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CHAPTER 1

The History of Life

Introduction

The birth of Darwinian Evolutionary Theory was accompanied by the final burial of the competing ideas of the poet-naturalist Johann Wolfgang von Goethe and the great French zoologist Étienne Geoffroy Saint-Hilaire. Darwin himself draws attention to this in his preface to later editions of The Origin of Species. Darwin provided a powerful insight into how new species arise and how they become different from their ancestors, whereas Goethe and Geoffroy were concerned with explaining why the disparate species of life retain so many similarities to each other. If the modern molecular biology of the past 20 years definitively proves that Goethe and Geoffroy were correct all along, how can Darwinian Theory stand unchanged?

There is a new and emerging transformation occurring in evolutionary theory today — namely, the inclusion of Modularity Theory (Schlosser and Wagner 2004). This is based in part on insights from our new molecular genetic understanding of how organisms progress through their embryological development. In the past, ideas and suggestions in this area were easily dismissed with rhetorical ripostes. Now, however, the new existence of powerful concrete molecular data has forced Darwinian biologists to come to grips with the conflicting view of Goethe and Geoffroy.

Explosions and Revolutions in the History of Life

This book is concerned with improving our understanding of three "explosions" and three "revolutions" in nature that are responsible for the emergence of most of the organisms that fascinate us in the biological world. From a number of points of view, these six events are the great moments in the History of Life on this planet, and each demonstrates the power of Modularity Theory for clarifying the mechanisms of evolutionary change.

Many biologists — such as the "cladists" (who have applied rigorous logical principles to the science of relationships among organisms) — believe that the History of Life is composed only of millions of individual and equally important branching events among individual species, with no one event more significant than another. Strict adherents to the Modern Evolutionary Synthesis — the "Darwinians"— are comfortable with some significance being attached to these six events, but see no special obligation upon Evolutionary Theory to explain them. A third major group of biologists — the "evodevo" enthusiasts (who look for explanations of evolution in the embryological development of organisms) — are substantially focused on the explanation of only two of these six events.

The Origin of Descent

The first revolution we will look at is the origin of biological descent. In the ancient seas of a billion years ago, transcription and translation to copy DNA and make proteins was already taking place in much the same way it occurs in our own bodies today. However, genes were passed horizontally — that is, from one adult organism directly to other adult organisms of various kinds. At some point in time, a new type of organism appeared that carefully guarded its own set of genes and passed them on only to its progeny vertically, as an intact set with only minimal modification. This was the start of the evolution of lineages and commencement of descent with modification. In other words, Darwinian evolution — in which generation follows generation and species follows species with gradual genetic change — only begins after the period of horizontal gene transfer begins to come to a close.

The fundamental mechnism by which genetic instructions in DNA are read out and converted to working proteins and enzymes (transcription/translation from nucleic acid to protein) was an intact functioning biological module — in other words, it was a self-contained, complete working process that later became a component of evolving organisms. This is a key concept in Modularity Theory. A very complex working system such as the molecular machinery that reads information in DNA to make proteins does not need to be reinvented from scratch in every organism. It is abundantly clear that in nature, once such an integrated complex process emerges, it is passed along as a complete functioning unit. This is true whether it is passed horizontally from an ancestor of a fungal cell to an ancestor of a plant cell, or whether it is passed vertically from a mother elephant to its calf. So it appears that the emergence of many extremely important biological modules predates the commencement of Darwinian evolution.

The Cambrian Explosion

The first explosion occurred at the dawn of the Cambrian period around 522 million years ago. The one-celled organisms had toiled for half a billion years to fill the world with oxygen. The ancestors of plants were now using photosynthesis to get energy by the capture of sunlight, and the ancestors of animals were now burning oxygen for energy. Multi-cellular organisms had emerged that could coordinate biological processes among a large number of specialized cells. Then, for reasons that remain obscure, all of the dozens of major animal phyla and many of the hundreds of classes and orders of animals suddenly appear on the scene, many with widely varying body plans. In the succeeding 500 million years, nothing remotely similar to this ever happens again. This explosive event has been a major focus for evo-devo scientists.

