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Embryos in Deep Time
The Rock Record of Biological Development
By Marcelo Sánchez
UNIVERSITY OF CALIFORNIA PRESSCopyright © 2012 The Regents of the University of California
All rights reserved.
Fossils, Ontogeny, and Phylogeny
Human history is a brief spot in space, and its first lesson is modesty.
Will Durant and Ariel Durant, The Lessons of History
I remember as a child being very impressed by a statement, attributed erroneously to Thomas Huxley, that claimed that if monkeys were left alone in front of typewriters, they would type by chance and, given enough time, would indeed type the entire Encyclopaedia Britannica. I had an abridged version of the Encyclopaedia in Spanish, fifteen thick volumes, so I had an idea of the extent of text involved. I read the statement for the first time in a creationism booklet, which pointed out the absurdity of the statement. But it made sense to me: unlikely, and yet, given infinite time, it could happen. While writing this book, I decided to investigate the matter a little and found out that this thought experiment about monkeys and typewriters has been seriously treated from philosophical and statistical perspectives and has been used in various popular accounts. In fact, this is one of the best-known thought experiments, dating to a 1913 essay by the French mathematician Émile Borel. Since then it has become a popular illustration of the mathematics of probability. Apparently the likelihood of monkeys typing the Encyclopaedia Britannica or Shakespeare's works is infinitesimally small. What is the relevance of this to a discussion about evolution and development? There are two points to discuss: the length of time and the probability of evolutionary change occurring during it.
Geologic time is not infinite, but it is long, very long, or deep—a good descriptor considering that it is in the depth of rocks that we can learn much about this distant past. The term deep time originated with the American writer John MacPhee's popular account of geology titled Basin and Range. There he discussed how geologists develop a sense of the vastness of time intellectually and emotionally. Consideration also of the vastness of the extinct biodiversity raises a transcendent perspective, and it may be the most fundamental contribution to human understanding of the universe that geologists and paleontologists can provide.
Thanks to dated fossils placed in evolutionary trees and to molecular estimates, we know that life originated at least 3.5 to 3.2 billion years ago and that multicellular life is at the very least 700 million years old. In the twentieth century one of the major achievements in geochemistry was the development of several methods of rock dating, based on isotopes of different chemical elements, leading to the secure establishment of an absolute time dimension of the vast history of earth and life. The evolution of biodiversity is said to have needed long periods of time. For Darwin, it was important to gather information about the antiquity of the earth and of life. He was concerned that there was "enough" time for truly complex structures, such as the eye, to have evolved. The later discovery of mutations and their "randomness" would at first glance seem to have made Darwin's worries justified.
Evolution is far from random, and monkeys in front of typewriters are not a good analogy for evolutionary processes. One of the main points made in recent books on evolution is the non-randomness and predictability of evolution. This is not a theoretical conclusion but rather something shown empirically by the patterns seen in living phenotypes and genotypes and also in fossils. The fact that mutations in some genes have a high probability of being selected repeatedly in independent lineages facing similar environmental conditions is called "parallel genotypic adaptation." This makes genetic trajectories of adaptive evolution predictable to some extent, leading to the reconstruction of molecular processes that most likely operated in extinct species in spite of the contingency of evolution. Development in extinct species can also be reconstructed thanks to principles that have been discovered to be shared by huge groups of species, even involving the the same developmental genes. And yet diversity, in terms of both species and breadth of form, is vast, like the deep time in which it evolved.
EXTINCTION OF MOST LIFE ON EARTH
The theory of evolution provides a rational explanation for the rich biodiversity that surrounds us. Every day across the world more and more people live in cities, but even those who are rarely confronted with nature are aware of at least some of this diversity thanks to television or a visit to a zoo. Approximately 1.5 million species have been described, and 10 million to 100 million species are estimated to inhabit the planet. But this huge number of species is only a small fraction of the total number of species that have existed on the planet; conservative estimates suggest that as much as 99 percent of the entire diversity of life that has existed on earth is extinct. Most of that past diversity is largely undocumented in spite of the work of paleontologists, as remains of most of those species no longer exist or await discovery and study.
The portion of this large diversity that is most directly obvious to us is vertebrates, the group of backboned animals to which we belong. There are about 59,000 living vertebrate species, but many more species have been described. Some vertebrate groups are better known paleontologically than others. Dinosaurs, for example, are known from some 550 described genera, but it is estimated that approximately 1,850 genera must have existed. Not all new genera and species of fossil organisms that are described are valid, as paleontologists continuously revise their decisions on taxonomy and new studies of variation and anatomy help to refine criteria on which to base decisions. A survey in 2003 determined that the then-valid 4,399 genera of fossil mammals represented 80 percent of the total number of genera ever described. This figure was 67 percent in 1945, as reported by the American paleontologist George Gaylord Simpson in a classic paper. More and more valid fossil genera of mammals and of other vertebrates are being named.
