by Brian Switek
9 The solution to this problem would only become possible with the invention of radiometric dating. After the discovery of radioactivity in the late nineteenth century it became apparent that isotopes of certain elements contained within certain rocks, such as uranium 238 and potassium 40, break down or decay into isotopes of other elements at constant rates over time (lead 206 and argon 40, respectively, in the case of the two mentioned here). By observing the rate at which the original radioactive material decayed into the daughter isotopes scientists could then look at the ratio of “parent” to “daughter” material in rock and determine how long it would have taken to produce that amount of daughter material, thus yielding the rock’s age. Unfortunately, most fossils are contained within sedimentary rocks that do not contain radioactive materials (with the exception of strata made of ash from volcanoes), but many times sedimentary strata are embedded between lava flows or rocks with radioactive materials. By dating the rocks above and below the fossil layers, then, geologists can establish a time frame. In this way “relative dating” based upon fossils and “absolute” dating work together.
10 Charles Darwin would later be a student of Jameson, but the budding sixteenyear-old naturalist found Jameson to be a rather dull teacher.
11 Jameson’s influence on this point has been known by historians of scientists for some time, but the myth that Cuvier was a religious fundamentalist who set out to prove a global Flood remains widespread. I heard it myself several times during my education. The fact that Cuvier argued forcibly against the evolutionary ideas of Lamarck and Geoffroy St. Hilaire bolstered this misunderstanding, for surely anyone who opposed evolution at the time must have done so on religious grounds (or so the myth goes).
12 Nor was Buckland especially credulous. He had gained fame for his ability to debunk fantastic claims. Perhaps most notably, during his honeymoon with his new wife in Italy in 1825, Buckland visited the shrine of Santa Rosalia, a Catholic saint whose bones were said to have eradicated the plague from the town of Palermo in 1624. When Buckland saw the bones, however, his expertise in comparative anatomy allowed him to immediately recognize them as goat, not human, bones. The attending priests replied that only the truly devout (i.e., Catholics) could see the bones, and since that time no one has been allowed to see “Rosalia.”
13 Today we know that Buckland was indeed studying evidence of flooding, just not of the sort he imagined. The gravels, loams, and clays were deposited by melting ice sheets during glacial cycles over the previous two and a half million years. When the climate warmed, the ice sheets melted, flooding the surrounding area and dropping mixtures of geological bric-a-brac that had been carried along from places far distant.
14 Traditionally Cuvier and Buckland’s view has been known as “catastrophism” and the opposing view as “uniformitarianism.” I have eschewed these labels here as I believe that they are more often abused these days than used properly. Even though Buckland and Cuvier spoke of quick revolutions they also relied on geological processes now in action to understand less drastic changes. Likewise, processes now operating cannot entirely account for geological events of the past, particularly events like asteroid strikes. Modern geology is thus a combination of both systems and it is not profitable to identify one side as the victor despite a textbook preference for the label uniformitarianism.
15 There is so much to discuss about Charles Darwin that it cannot all fit in one chapter. Some readers will surely say, “But you forgot ...” by the time they get to the end of it. I can only offer my apologies that I did not have more space. Given the interests of this book I have decided to focus on the importance of geology and paleontology to Darwin’s work. There is no shortage of biographies on Darwin for those who desire a more comprehensive account, among the best being Adrian Desmond and James Moore’s Darwin: The Life of a Tormented Evolutionist and Janet Browne’s two-volume study, Darwin: Voyaging and Darwin: The Power of Place.
16 My favorite tale is that of the beaver. The testicles of male beavers were useful as medicine, the Physiologus says, and so male beavers were often hunted for this resource. Rather than let themselves be slaughtered, however, the beavers would castrate themselves and throw their genitals to the hunter so that they might escape with their lives. Should they be harassed by another hunter later they would simply roll over to show that they had already been snipped. And the moral of this tale of selfmutilation? That we must likewise excise sin from our lives and cast it before the devil so that he will leave us alone. Never mind that the testicles of beavers are actually held inside their bodies, making them incapable of exemplifying such a lesson.
