a fork low down in a tree.” Its role as a “persistent” form was a more important than its potential spot as a transitional form.

  FIGURE 20 - The skull of Archegosaurus, an early amphibian thought by Richard Owen to be relevant to the origin of land-dwelling vertebrates.

  The finely graded series of fossils documenting the evolution of fish to the first amphibious vertebrates was still missing. While it initially seemed that a fish akin to Lepidosiren had been gradually modified into something like to Archegosaurus, this evolutionary trajectory had a serious problem. The evolution of the first terrestrial vertebrates required the evolution of limbs, a change that the lungfish was in a poor position to initiate.

  For all their other amphibianlike traits, lungfish did not possess anything even approximating a primordial limb. All they had were small, bone-filled tendrils that were difficult to envision as being the forerunners of arms and legs. Naturalists debated whether lungfish or other forms were the type from which tetrapods, the first four-legged and land-dwelling vertebrates, had been derived, and it was not until the latter part of the nineteenth century that a good candidate was found. Described by J. F. Whitleaves in 1881, the fossil fish Eusthenopteron had a branching series of bones that would have been enclosed in flesh and suppo
rted its pectoral fin. It had a single bone connecting the fin to the body, akin to our humerus, and in the group of smaller bones that followed anatomists could see the building blocks for our own limbs. This arrangement seemed adequate for an early fish that was going to venture out onto land and would require stout limbs to support itself.

  With fish like Eusthenopteron as a morphological starting point, anatomists began to envision what the hypothetical transitional stages between its limb and those of the earliest tetrapods would have looked like. Frustratingly, however, year after year rolled by without the discovery of any fossils that would test these ideas. In his 1922 study The Origin and Evolution of the Human Dentition, the American anatomist W. K. Gregory was forced to admit that even though Eusthenopteron and its relatives could be thought of “standing relatively near to the ancestors of the Tetrapoda . . . the connecting links are lacking from the geological record.” Just as resistant to solution was the functional question of why fish had hauled themselves out of the water.

  That early tetrapods evolved from fish was without a doubt. The fossil record clearly showed that the Devonian world, in which vertebrates only lived in the sea and now dated between 416 to 359 million years ago, had been followed by a time of rapid radiation of early terrestrial vertebrates. What could have effected such a massive change?

  In his 1917 textbook Organic Evolution, the American paleontologist Richard Swan Lull reviewed the competing hypotheses. Could the fleshy-finned fish have crawled onto land to escape predators? No, for the fish would still be amphibious and have to raise their young in the water, thus leaving the offspring vulnerable even if the parents were safe. Perhaps the lure of food on land enticed the fish to crawl onto the muddy banks of the Devonian swamps? It sounded plausible, but food in the water was just as abundant, if not more so, than on the shores. Nor did it seem that the “lure” of oxygen in the air could have triggered the change paleontologists knew had to have taken place.

  The only idea that Lull felt stood up to scrutiny was that the end of the Devonian was marked by intense fluctuations in climate, a notion that had been proposed the previous year by the geologist Joseph Barrell in a lecture before the Geological Society of America. For Barrell, the ancient deposits of the Late Devonian, often containing rocks the color of dried blood, did not indicate any kind of watery paradise for tetrapods and their ancestors. Instead, he took them to represent a dry, semi-arid climate similar to that of modern equatorial Africa, where there were dramatic differences between the wet and dry seasons.

  The kind of climate in which Owen’s Protopterus lived provided a good analog for the conditions endured by the Devonian ancestors of tetrapods. Fish flourished during the rainy season in equatorial Africa, but with the onset of the dry season rivers, lakes, and streams disappeared, leaving behind nothing but cracked earth and fish skeletons. It would have been the same in the prehistoric past, and the fact that many Devonian fish skeletons were found together, Barrell argued, meant that they had been crowded together in shrinking, stinking ponds in just such a manner.

  In such harsh circumstances most fish would perish, but Owen’s lungfish showed that survival was possible even if escape was not. As bodies of water turned into miasmas of toxic, oxygen-depleted muck, lungfish were able to draw oxygen from the air via their lungs. Then, when all moisture was depleted, they went into a kind of hibernation beneath the soil until the rains returned to release them from their earthen cocoons. The fishy ancestors of the first tetrapods likely had the same defense against drying out.

  Eventually, however, there came a time when the rains did not return quickly enough. If the fleshy-finned fish trapped in the isolated ponds were to survive they would have to set off across the baking mudflats in search of more water sources. They would have propped themselves up on their fins and dragged their bodies over the searing mud, and those that could wriggle the farthest would survive to perpetuate that strength in the next generation. After this cycle had been repeated enough times, limbs would have carried the first amphibious creatures further than any of their fish ancestors. It was this struggle for existence that set the stage for the later pageant of tetrapod evolution. After the first tetrapods evolved, vertebrates could exploit all that the land had to offer; we, too, owed our own existence to the success of the intrepid Devonian fish.