The case of the evolution of our own genus, Homo, illustrates the controversy and diverging opinions on the taxonomy of fossil forms. Yet on many issues there is broad agreement. For example, most anthropologists acknowledge that about two million years ago, at least five different species of humans inhabited the planet. We are the last branch of a once much richer evolutionary tree since our ancestors diverged from chimpanzees some seven million years ago.
The numbers I present here serve simply to illustrate the fact that if we wish to understand the evolution of biodiversity, looking at the past is fundamental. The role of paleontology in describing extinct biodiversity is obvious, but there are many other ways besides mere description in which paleontology contributes to evolutionary biology.
THE FOSSIL RECORD AND WHAT IT TELLS US ABOUT EVOLUTION
The new fields of inquiry created by the emergence of molecular biology in recent decades brought about much well-founded enthusiasm. It also led in some circles to questioning the justification for continuing older disciplines such as paleontology. In the context of these discussions, many of my colleagues have reflected on the many specific aspects that paleontology alone can address to help other biological disciplines and establish an integrative research agenda. These include the following.
1. Fossils provide a time scale for evolution. The oldest representatives of groups from datable rocks provide numbers in which at least a splitting among groups of organisms must have occurred. If we have the DNA sequences of two living species and thus know how much they differ, and if we know when their forms became distinct in the fossil record, we can establish a rate of DNA change. This rate can then be used to estimate the divergence dates of other, related groups for which fossils are not yet known.
2. Fossils provide information on organisms' historical distribution in space. The young Charles Darwin himself noticed this during his voyage around the world. He recorded fossil marine molluscs in the Andes, with obvious implications for the dramatic changes occurring in geologic time. The discovery of fossil platypus relatives in South America several years ago, among other discoveries, provided hard evidence of presupposed faunal connections with Australia. The distribution across Southern Continents in the Triassic of a dog-sized early mammalian relative, Lystrosaurus, and of extinct marine reptiles called mesosaurs in the Permian provided independent evidence for continental drift. Fossils document the past presence on islands of many forms greatly affected by human presence and driven in many cases to extinction in prehistoric times, as is the case for the many giant lemurs found in caves in Madagascar. The list of examples is indeed very long, and the significance of fossils in solving (and creating) biogeographic puzzles or serving to test hypotheses based on living species alone is indisputable.
3. Fossils can document the order in which a suite of features that now diagnose a modern group of organisms or a species arose. Modern mammals uniquely have, in addition to many other traits, hair, mammary glands, and two tooth generations. However, we know that these traits did not all appear at once, as documented by a rich fossil record going back some 315 million years, the minimal time established by fossils for the splitting of the mammalian and reptilian lineages from their common ancestor.
4. Fossils documenting the origin of groups of living species are crucial for testing relationships among organisms. Without the extinct theropod dinosaurs illustrating the origin of birds, for example, it is more difficult to understand how a chicken can be more closely related to a turtle than to a mammal. Fossils improve the accuracy of evolutionary tree reconstruction through the recognition that features can evolve in parallel, such as the constant body temperature of birds and mammals. They can also provide direct information on what the first feathers and hairs of these animals looked like early in their evolution. Fossils often document organisms with a unique mosaic of features, which is very revealing for our understanding of the origin and function of living organisms' traits. Theropods show a unique combination of features that we would not know about just by looking at living species. Who would have guessed that creatures living in the Cretaceous, such as the terrestrial Tyrannosaurus rex, had colorful feathers?
The fossil record, limited as it may be, should be cherished as the most important source of evidence for what really happened on large time scales involving major transitions. Fundamental questions about evolution can be addressed with paleontological studies. For example:
Does evolution proceed at a relatively constant rate, through the exponential accumulation of lineages, punctuated only by extinction events? Or does it proceed through bursts of speciations, including "adaptive radiations," otherwise remaining relatively constant?
How can a very large number of characters of a phenotype change dramatically while not losing their structural and functional integration that any viable organism must possess?