17 Two days before the publication of On the Origin of Species Darwin would write to his neighbor John Lubbock that “I do not think I hardly ever admired a book more than Paley’s Natural Theology: I could almost formerly have said it by heart.”
18 By this time Darwin had become a well-respected young naturalist, so his father organized investments so that Darwin would be financially supported.
19 Natural selection was the core mechanism in Darwin’s evolutionary framework, but it was not the only one. He believed that there were other supplementary mechanisms that also contributed to species change, and he was often frustrated when critics asserted that his vision of evolution was based on natural selection and nothing else.
20 Owen had a habit of changing species and genus names if he thought the original title was somehow inaccurate or not evocative enough. In this particular case the name change was not warranted, and today the lungfish from Africa is known by the name Protopterus.
21 It should be noted, however, that not all tetrapods develop fingers from this arc. There are other patterns of finger development, usually associated with a reduced number of fingers, but the mode of digit formation as proposed by Shubin and Alberch is the most relevant for the discussion of early tetrapods.
22 There is still some debate whether the hands and feet of tetrapods were entirely novel features. Some scientists have argued that that they are modified versions of structures seen in the ancestral fish, while others have said that the differences between zebrafish and tetrapod embryos show that hands and feet were entirely unique to tetrapods. The problem with this argument is that zebrafish are not particularly closely related to tetrapods and might not be very informative on this point. Research and debate will continue, but the basic developmental patterns outlined in this chapter have been confirmed by multiple lines of evidence.
23 And for those who are about to object “We don’t have tail bones!” I must point out that you are probably sitting on them right now. The coccyx in our skeleton is the remnant of a tail possessed in our monkey ancestors and remains an attachment point for sinew and muscle.
24 In January 2010 a team of scientists led by Ahlberg described what appeared to be tetrapod trackways over 10 million years older than Tiktaalik in the journal Nature . If these traces were made by tetrapods then the evolution of the first vertebrates with limbs was far more complex than we presently realize, but whether the “tracks” were made by tetrapods at all is still controversial. Many tracks attributed to tetrapods have later turned out to be traces left by invertebrates, from giant centipedelike arthropods to sea stars that left fingerlike impressions, so further research will be required to determine what animals left the traces.
25 The historian Adrienne Mayor has done more than anyone else to elucidate the connections between fossils and mythology. Her books The First Fossil Hunters and Fossil Legends of the First Americans are essential reading on geomythology.
26 The association of the Lenape with Europeans led to the near-destruction of the tribe. Disease, confusion over property rights, and reliance on trade with the Europeans all contributed to the downfall of the Lenape, but there are still Native Americans today who can trace their ancestry back to this tribe.
27 Deane did not publish his first paper on the footprints until 1843, eight years after he became acquainted with them. The fa
ct that Hitchcock published earlier and more frequently than Deane, however, led to a public row over who had priority. Hitchcock tried to strike a conciliatory note, while maintaining that he had launched a new branch of science, but Deane was not satisfied. Hitchcock ultimately won most of the recognition, but Deane’s contributions should not be forgotten.
28 The more famous poet Henry Wadsworth Longfellow was also so inspired by the tracks. While he did not devote an entire poem to them, they are mentioned in both “To the Driving Cloud” and “A Psalm of Life.” These were the “footprints on the sands of time.”
29 Owen did ascribe it to a new species on the basis that the feathers with the skeleton looked slightly different than the feather von Meyer described. It was not unreasonable to think that there could have been more than one bird present at the same place at the same time, but even though there is still controversy today most paleontologists ascribe all the known specimens to the species established by von Meyer, Archaeopteryx lithographica.