  Unfortunately there were no fossils to document this change. The oldest known trace of a tetrapod, an enigmatic track named Thinopus antiquus, suggested that by the end of the Devonian limbs and fingers had already evolved. (Though there has long been doubt as to whether the ambiguous impression is a track at all.) The Drying Pond Hypothesis was the most popular explanation for what occurred during the undocumented part of the fossil record, and it would be most closely associated with one of the most influential vertebrate paleontologists of the twentieth century, Alfred Sherwood Romer.

  Romer worked during a time when evolutionary science was undergoing a major change. From the last decades of the nineteenth century into the 1930s many paleontologists rejected Darwin’s idea that natural selection was the primary driving force behind evolutionary transformations. Darwin’s vision predicted orderly, graded steps between forms, while many paleontologists saw that some species appeared abruptly in the fossil record and others persisted virtually unchanged for millions of years. Hence they preferred to believe that evolution was guided by internal forces that pushed organisms toward particular endpoints. (Even Barrell, in proposing what sounded like a Darwinian scenario in his hypothesis for the origin of tetrapods, preferred not to say whether it was natural selection or some other evolutionary force that had modified the desperate fish.) These ideas fell broadly under the banner of orthogenesis, the hypothesis that evolution was striving toward particular goals, but no one could say how these alternate mechanisms worked.

  Romer was among the naturalists who began to reestablish the importance of natural selection to evolutionary science. Taught by W. K. Gregory, who was more sympathetic to natural selection than many of his peers, Romer took a keen interest in the relationship between form and function in the vertebrate body, something best understood in terms of Darwinian mechanisms and not internal driving forces. Along with his colleague George Gaylord Simpson, Romer helped to reconcile the fossil record with what Darwin had predicted, and their findings were combined with those of geneticists and population biologists to form the Modern Evolutionary Synthesis. By the 1950s the sloppy orthogenetic ideas that had so pervaded paleontology were all but eradicated.

  The Drying Pond Hypothesis, with its emphasis on the evolution of new forms to perform the novel function of walking on land, fit neatly within Romer’s view of evolution. What many paleontologists called the “invasion of the land” was not a hopeful step toward “higher” forms of vertebrate life but a dire risk that only seemed glorious in hindsight. And the evolution of limbs was only part of the story, Romer argued. It was not until the erstwhile fish started to consume arthropods (which had made the transition to terrestriality long before vertebrates did) that the true explosion of tetrapod forms began. The basic equipment to move on land had probably evolved by the close of the Devonian, but the impetus to use it did not come until there was something on land for early tetrapods to go trundling after.

  Alternatives to this adaptive story were still proposed from time to time, but they could not compete with the romantic imagery of a lowly fish struggling against the elements. In popular restorations Eusthenopteron , the fleshy-finned fish from the Devonian of Canada, was often placed with its tail in the water and its forefins on the muddy bank, literally spanning the divide between the aquatic and terrestrial realms. Yet the next phase of the evolutionary series remained elusive. A fish akin to Eusthenopteron must have initiated the changes that culminated in early amphibians such as Eryops, a salamander on steriods from the 295-million-year-old rock of Texas, but these forms were just bookends to the evolutionary series. The osteological volumes that could be slotted between them had yet t
o make their full public debut.

  The first signs of fossil creatures that would fill the evolutionary gap were not found in the accessible deposits of Europe and North America, but in a much harsher landscape. A series of failed attempts to reach the North Pole by Swedish explorers in the 1890s led to the chance discovery of Devonian-age fish fossils in the icy rock of Greenland. Vestiges of warm, primordial pools were locked in the frozen rock in the shadow of the Celsius Berg, but the petrified scraps were not enough to merit any expeditions to recover more fossils.

  As the years rolled by, however, Greenland attracted international attention for its possession of a more useful kind of fossil treasure: oil. Crude petroleum was becoming the lifeblood of developing nations, and in 1929 Norway and Denmark were racing to map and control Greenland’s resources. Once again, science was pressed into empire’s service, and those who best understood the geology of Greenland stood a better chance of controlling the natural riches contained within its boundaries.

  Thankfully some of the geologists sent to Greenland were curious enough to do more than just look for signs of oil. As they mapped the island’s stratigraphy some noticed the same kinds of fossilized fish the Swedish explorers had found decades before. One of the geologists employed by the Danish expedition of 1929, a Swede named O. Kulling, even found some fishlike scales that did not quite match any prehistoric fish then known.