To illustrate the importance of fossils, I referred to the origin of feathers and hair. These are considered "evolutionary novelties": features that are very different and innovative and that somehow have led to, or are correlated with, a new chapter in evolutionary history. Another example is the evolution of hands and feet, as has been documented in the earliest representatives of the group to which land vertebrates belong. What biologists have agreed on and emphasized again and again is that to understand these innovations, you need to know how they developed in the individual history of the organism that possesses them. We recognize the human hand as a formed and flexible mixture of skeleton, muscle, skin, nerves, blood vessels, and other tissues. To address the origin of the hand you need to go back within a lifetime to our fetal period, during which its various components form. What are their connections with other parts of the body? Which genes are involved in their development? How do muscles, tendons, and bones come together? As stated by the evolutionary biologist Günter Wagner, "The explanation of evolutionary novelties is identical to the identification of the developmental changes that make the novel character possible." There is another trip back in time we need to take in order to understand the origin of the hand, and that is a trip in geologic rather than individual time. The first fossil vertebrates with muscular hands and feet lived nearly 400 million years ago. By studying those fossils, we can learn the ecological context in which hands evolved and the combination of anatomical features those animals possessed. We know that the earliest vertebrates with hands and feet had more than five digits on each and were aquatic. Now, imagine we could go back in geologic time and then go back in the individual developmental time of the now-extinct animals to understand how their hands arose. This hypothetical journey epitomizes the coming together of two disciplines that at first may seem quite distinct: paleontology and developmental biology. But their relation is exceedingly close. We can go back in time in the history of these disciplines and their relations and see how the parallels between individual and transformational change, as documented by fossils, have long been recognized.
THE RELATION BETWEEN PALEONTOLOGY AND THE BIOLOGICAL DISCIPLINES OF DEVELOPMENT
Whole disciplines or ideas arise or solidify in the canon of human knowledge thanks to the efforts and accomplishments of exceptional individuals with the interest, drive, and opportunity to pursue a particular subject. This was certainly the case with Georges Cuvier, often referred to as the father of paleontology. Cuvier was educated largely in Stuttgart and lived in Paris in the late 1700s/early 1800s. Over the course of several decades, he documented the anatomy of hundreds of fossils, establishing the reality of extinction and change over geologic time. He lived before Darwin and was never sympathetic to the evolutionary ideas of his former teacher Jean-Baptiste Lamarck, or those of Cuvier's peer, a person very much interested in development, Etienne Geoffroy Saint-Hilaire. Statues or busts of all these past glories of French science can be admired by any tourist in Paris visiting the wonderful Museum of Natural History and the Jardin des Plantes. Cuvier's anatomical work was widely recognized, and his prominent political status during and after Napoleon's reign also contributed to his visibility and to the spread of his encyclopedic work.
Cuvier's magnus opus, Leçons d'anatomie, was translated into German by a famous anatomist working in Halle, Johann Friedrich Meckel the younger. Meckel's own work also concerned anatomical comparisons, but these were among embryos, and he was especially interested in malformations during development. What Cuvier did for fossils in Paris, Meckel did for embryos in Halle, as he amassed a huge collection of fetuses of many kinds of vertebrate animals. His legacy can be seen even in basic anatomical terminology: Meckel's cartilage, a major skeletal feature of the lower jaw in embryonic humans; and Meckel's diverticulum, a portion of the small intestine present in some people. Meckel saw his task as that of completing the work of Cuvier on anatomy by looking at embryology. With him, a long tradition of biologists studying embryos began. Meckel saw a parallel between the differences among adults of living and fossil organisms, on the one hand, and those among embryos of the same species at different time intervals during individual development, on the other. Figuring out the extent or truth or significance of this fascinating fact is what much of biology has been about for a couple of centuries.
It is a pity that Cuvier and Meckel did not live to see a fossil that would have given them the chance to ponder extinction and malformations at the same time. A team of French and Chinese paleontologists working in northern China published in 2007 the discovery of a very singular specimen of a choristoderan reptile, representing a group of semiaquatic reptiles of uncertain affinities. Found in Cretaceous rocks known for yielding spectacular fossils of feathered dinosaurs and early mammals, the beautifully preserved skeleton has two heads, each connected to its own set of neck vertebrae, with the rest of the skeleton also completely preserved. The curled-up position of this and other similar but single-headed specimens suggests that this individual was a malformed embryo. Assignment of these fossils to the Choristodera was based mostly on the long neck vertebrae and the short limbs, two features characteristic of this group known from fairly complete and numerous adult specimens in this formation. The two heads/two necks malformation is common in living reptiles and is called axial bifurcation. This condition is not necessarily fatal, as many turtles and snakes have survived with it for many years in captivity. Obviously, choristoderes shared with living reptiles a developmental system in which this kind of developmental anomaly could occur.
Excerpted from Embryos in Deep Time by Marcelo Sánchez. Copyright © 2012 The Regents of the University of California. Excerpted by permission of UNIVERSITY OF CALIFORNIA PRESS.
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Table of ContentsAcknowledgments Prologue 1. Fossils, Ontogeny, and Phylogeny 2. Evo-Devo, Plasticity, and Modules 3. Fossilized Vertebrate Ontogenies 4. Bones and Teeth under the Microscope 5. Proportions, Growth, and Taxonomy 6. Growth and Diversification Patterns 7. Fossils and Developmental Genetics 8. “Missing Links” and the Evolution of Development 9. Mammalian and Human Development 10. On Trilobites, Shells, and Bugs Epilogue: Is There a Moral to Developmental Paleontology? Notes Bibliography Index
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