30 Phillips had been suspicious that parts of the Megalosaurus were out of order prior to Huxley’s visit, but he had not yet completed his research on the problem. This story, recounted by Huxley, also runs counter to the apocryphal story that Huxley first recognized the resemblance between birds and dinosaurs while carving a Christmas goose. I do not know the origins of this tale, but I have found no evidence for its veracity. Even so, many paleontologists do subject their friends and relatives to anatomy lectures every Thanksgiving and Christmas.Additionally, a recent reanalysis found that many of the fossils attributed to Megalosaurus actually belong to other dinosaurs, meaning that the original jaw fragment Buckland described is all we presently know of it.
31 Owen was able to give form to his view through the artistic talent of Benjamin Waterhouse Hawkins, who was commissioned to create life-size sculptures of the dinosaurs (and other prehistoric monstrosities) for the Crystal Palace exhibition of 1851 in Hyde Park, London. Hawkins’s sculptures were later moved to Sydenham Hill, where they still can be seen today.
32 Of Hesperornis the ornithologist William Beebe would later write, “When in the depth of the winter, a full hundred miles from the nearest land, one sees a loon in the path of the steamer, listens to its weird, maniacal laughter, and sees it slowly sink downward through the green waters, it truly seems a hint of the bird-life of long-past ages.”
33 Even creationists offered ideas for the point of origin of birds. In an article published in 1897, W. T. Freeman wrote, “I suggest that in the earlier days there were ill-developed, low-typed, wallowing birds, also some highly developed reptiles. Perverted sexual instinct exists now, why not then, and as a result of this, why has not the archaeopteryx been an anomalous false hybrid that has been incapable, like other mongrels, of reproducing its kind?”
34 Although he doubted its ability to fly, Beebe thought that Archaeopteryx might have been able to sing (or at least squawk). A naturalist walking through a Jurassic forest might have heard “an archaic attempt at song—a lizard’s croak touched with the first harmony, which was to echo through all the ages to follow.” This same idea was expounded several years earlier in Eden Phillpotts’s fictional work Fancy Free. In the work, a fictional clergyman recounts a Mesozoic safari, during which he spies an Archaeopteryx, “He was the very first thing of his kind that Nature had managed; naturally he could conceive of nothing finer than his primitive self and preposterous voice. He gurgled and hissed, and squeaked, and even tried to trill.”
35 This is an oversimplification of Dollo’s Law, stated here as it best reflects Heilmann’s reasoning. As it is presently understood, Dollo’s Law has more to do with the closing of certain evolutionary pathways as creatures change. Snakes, for example, lost their legs through evolution and cannot revert to an ancestral state. Descendants of living snakes could evolve legs, but they would be entirely new structures and not duplicates of the ones their distant ancestors lost.
36 In 2007 paleontologists announced that they had finally found evidence of group behavior in these kinds of dinosaurs. A trackway from China not only recorded the tracks of Deinonychus relatives, confirming that they held their large claws off the ground, but the trackway also preserved the movements of several dinosaurs together. The intricacies of the preservation and spacing of the tracks supported the idea that these dinosaurs were moving together, and it seems that at least some dinosaurs like Deinonychus were gregarious.
37 Even this is an oversimplification. Some animals, like small bats and birds, have high constant body temperatures for part of the day or year but have their body temperatures affected by the surrounding environment at other times. Others, like some fish, have body temperatures that fluctuate with the environment but are maintained several degrees higher than the surrounding water.
38 This was later reinforced by the discovery of another Archaeopteryx without clear feather impressions that had been labeled Compsognathus. What so many naturalists said was right; without feathers Archaeopteryx looked just like a dinosaur.
39 Artist Gregory Paul illustrated what some of these downy dinosaurs might have looked like in his 1988 book Predatory Dinosaurs of the World.
40 Other groups of predatory dinosaurs, represented by theropods, such as Allosaurus , Spinosaurus, and Carnotaurus, were not coelurosaurs, and to date there is no evidence that they had feathers.
41 This is no more fantastic than suggesting that our own prehistoric ancestors were covered in hair, based upon their relationship to us.
