Genetics of Original Sin (6 page)

First to be adopted in this way were the mitochondria, which are the main sites of oxidative energy production, the central “power plants,” in the vast majority of eukaryotic cells. These organelles are derived from bacterial ancestors that must have ranked among the most efficient prokaryotic oxygen utilizers at the time they were adopted and have left similarly endowed, present-day descendants showing many signs of kinship with mitochondria.

The second eukaryotic organelles of established endosymbiont origin are the chloroplasts, which harbor the light-utilizing systems of all photosynthetic eukaryotic cells, to wit, all unicellular algae and green plants. The bacterial ancestors of these organelles have been identified as belonging to the group of cyanobacteria, encountered above as the “inventors” of oxygen-generating photosynthesis. These ancestral organisms were first adopted by cells that already possessed mitochondria, which are thus present in all photosynthetic eukaryotes (except when lost in the course of evolution).

For endosymbiosis to take place, there must first have existed cells with a size and functional properties that allowed them to harbor the bacteria that gave rise to the organelles. This question has been a fertile ground for all kinds of hypotheses,
one more ingenious than the other. For my part, I stick to the simplest possibility, directly inspired by what we know and using a common cellular function, called “phagocytosis,” whereby, for example, white blood cells capture infectious bacteria that invade an organism. We need merely to suppose that a “primitive phagocyte” possessing this property already existed at the time we are talking about and that, exceptionally, the bacterial ancestors of the endosymbionts captured by this organism were not killed and destroyed, as happens in white blood cells, but survived to become the endosymbionts. Such a phenomenon would hardly be surprising, as several present-day instances of it are known. According to the hypothesis I propose, this phenomenon would have happened at least twice, first to the ancestors of mitochondria and then, again, to the cyanobacteria that evolved inside the host cell to become the chloroplasts.

According to this scenario, formation of the “primitive phagocyte” from a prokaryotic ancestor appears as a crucial step in the development of eukaryotic cells. A detailed discussion of the manner in which this key transition could have occurred would take us too far. Let me simply emphasize the important role that may have been played by the passage from extracellular to intracellular digestion. All living beings that feed on nutrients provided by other living beings must start by digesting their foodstuffs, that is, cutting the big molecules of which these are made into small molecules that can be assimilated. This is what happens in our stomach and intestines. For single cells, this function is carried out in two different ways, depending on whether they are prokaryotic (bacteria) or eukaryotic. The former universally digest their foodstuffs with the help of enzymes that they discharge into their immediate surroundings, a process that requires prokaryotes to reside within
their food source, like worms inside an apple or a piece of cheese. Eukaryotic cells, on the other hand, almost all feed by phagocytosis and digest their food within small intracellular pockets called “lysosomes”; they are thereby freed from the residential constraints to which bacteria are subjected. Thus, the development of the phagocytic mode of cellular feeding probably represents one of the key events in the birth of eukaryotic cells, the source of their emancipation and their ability to adopt endosymbionts.

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.

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.

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 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
).

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.

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.

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.