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Evolution of Geosciences

"I look at the natural geological record, as a history of the world imperfectly kept, and written in a changing dialect; of this history we possess the last volume above...  Of this volume, only here and there a short chapter has been preserved; and of each page, only here and there a few lines."
– On The Origin of Species
Evolution of Evolution: Interview with Geologist Lynn Soreghan. Darwin's acceptance of the possibility of large changes in Earth's climate system was prescient. Now, a century and a half later, we continue to grapple with the concept of major, even otherworldly, climate extremes. Provided by National Science Foundation
In 1859, Charles Darwin hit on the idea of natural selection to explain how organisms change over time, but the idea was not confined to biology. It swayed modern thinking about science that deals with the Earth as well. This was a likely outcome. Darwin's theory of evolution came from his work as a naturalist, one who advocates that the world—its mountains, oceans, rivers, plants and animals—can be understood in scientific terms. He wrote extensively about the Earth and its environments and theorized how they, coupled with the long expanse of time, contributed to change.

Evolution of Evolution: Interview with Paleoclimatologist Judy Totman Parrish. One of Darwin's contemporaries, Charles Lyell, published "Principles of Geology"; his picture of Earth history as a grand procession of change would greatly influence the thinking of one of the world's foremost naturalists, Charles Darwin. Provided by National Science Foundation
Interview with Paleobotanist Kirk Johnson

Interview with Paleontologist Charles Marshall and Evolution Historian David Sepkoski


Darwin's Missing Rock and the Increasing Precision of Earth Time —
Darwin's understanding of geologic time helps zero in on elusive rock layer

By Kirk R. Johnson

Darwin began to conceive of the immensity of Earth's time when he observed the rate of coastal erosion and compared it with the vast thickness of geologic strata. "What an infinite number of generations which the mind cannot grasp, must have succeeded each other in the long roll of years! Now turn to our richest geological museums, and what a paltry display we behold" (On the Origin of Species," p. 287). At the same time, he realized how few of the lives that have been lived were actually preserved as fossils. Time is long, the Earth is large and only locally, and occasionally, is the subsiding coffin of sedimentary deposition in operation.

As a museum guy, Darwin's words still ring true to me 150 years after they were written. I am, as he was, greatly impressed by the thickness of geologic strata, the amount of time represented by the strata, and even more so, the time that passed while unconformities formed between the depositions of subsequent formations. Darwin sought to explain why intermediate forms were not usually seen in geologic formations and came, in part, to the conclusion that species lasted longer than it took for formations to accumulate and that formations themselves were relatively sparsely distributed in time. He also realized that time within formations was not uniform: "It would seem that each separate formation, like the whole pile of formations in any country, has been intermittent in its accumulation" (Ibid, p. 295).  Both of these insights, like many made by Darwin, were strikingly modern.

Now flip this around and ask the question, what do you do if you want to find a rock layer that was deposited in a specific year, let's say 65,510,023 years ago? This is a problem that did not trouble Darwin but it troubles me because I search for the K-T boundary, an event that appears to have happened one day at the end of the Cretaceous period. It is amazing that we have found the K-T boundary at all. The fact that we can search for and find it in specific spots like the Denver Basin or the seafloor says a lot for our vastly increased resolution of geologic time. A stack of 65,510,023 pennies would be 56 miles high. Imagine the challenge of finding a specific penny in that stack and you have a concept of why it is not easy to find the K-T boundary. Darwin said little about dinosaurs, and his discussions about extinctions never imagined ones that happened in a single afternoon, yet he truly understood the ramifications of deep time and the fact that tiny changes over huge time make for huge change. When confronted with the immensity of time it is sometimes easy to forget that abrupt moments occasionally leave their mark and things that happened a long time ago may have happened really fast.

In some places we can find the K-T boundary because of its trademark mix of cosmic particles and properties; however, we are still unable to precisely measure its age. To be sure, the 500,000-year-error bars of the 1980s have been replaced with 50,000-year-error-bars, and cyclostratigraphy and high-resolution uranium-lead analyses are now pushing toward errors less than 20,000 years.

