Genetics of Original Sin Read online

Page 5


  Protists are the ultimate champions of unicellularity

  We lack reliable fossil traces and thus do not know for sure when eukaryotic cells first arose. But we do know they have given rise to a multitude of unicellular organisms, or protists, which have gone on evolving up to the present day and exploiting the potentialities of unicellularity to their utmost, spreading into an extraordinary variety of organisms. These include the most elaborate and remarkable unicellular forms known, which have fascinated their observers by the multiplicity of their specializations, the elaborateness of their adaptations, and the beauty of their structures.

  Multicellularity allowed division of labor

  There is a limit, however, to what can be accomplished by a single cell, obliged to carry out all the functions needed for independent life. At some stage, the advantages of a “division of labor” must have favored the emergence of organisms genetically predisposed to form multicellular associations. Many mutually advantageous associations among members of the same or of different species no doubt formed, as they do today. But true multicellular organisms were apparently late in appearing. Possibly accounting for this delay is the fact that true multicellular organisms are derived from a single egg cell, which gives rise to two or more distinct cell types by division and differentiation. Here is the key word: “differentiation.” Starting with a single genome, different cells are generated by a process dependent on certain genes being expressed and others silenced, in a manner different for each cell type. Mechanisms for turning genes on and off are already present in the simplest of prokaryotes. But it probably took special circumstances to convert such primitive mechanisms into a developmental pattern. There will be more on this subject in chapter 6.

  According to presently available evidence, multicellular forms of life appeared only about one billion years ago. Plants came first, soon followed by the fungi, or molds. Animals arose much later, about six hundred million years ago—that is, at the time when the atmospheric oxygen level went through its second rise, from 1 percent to 21 percent of the atmosphere. This is probably more than a coincidence, considering the absolute dependence of animals on oxygen. The three lines evolved in parallel, following comparable courses within the constraints imposed by their respective modes of life.

  One common trend was a progressive rise in complexity, a quality that, to avoid the accusation of subjectivity and personal value judgment made by some philosophers, can be defined objectively by the number of different cell types of which organisms are made. This number increased from an original two to several tens in plants and fungi, and up to some 220 in animals. This rise in cellular diversity went together with increasingly elaborate arrangements of tissues and organs. Particularly intricate body plans were achieved in the animal line, with, among others, the appearance of neurons and their association into increasingly complex polyneuronal systems, of which the human brain is the most highly developed extant form.

  Born in water, plants were the first multicellular organisms to invade land

  A second feature common to the evolution of the three lines is that they all started in water and eventually invaded land, thanks to a variety of adaptations. Plants led the way, as they had to, since only they could do without other living organisms, being capable of constructing all their substance from water, carbon dioxide, and a few minerals, using light as energy source. The other two lines, being dependent, directly or indirectly, on the plants for food, could only invade land that had already been colonized by plants. I leave out here prokaryotes that could have served to feed very primitive forms of life.

  Inaugurated in water by simple seaweeds, plants started to move out of their birthplace by way of coastal varieties periodically exposed to dryness at low tide and thus likely to benefit from traits favoring survival under dry conditions. These acquired attributes included rootlets capable of drawing water and minerals from the soil and coverings that both protected the plants against desiccation and allowed them to draw carbon dioxide from the surrounding air. Thus were born primitive mosses, the first multicellular organisms to invade land.

  The mosses further evolved into the first vascularized plants, fitted with roots and leaves linked by a double set of conduits. One set of conduits, leading upward, served to bring to the leaves the water and mineral nutrients taken up from the soil by the roots. In the leaves, these nutrients were then combined with atmospheric carbon dioxide into various organic compounds with the help of sunlight energy. The other set of conduits served to convey the products of these syntheses from the leaves to the roots and other nonphotosynthetic parts, to be used for metabolism and growth. This basic design has been preserved in the entire further evolution of plants, leading, largely by way of improvements in reproductive strategies (see chapter 5), first to organisms represented today by ferns, then to organisms related to conifers, and, finally, to flowering plants, which make up much of the plant world today. An important development in this history was the “invention” of lignin, the hard substance of wood to which trees owe their remarkable strength.

  The plants were soon followed on land by the fungi (mushrooms and molds), which, though being both dependent on other living organisms for their food supply and unable to move and hunt for food, have acquired the means to survive by utilizing whatever organic support, whether living or dead, they can stick to, deriving nutrients from it with the help of powerful digestive enzymes that they secrete in contact with their support.