It is now clear that a major aspect of this explosion was the emergence of an embryological process called terminal addition — the repetitive process of construction of similar body components tacked onto one end of the embryo like beads on a string. Both insect segments and our own vertebrae are echoes of this event. Each of these developmental modules could be composed of a wide variety of tissues with smoothly interacting functions — muscle, nerve, skin, and so on. The various resulting body modules or groups of modules could then be assigned an individual specialization. Examples of this include the differentiation of some insect segments into wing-bearing components and some into antenna-bearing components.

The Inverted Insect and the Origin of Vertebrates

The second revolution is the abrupt and radical emergence of the "inverted insects" we call the vertebrates. One species of one of the numerous Cambrian phyla appears that is literally flipped upside down and is, at least in part, internally inverted. I say "radical" because this appears to involve a true 180-degree flip in body construction plan, and I say "abrupt" because it almost certainly took place in a single generational event. This revolution has also been a major area of focus for evo-devo.

The Dinosaur Domination

The second explosion is the appearance of a vast array of dinosaurs beginning around 235 million years ago. This now appears to be due to an abrupt change that took place in the body plan of one species of reptiles. The ancestral dinosaurs experienced a modification of their body axis — the vertebral column and surrounding structures — that suddenly altered the balance of power of life on earth, giving them the lead in energy, speed, respiration, and size. This change in the dorso-ventral (back-to-front) organization of the body axis in development simultaneously altered all the body modules of the ancestral dinosaur to produce a sudden and overwhelming leap in effectiveness of adaptation of several body systems all at once.

The Mammalian Takeover

The third explosion is the diversification of mammals 70 million years ago that led to the origin of all of the major mammalian orders that we know today. This explosion also appears to be associated with a change in the way in which the main body axis is organized. At a key point in the emergence of mammals, two modular reorganizations of the body axis occurred: One matched the effect of the dorso-ventral change in the dinosaurs, and the other provided greater distinction in the differentiation among the body axis modules. These two changes made mammals successful competitors in a variety of environmental niches. Simply put, these changes allowed mammals to run faster and to have more energy than the dinosaurs and lizards.

The Transformation That Launched the Human Lineage

The third — and, for many people, the most important — revolution is the body plan change that set a species of primates on the path toward becoming human 20 million years ago. What actually happened at this time is not what the scientific orthodoxy has taught over the years. The data presented in this book lead inevitably to the conclusion that the key initial event in human origins was similar to the key event that drove the dinosaur explosion and the mammalian explosion. Once again it was a sudden dorso-ventral transformation of the modules of the body axis — a remarkable reorganization of the vertebral column in which some parts moved forward and others shifted backward — that generated an upright species. In fact, the evidence shows that this upright species — most likely a ground walker — was the ancestor of both the great apes and man.

The fact that this astonishing reorganization of the primate body design in the human ancestor has escaped the understanding of thousands of scientists across hundreds of years does not diminish its profound importance. The major plane that separates the front part of the body from the back part changes from its standard primate position (in front of the lumbar spinal canal) to a revolutionary position behind the spinal canal. The first fossil evidence of the transformation was discovered more than 40 years ago (Walker and Rose 1968). A new fossil discovery made in 2004 (Moya Sola et al. 2004) confirms this remarkable event and will now force a dramatic reinterpretation of human evolution. In short, understanding the way in which major modular transformations impact subsequent evolutionary events is essential to understanding our own special history as part of a unique lineage of upright primates.

Darwin, the Modern Synthesis, and the Arrival of Modularity Theory

Certainly there are various other revolutions and explosions in the History of Life, and a number of these will be discussed in this book for context and comparison. However it is these six major events that are critical to our understanding of our own existence, as well as our understanding of how life evolves. If these six events are to be fully understood, however, it is my contention that Darwinian Theory and the Modern Synthesis of the 1940s (the addition of population genetics to the original theory) must be revised and expanded to incorporate the modern molecular genetics of biological construction.

We can safely say that Darwin simply specified descent with modification (gradual change of a species due to shifts in the genetic makeup of the total pool of genetic varieties from generation to generation), and that the various revolutions and explosions fall within this dictum. For example, Darwinian Theory and the Modern Synthesis really do an excellent job of explaining how and why the shape of the beak of a group of finches slowly changes over time. There is variation (slight differences) in beak shape among the population (the total interbreeding group) of individuals in a species. There are also various environments, and changes within these environments. Natural selection, or "survival of the fittest," will "prefer" some variants of the species over others, and gradually the gene pool of the species will change. Speciation events (the origination of a new, separate species) will wall off one group of individuals from another. If each has a different gene pool, then the two descendant species will appear different from each other, and each will be optimized for survival in the environment in which it lives.