42 There are a number of ways to distinguish the ornithischians from the saurischians, but the easiest is to look at their hips. The ornithischians have a backward-pointed process called the pubis, while the same process in many saurischians is oriented forward (though it was secondarily rotated to point backwards in dinosaurs closely related to birds). Huxley was wrong when he thought the hips of Hypsilophodon represented all dinosaurs. We now know it was a type of ornithischian, unrelated to the earliest birds.
43 Some marine reptiles, like the ichthyosaurs, had only one temporal fenestra. They differed from synapsids, however, in that they had evolved from ancestors with two such openings and the one that was retained was higher up on the skull.
44 Today the term “pelycosaur” has fallen out of fashion because it is thought to be paraphyletic, or a group that does not contain all the descendants of a common ancestor (in this case, the therapsids). The name is retained here, however, because it allows creatures like Dimetrodon to be distinguished from other synapsids, and the connection between the pelycosaurs and the earliest therapsids is explained.
45 An exception to this may be a potential early therapsid fossil from the Lower Permian called Tetraceratops. More complete remains will be required to determine its relationships, but if it is an early therapsid, as some paleontologists have proposed, it may indicate that the early therapsid form existed for millions of years before the adaptive radiation at about 267 million years ago. At present, though, it appears that therapsids diversified quickly after the first members of the group evolved, fitting a more punctuated evolutionary model than a “gradual” one.
46 Interestingly, the separation of ear bones from the lower jaw of synapsids may have occurred more than once. In 2005 a team led by Thomas Rich described the jaw of an early platypus relative called Teinolophos that lived after the split between monotremes and other mammals but still had ear bones connected to its lower jaw. This suggests that the separation of the ear bones from the lower jaw happened once among monotremes and again in the lineage leading to the other two groups of mammals.
47 The levels at which natural selection might work have been hotly debated in recent years. Some scientists, such as Richard Dawkins, have argued that natural selection primarily works at the level of the gene, thus rendering organisms only as gene-propagation vehicles. Scientists such as Stephen Jay Gould, however, have argued that natural selection can act on a variety of hierarchical levels, from genes to individuals to populations to
species and maybe even entire evolutionary groups. I am more inclined to agree with the latters camp on this point, and the details of mass extinctions and their aftermath might be able to tell us how natural selection acts at a level above that of individual organisms.
48 The intense eruptions of the Deccan Traps volcanoes have also been proposed as extinction triggers, but they do not fit the pattern of extinction as presently understood. Even so, the cause of the end-Cretaceous mass extinction is still being debated, and future discoveries will help paleontologists better understand what happened.
49 H. N. Hutchinson also mourned the loss of so many specimens in his book Extinct Monsters and Creatures of Other Days: “How little [do the farmers] know that hundreds of museum curators all over the world would be only too glad to procure some of this ‘rubbish’!”
50 Like many fossils in German museums, Koch’s original Hydrarchos was destroyed in an Allied bombing raid on Berlin in 1945.
51 There are some notable exceptions to this. Extinct archosaurs such as Tyrannosaurus and Malawisuchus had heterodont teeth, and some mammals, like dolphins, have homodont teeth (or teeth that are the same throughout the jaw).
52 Agassiz later changed his mind when he saw them himself and said that the jaws came from mammals.
53 Interestingly, remains of Pakicetus had been described just the year before as a new species of Protocetus called Protocetus attocki. The fact that those particular fossils indicated a second species of Pakicetus could not have been known until more complete remains of the genus were discovered and compared.
54 Sadly, the Yangtze River dolphin, also known as the Baiji, was declared extinct as this book was being prepared. There may be a few individuals left, but the population is so small as to no longer be viable. The reason for the extinction of the Baiji is directly attributable to human activities, particularly hunting during the humanitarian disaster known as the Great Leap Forward, pollution of the river, and habitat reduction as a consequence of the construction of the Three Gorges Dam. All of the other freshwater dolphins in the world are endangered as well.