  The evolution of animals developed around the alimentary function

  The story of animals is more complicated. Being obliged, like the fungi, to obtain their food from other living organisms, animals developed, like these organisms, around the indispensable functions of feeding and digestion, but in a different way. Their first ancestors, born in water, initially arose by exploiting the primeval phagocytic mechanism of feeding common to all protists. From first serving to support individual cells, as in sponges, this mechanism became communal in the digestive pouches of polyps and jellyfish, using enzymes secreted by the cells surrounding the pouch. Conversion of the pouch with a single opening—serving both for the entry of food and for the exit of waste—into a one-way canal, fitted with a mouth at one end and an anus at the other, completed the basic design of the animal alimentary tract, which has been maintained in all the forms that followed.

  All other animal functions developed around this central alimentary core, in relation with the presence of cells that were increasingly distant from the digestive tract, while remaining dependent on it for their feeding. Thus were born circulation and, with it, respiration and excretion. Circulation served for bringing to the cells the foodstuffs and oxygen they needed and for clearing them of waste products. Respiration acted as a means, by way of gills and other organs, to capture oxygen and introduce it into the circulation for delivery to all cells. The function of excretion was to discharge, by organs such as kidneys, cellular waste carried by the circulation.

  Another characteristic animal acquisition was motility, which was ensured by a variety of mechanisms, mostly dependent on the operation of special organs, the muscles. Organisms were thereby provided with all sorts of ways to seek food, find mates, join in groups, escape or fight predators, and soon. With motility came the neurons and the beginnings of a nervous system, serving first to adapt motile responses to sensory influxes and developing further into increasingly complex regulatory networks, thanks to the ability of neurons to establish connections (synapses) with each other. Chemical transmitters evolved as a means to use these connections to transmit signals from neuron to neuron, and these transmitters eventually developed into hormonal systems. Finally, all kinds of specializations were built around the all-important function of reproduction (see chapter 5).

  Marine invertebrates inaugurated animal life

  These events gave rise first to the rich world of marine invertebrates, which include the sponges and jellyfish already mentioned, corals, sea anemones, different kinds of
worms, mollusks—characterized by a great variety of solid outer shells—arthropods, such as lobsters, crabs, and other crustaceans—distinguished by an articulated outer skeleton made of a very tough substance called chitin—and, characterized by a peculiar fivefold symmetry, echinoderms, of which starfish and sea urchins are the best-known representatives.

  Body segmentation opened the way to vertebrates

  A key event that occurred at some early stage of this development was the repeated duplication of a central set of genes (see chapter 6), which led to segmentation, the building of bodies made of a large number of similar units. Almost identical at first, as in the familiar earthworms, these units later evolved into a wide variety, illustrated, for example, by the antennae, claws, and other appendages of crustaceans. Eventually, the units produced the characteristic segments of vertebrates, starting with primitive fish, which further evolved into more advanced fish and, from these, into all the forms that followed.

  Several distinct animal lineages moved from water to land

  Adaptation of animals to living on land involved several key acquisitions: a skin capable of protecting against desiccation, a mechanism for deriving oxygen from air instead of from water, and a motor system allowing movement on land. Ability to reproduce on land, as we shall see in chapter 5, was another essential requisite. Remarkably, several distinct such adaptations developed at different stages of animal evolution. For example, marine worms turned into nematodes and, in a later, segmented line, into earthworms; aquatic mollusks evolved into snails; and arthropods gave rise to the vast group of insects and arachnids (spiders, scorpions, and the like). As to vertebrates, their transition from water to land probably took place in shallow tropical lakes that periodically evaporated during the dry season and regained water during the rainy season. Some fish, known as lungfish, of which species still exist today, became able to survive on land thanks to a dryness-resistant skin, rudimentary lungs derived from the swim bladder, and modified fins converted into primitive limbs. Thus arose, some 400 million years ago, the first amphibians, represented today by animals such as frogs, salamanders, and toads, which still depend on water for their early development. Then, about 350 million years ago, some amphibians evolved into the first vertebrates fully adapted to live and reproduce on land, the reptiles, made famous by the giant dinosaurs, which fill museums with their spectacular remains and have inspired innumerable works of fiction.

  Dinosaurs gave rise to birds and mammals

  Further vertebrate evolution took place on land. Some dinosaurs acquired feathers, perhaps serving initially as a protection against a cold climate, and eventually turning into primitive wings that allowed the animals to glide and, later, to fly. First revealed by archaeopteryx, the fossil of a feathered, presumably flying dinosaur, discovered in 1864 in a Bavarian schist quarry, this story has since received confirmation from a number of fossils found in China. Its outcome is the appearance of birds, about 150 million years ago.