Darwin ended his chapter on the imperfection of the geological record with a nod to Lyell's metaphor of geologic time as a giant multivolume history of the world, where only a few scattered words from a smattering of random pages were preserved. It is amazing that we are now able to begin to place those few words in their proper position in Earth time.

Kirk R. Johnson is chief curator and vice president of research and collections at the Denver Museum of Nature & Science.  He is a paleobotanist who has been on a 25-year, worldwide search for the K-T boundary, trying to understand its impact on plants.  In the process, he has learned that geologic time is simply time that is hard to measure and even harder to conceive. A summary of his efforts (with colleague Douglas Nichols), "Plants and the K-T Boundary," was published by Cambridge University Press in 2008.  The National Science Foundation supports his research on vertebrate fauna from the late Cretaceous (Campanian) of Utah and on critical transitions in the history of life in and around Denver, Colo.

Please see the Resources section for the Bibliography/Additional Reading list for this essay.

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Modern Paleobiology: Out of Darwin's Shadow
Knowledge of early 21st century fossil record bears little resemblance to 1859

By Charles Marshall and David Sepkoski

Charles Darwin recognized that the fossil record—evolution's 'time machine'—provided the best available test of his hypothesis of descent with modification.  "On the Origin of Species," however, bears witness to his disappointment that the fossil record did not reveal the abundance of intermediate forms he expected to see.  In fact, Darwin devoted an entire chapter in Origin, 'On the Imperfection of the Geological Record,' to explain why the absence of such forms might be expected as the inevitable result of an incomplete fossil record.  Thus, while the central importance of the fossil record was recognized from the outset, its perceived failure in 1859 left a deep pall over its value to our understanding of evolution.

However the paleontology of Darwin's era bears little resemblance to modern paleobiology.  First and foremost, our knowledge of the fossil record has increased enormously since Darwin's time, and is still accelerating.  For example, in 1859 only three genera of non-avian dinosaurs were known; in 1990 there were 285 valid genera, and now there are over 527.  Secondly, we have developed methods for quantifying the incompleteness of the fossil record that enable us to determine whether the fossil record will be of value for a given question.  In primates, for example, it appears that only about 5 percent of all species that have ever existed have been found in the fossil record.  At this preservation rate, we only expect about 9 percent of these fossil species to be directly ancestral to another fossil species.  Indeed, for many groups, as Darwin observed, we do not expect to see direct evidence of descent with modification.

Paleobiology also has altered the character of modern evolutionary biology.  Major contributions include punctuated equilibrium, the discovery that stasis is a major feature of evolution, and the development of hierarchical approaches to understanding the large-scale patterns of origination and extinction, including the invigorating debate over species-level selection—i.e., determining whether the process of natural selection shapes entire biological groups as well as smaller units such as individuals, genes and cells.

Less controversial, but equally vital, are paleobiology's ongoing contributions to our understanding of the assembly of ecosystems, and to the nature of biotic response[s] to short- and long-term climate change. Moreover, paleontology has been enriched by the integration of data and ideas from geology, evolutionary biology, ecology, molecular biology, evo-devo and genomics.

While Darwin's legacy may have been bittersweet, it had an undeniably salutary effect on the modern field of paleobiology.  Many of the 20th century's greatest paleontologists, from George Gaylord Simpson to Stephen Jay Gould, took Darwin's gloomy assessment of the incompleteness of the fossil record as a challenge, rather than a discouragement.  In many ways, the ongoing research program of paleobiology is a testament to the central importance Darwin placed on the fossil record for the study of evolution, even though the science of paleobiology has vastly outstripped anything Darwin could have imagined in 1859.  Darwin's assessment of the value of the fossil record would most likely have been very different if he knew what we now know today.

Charles R. Marshall is a professor of organismic and evolutionary biology and a professor of Earth and planetary sciences at Harvard University.  He is also curator in the Department of Invertebrate Paleontology at the Museum of Comparative Zoology.  He uses techniques in paleontology, developmental biology, statistics and molecular and morphological phylogenetics to understand the nature and causes of evolutionary innovation and extinction over geological time scales.  Marshall is widely published.