It was 200 years ago that the great French zoologist Étienne Geoffroy Saint-Hilaire published the fantastical proposal that vertebrate animals were flipped-over versions of invertebrates. Between 1818 and 1996, no respectable biologist took this proposal seriously. Now we know it is fact. The problem for Evolutionary Theory is that this critical event did not appear to involve variation, populations, or even the accumulation of gradual (small, incremental, generation-by-generation) changes due to natural selection. Because the invertebrate ancestors remained far more prolific and varied, it is difficult to argue that this body inversion event provided improved survival for these ancestors of the vertebrates. In fact, there is a strong case to be made that what happened was a freak or "monstrous" event. It was sudden; it was lucky to have occurred in a way that allowed for it to be passed along into descendants; and it was lucky to have produced a type of creature that was at least capable of surviving to reproduce.

Of course, luck implies chance and chance implies randomness. Actually, although this revolutionary event suggests randomness from the point of view of natural selection, it is rigidly ordered and formal from the point of view of developmental biology and functional morphology (the study of how body components actually work in the life of the animal). The ancestral vertebrate was no monster, because it had an altered but complete and properly functioning developmental embryological sequence — in other words, unlike a conjoined twin or a mutated stillborn with no head or mouth, this organism had a full, working set of body components. It did not survive simply from luck, but rather because it had a novel design that met the basic functional requirements for sustained existence — namely, the capability of obtaining and processing food, avoiding predation, and successfully reproducing. There has been a fear in Evolutionary Theory that if natural selection does not act, then evolution will be seen by its opponents to have progressed at random. However, what actually seems to occur is that some steps in evolutionary history are guided by a different mechanism than the standard of population variation, selection, and speciation. It is not that evolution is progressing under no direction; rather, it is that the Modern Synthesis mechanism is not the prime player in some key events.

Why Are the Mechanisms of These Grand Events Unresolved?

By now it's probably fairly obvious that these three revolutions and three explosions are significant. But how have they been considered in the past? The first revolution — the origin of descent — simply was not known about or convincingly demonstrated until the past five or 10 years. It only became apparent as our understanding of molecular biology progressed. Though evidence of the Cambrian explosion has been accumulating for 200 years, its existence has been questioned in two ways: One suggestion is that all we are seeing is a change in fossilization — all the different types of animals emerged gradually over eons, but their fossils appear all at once because of an environmental change. (Recent molecular biology tends to support an abrupt event, however.) Another suggestion is that, because it all could have taken 1 million years or more, it was really a stately and gradual event — an evolution and not a revolution. However, when we look at the issue of remarkable generation of new body plans, it becomes apparent that even if it took 1 million years, it was a very different million years than the other 999 1-million-year periods in the billion-year History of Life on this planet.

Almost all biologists believed that the origin of vertebrates was a gradual selective process similar to the origin of any other group. Then, in one of the great ironic events in the history of science, data has recently come pouring in from the molecular analysis of the genetics of embryo formation confirming Geoffroy's theory of "flipped-over" vertebrates.

The basis for the success and diversification of dinosaurs was not understood until very recently; a similar situation prevailed for mammals. Neither of these events — the diversification of dinosaurs and the diversification of mammals — seems to have been nearly as rapid as the Cambrian explosion. If you assume gradualist Darwinian action, there is a concept of a "crown radiation" — an extensive branching into numerous different types of species in a kind of animal (for example the mammals) that has only existed in the form of limited numbers of species in the past.

An unexplained vast extinction opens up an array of environments — think of the dinosaurs dying out and making way for the mammals. At the moment of extinction for one group, a new and entirely different type of animal appears with improved features, and this group then generates a wide array of new species that become optimized for the various niches in the new environment. This argument is somewhat plausible for the dinosaur diversification, but it is difficult to explain why the dinosaurs did not come roaring back 70 million years ago (when instead the mammals became the dominant large land animals).

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Excerpted from "The Upright Ape"
by .
Copyright © 2007 Dr. Aaron G. Filler.
Excerpted by permission of Red Wheel/Weiser, LLC.
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