  Other dinosaurs became covered with hair and acquired milk-secreting glands on their chest, allowing females to feed their young. This acquisition led, some 225 million years ago, to the first mammals. These creatures remained small, enjoying a relatively modest existence in the shadow of the monstrous dinosaurs, until some 65 million years ago, when a planetary catastrophe, probably initiated by the fall of a large meteorite on the Yucatán Peninsula in Mexico, precipitated the massive extinction of dinosaurs and many other animal and plant species. Subsequent to this cataclysm, mammals underwent an extraordinary development and came to occupy all environments, even returning to the sea in some cases, as happened to the ancestors of seals and whales. Mammals gave rise, some 70 million years ago, to the primate group, out of which a line detached, some 6–7 million years ago, that was to lead to the human species.

  Viewing this grand history (fig. 3.1), or rather its present outcome, through the eyes of the prophets who wrote the Bible or of medieval scholars, who didn’t even know about microbes, one can readily understand how this whole pageantry was viewed as given once and for all, brought into being by a Creator for the sole benefit of humankind. Even the eighteenth-century Swedish naturalist Carl von Linné (1707–1778), who did know about microbes and who spent his entire career observing and describing living organisms, patiently classifying them into species, genera, families, orders, classes, phyla, and kingdoms, failed to see that the kinships he was recognizing rested, like those of human families, on a vast genealogical tree springing from a single root. Linné remained all his life an unconditional defender of “fixism” and adhered staunchly to the biblical story. Even his later French successor Georges Cuvier (1769–1832), the founder of comparative anatomy and paleontology, adamantly refused to accept the transformist hypothesis proposed by his rival Lamarck, even though he was hardly influenced by biblical creationism. We don’t have their excuses today. Evolution, as we have seen, no longer calls for demonstration.

  Fig. 3.1. The main steps in the history of life, in particular of animals. Note that life remained exclusively unicellular during 2.5 billion years. The first animals appeared 600 million years ago, after life had already accomplished five-sixths of its history. The human species dates back a mere 200,000 years, the equivalent of the last half-hour if life had started one year earlier (and animals two months earlier).

  II

  The Mechanisms of Life

  Introduction

  In the first part of this book, I sketched a descriptive picture of the main steps of evolution. But describing is not enough for understanding. One must explain. All historians know this. That is what I try to do in this second part. It starts with three chapters devoted to three fundamental biological mechanisms that need to be known by anybody wishing to understand life and its history: metabolism, the entire set of chemical reactions that underpin the functioning of living beings since their first appearance; reproduction, which has served as a link between generations all along evolution and ensures hereditary transmission; and, finally, development, which covers the processes whereby, in multicellular beings a fertilized egg gives rise to an organism.

  Natural selection, the topic of chapter 7, represents the central theme of the book, the conducting thread between past and future that leads to the warning at the end of this book, the beacon that illuminates the entire history of life, up to its most recent steps and, even, its future prospects.

  There will be a brief mention, in the last chapter of this part, of some of the other evolutionary mechanisms that have been proposed, including “intelligent design,” which is not properly speaking a scientific mechanism, but warrants attention because of the media upheaval it generates.

  4

  Metabolism

  The New International Webster’s Comprehensive Dictionary defines “metabolism” as “the aggregate of all physical and chemical processes constantly taking place in living organisms.” The key words are “physical” and, especially, “chemical.” There is no escape. If one wishes to understand life, one has to go through some chemistry. In a book like this, we can’t examine all the details that fill biochemistry textbooks with formulas of daunting complexity. Fortunately, it is possible to give an idea of how metabolism works without calling on a single formula. This is what I try to do.

  Living cells are chemical factories

  Have you ever visited a chemical factory? If you’ve seen one, you’ve seen them all, for all are constructed on the same model: a collection of closed vats linked by pipes. Each vat is the site, under specified conditions of temperature, acidity, and so on, and with the eventual addition of a catalyst to facilitate the reaction, of a given step in the specified process. The pipes feed reactants into the vat and allow exit of the products. Raw materials are introduced into the system. They circulate from vat to vat, while undergoing progressive transformations, finally to exit as finished products. The pathways thus followed vary with the nature of what is manufactured. They can be more or less complicated but rarely compr
ise more than a few tens of steps.

  Living chemical factories follow the same model, except that they carry out a large number of different production programs simultaneously, that they include many more steps, and that, aside from a few compartments, such as mitochondria, that house a large number of reactions, there are no vats and no pipes, or their equivalent. It all takes place in a single phase, or metabolic pool, containing all the participating substances. This is possible because these substances may rub each other without in the least interacting. The circulation of matter through the system is entirely ensured by the catalysts of the reactions, most often enzymes of protein nature.