David Sepkoski is an assistant professor of history at the University of North Carolina Wilmington (UNCW).  He received his Ph.D. from the Program in History of Science and Technology at the University of Minnesota in 2002.  Before joining the department at UNCW in 2006, he taught in the history department at Oberlin College in Ohio.  Sepkoski specializes in the history of evolutionary theory, particularly the interaction of paleontology and biology during the 20th century.  He recently published "The Paleobiological Revolution: Essays on the Growth of Modern Paleontology" with Michael Ruse, a professor of philosophy and director of the Program in the History and Philosophy of Science at Florida State University.

Please see the Resources section for the Bibliography/Additional Reading list for this essay.

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Life, Climate and the Disguise of Change
Darwin's ideas apply to evolution of climate too

By Gerilyn Soreghan

The concept of recent glaciation over parts of the planet now ice free so pervades common knowledge as to inform pop culture and art—witness the animated "Ice Age" movies, for example. But this took time. Although arguments on the concept of a recent "ice age" date to the 1830s, and Louis Agassiz's Etude sur Glaciers appeared in 1840, the idea of Pleistocene glaciation, or widespread ice as recently as 10-20,000 years ago, was not widely accepted until the late 19th century. Agassiz's insights proved serendipitous to Charles Darwin, who drew upon them to explain the geographical ranges of species. They revealed to him the possibility that changes in climate could have been profound, and the impact on life similarly profound.

In turn, Darwin's acceptance of the possibility of large changes in Earth's climate system was prescient. Now, a century and a half after the landmark publications of both Agassiz and Darwin, we continue to grapple with the concept of major, even otherworldly, climate extremes.

Murmurings of the possibility of global ice in Earth's past, for example, began surfacing as early as the mid 20th century. Predating the paradigm shift of plate tectonics, these ideas were truly radical, ultimately leading to the hotly contested hypothesis of the so-called "snowball" earth—episodes 600-800 million years ago when Earth's surface may have been nearly consumed by ice. The impacts on the biosphere would have been severe, perhaps even presaging the so-called Cambrian Explosion of life—a sudden radiation of complex multicellular life approximately 540 million years ago, as evidenced by their seemingly rapid appearance in the fossil record.

Oddly, initial suggestions of another great glaciation appeared in scientific literature about the same time as publication of "On the Origin of Species," in 1859. Although absolute dates as well as the extent of this glaciation initially remained unknown, the existence of this "Late Paleozoic Ice Age" is now indisputable, with evidence of ice sheets throughout the "Gondwanan," or southern polar continents.

Ice confined to relatively high latitudes is an acceptable concept, owing to its analogy with the behavior of the recent planet. Ice in the low-elevation tropics, however, presents a dilemma. Allowances could perhaps be granted for the "snowball" time, a truly dark age of Earth's history. The late Paleozoic, however, archives a more familiar interval a mere 300 million years distant; a planet with a fully developed terrestrial biosphere, a favorite of museum dioramas populated with ferns, über-insects and mammal-like reptiles. Could it, too, have hosted tropical ice?

Still controversial, emerging evidence has come in the same form as that illustrated by Agassiz so long ago: an ancient landscape, hypothetically carved by ice. In this case, however, within a place formerly situated squarely upon the equator.

Darwin's admonition in "On the Origin of Species" that "there is reason to believe that in the course of time the effects [of changed conditions] have been greater than can be proved by clear evidence" applies to climate as to life. Ice, like life, rarely leaves a clear record. The fossil evidence is elusive; the soft parts vanish, gaps fill the archives. Bits and pieces of fossil landscapes, scattered, ambiguous debris, or mysterious shifts of sea level form the auxiliary clues. We've learned the lessons of change many times in many forms. Yet even now we struggle to imagine the vastly different types of climate that could have prevailed in Earth's past. Darwin's ability to imagine the unimaginable and to conceive of life as having evolved so fundamentally and profoundly over the course of geologic time continues to inspire us to imagine how climate could have similarly evolved so profoundly.

Gerilyn (Lynn) Soreghan is Brandt Professorin the Conoco Phillips School of Geology and Geophysics at the University of Oklahoma in Norman. She has published widely on climatic and tectonic issues of the Late Paleozoic, based primarily on research in the western United States. The National Science Foundation supports her research on the paleoclimate of western equatorial Pangaea.

Please see the Resources section for the Bibliography/Additional Reading list for this essay.

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Charles Darwin's Impact on Geology
Evolutionary ideas have both direct and indirect effects on geosciences

By Judith Totman Parrish

One of Darwin's contemporaries, Charles Lyell, published "Principles of Geology," the first book to discuss the Earth as we do today. Lyell described Earth as a series of gradual processes, not solely the catastrophes envisioned by previous workers. He established important ideas about earthquakes and volcanoes, and his picture of Earth history as a grand procession of change would greatly influence the thinking of one of the world's foremost naturalists, Charles Darwin.

"Principles of Geology" appeared just before Darwin's voyage on the Royal Navy ship HMS Beagle, prompting him to make careful observations of geological events including earthquakes and volcanoes during his journey. Darwin's most direct geological contribution was his explanation of the origin of atolls, the annular coral islands that dot the western Pacific Ocean. He observed many different kinds of islands and realized each represented a developmental sequence—from the earliest stages as reef-ringed, extinct volcanic peaks, to the latest stages of barely submerged coral reefs with central lagoons. Although Darwin was only partially correct when he attributed the evolution of the islands to reef growth and erosion, his explanation played an important role in future understanding of seafloor spreading and the now widely accepted theory of plate tectonics, which describes the large-scale movement of the Earth's outermost shell.

Most people think of evolution as relating to biology and paleontology. What they don't realize is that most paleontologists are also geologists, who use fossils to date rocks and study past environments. Darwin's Theory of Evolution is the structure that supports these efforts.

Biostratigraphy is the science of dating rocks by the fossils they contain, and it was just getting started in Darwin's time. Scientists noticed that certain types of fossils appear in the same sequences everywhere in the geologic record. They reasoned this was because certain organisms lived only at specific times. This meant the sequence of fossils could be used to date rocks relative to each other and to correlate the age of rocks from place to place. It was application of a Darwinian idea—that the sequence of fossils through time represents successive events of evolutionary change and extinction—that put the geologic science of biostratigraphy on a much stronger footing. Later, this relative time scale of fossil sequences would be calibrated with radiometric dates.

Other geologic sciences also use Darwinian principles. For example, taphonomy and paleoecology rely on understanding evolution and natural selection to determine past environmental conditions. Taphonomy looks at how organisms decay and become fossilized over time, and paleoecology uses fossils to reconstruct information about past ecosystems.

Much of what we know about past climates, including the temperature record, humidity, rainfall and other factors, comes from our understanding of fossils. Evolution and natural selection allow us to interpret changes in the form and species of plants as the climate changed. We rely on this knowledge to understand how plants respond to climate and how that response might be recorded in the fossil record. Because the Theory of Evolution helps us understand fossils in their environments, geologists' interpretations of the rocks are far richer and more nuanced than they could be without it.

Darwin's theory revolutionized and became the foundation for all of biology, but for the reasons mentioned above, it was just as much the foundation for geology as well. The inexorable changes that Lyell wrote of were echoed by Darwin in the Theory of Evolution and, in turn, echoed back to geologists in a coherent sense of how Earth—and the life upon it—changed; when those changes occurred; and, how what we see today, is a product of that long history.

Judith Totman Parrish is professor of geological sciences at the University of Idaho. Her education includes a BS and MA in Biological Sciences and an MS and Ph.D. in Earth Sciences, all from the University of California, Santa Cruz; followed by a postdoctorate at the University of Chicago. Her field is pre-Quaternary Era paleoclimatology. She is currently (2008-2009) president of the Geological Society of America. The National Science Foundation supports her work on the evolution of the Pangean Megamonsoon: Plant Taphonomy in Triassic Sedimentary Rocks of the Ischigualasto Basin.

Please see the Resources section for the Bibliography/Additional Reading list for this essay.

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