Living Systems in Evolution
copyright © 1999 by Elisabet Sahtouris
Little more than one century ago, priests told people that the Earth was a few thousand years old; a few decades ago, scientists believed that life on Earth began only slightly more than half a billion years ago. Now we know that the Earth's skin was already swarming with fully evolved monera well over three and a half billion years ago. Two billion years, from about 3.7 to 1.7 billion years ago -- almost half the Earth's life -- belonged solely to bacteria. This chapter is the story of the dramatic leap into the other four kingdoms of life, a leap that took place about 1.7 billion years ago and which holds important lessons for humanity today.
Before we go into that story, however, let us recall the magnificent work of the bacteria in preparing the way for their own evolution into other forms of life. Let us recall that they invented all life's ways of making a living and created the conditions for life that we huge latecomers enjoy today. Without the bacteria, the Earth's atmosphere would be unbreatheable and its crust would have remained a cratered desert of glassy rocks. Without the activity of bacteria, even the oceans, as we saw, would have gassed off our planet.
When we left the story of evolution, the bluegreens were turning solar energy to use in making food, and turning food energy to use in the work of life. Their waste gas, oxygen, together with other waste gases, such as the methane made by the older bubbler bacteria, piled up, creating a new kind of atmosphere. Gaia had solved some big problems and was thriving.
The bluegreens tried out all sorts of new shapes and configurations for their one-celled bodies -- tiny balls, big blobs, long strings of individuals joined end to end, and even great sheets of them all stuck together with jellylike stuff and looking like seaweed. Some lived in large colonies, branching out into shapes that plants adopted when they evolved much later. Some even made spores in ways that remind us of plants making seeds. Perhaps the coded plans for such shapes and parts were stored in bacterial DNA and passed on for many millions of years until true plants evolved and found use for them. Other kinds of bacteria lived on their own as threadlike whips, lashing themselves from place to place. This was another early evolved structure that prefigured a way of moving that would evolve later in the dance with flexible backbones and muscles.
Besides trying out new shapes and movements, the monera evolved a division of labor that streamlined individual bacteria by reducing the amount of DNA and equipment each had to carry. Various kinds of monera became specialists at particular jobs, such as respiration, photosynthesis, or fixing nitrogen gas, yet all of them had access to the whole bacterial gene pool because they never lost their ability to trade DNA in their WorldWideWeb when necessary or desirable.
The new, streamlined specialists spread out into new habitats. Some did well in freezing cold waters; others lived in very hot springs. Some blew about in the air they had helped to create, then settled far from where they began. Some even found it possible to live and multiply on land, eating their way into rocks, where they started the processes that would eventually turn them into soil. But the more specialized the monera were, of course, the less independent they were, the more they depended on one another. Oxygen users now needed oxygen makers, food eaters needed food makers, nitrogen builders needed nitrogen fixers, and so on. In specializing, the monera were evolving what we call food chains, or ecological systems, in which each species provided for and took from others.
It seems that nature must always work out a balance between the independence and interdependence of individual creatures -- between their autonomy and their holonomy. Specialization -- whether in human society, ecosystems, multicelled creatures, or bacterial networks -- is a feature of whole systems that makes them more versatile and efficient through the interdependence it creates among parts. Specialization brings variety into the life dance, but increases holonomy at the expense of autonomy, since it increases interdependence. This balancing of autonomy and holonomy is very important to understand if we are to learn to manage our human affairs as well as Gaia has worked out hers.
We saw the bacteria multiplying to ever greater numbers, discovering ever more and different ways of surviving and making their living, adapting themselves to geological changes in their environment as well as to the changes they themselves brought about. Before the ozone layer had gathered, strong ultraviolet light often damaged their DNA, but they invented splicing enzymes to repair the damage, and they learned to share this information, as well as other information stored in DNA plans, with each other. Thus the genetic plans for many variations on their organization were kept available, to be borrowed and copied from one another.
The bacterial population as a whole could therefore respond to emergencies such as chemical changes in the environment by drawing on and quickly spreading the genes that best helped them cope with those emergencies. Nowadays, part of their coping in this manner is their developing resistance to man-made antibiotics, or learning to digest new foods in changing environments. We also observe, in the frequent failure of our genetic engineering, that organisms recognize and remove -- edit out -- implanted genes.
Huge teams of specialist bacteria were dividing up tasks, recycling Earth's materials, learning to balance the whole dance of life. But as specialists they also ran into trouble now and then. We can imagine that blue-green food makers needing light, for instance, must have found themselves stuck too long sometimes in dark places, and bubbling or breathing food eaters must have found themselves short of food because there was so much competition for it, so many mouths to feed, so to speak.
Ultraviolet light reaching the Earth had helped produce the sugars and other food molecules on which bubblers and breathers depended. As the ozone layer grew and began to screen out ultraviolet rays, this production was severely limited, increasing the competition for food to the point of crisis. The challenge of worldwide hunger seems to have pushed the monera to rediscover some old steps and patterns in their dance of life and weave them together into a new pattern that produced a very great leap in the dance of evolution.
This leap was accomplished when the ever more specialized bacteria got together within the same walls, where they could use their various ways of making a living cooperatively. In doing so, they evolved a very large and sophisticated new kind of cell -- a kind of cell so different from the bacterial moneron that it is more closely related to us than to any of its own ancestors.
· · ·
These new cells -- on the average a thousand times bigger than bacterial cells -- formed a second kingdom of life to join the monera: a kingdom of life we call protista. The name comes from a longer word, protoctists, Greek for first builders. Protists, like monera, are single-celled creatures, though much larger. Later in our story, they will go on to build multicelled creatures, but for now let us look at them as multi-creatured cells.
Although the big new protists, once evolved, were smoothly run cooperative ventures, they did not start out that way. In fact, the new step in evolution almost certainly began quite uncooperatively in the desperate search for food. With the growth of bacterial populations and a developing ozone layer, the time when free food lay or floated all around came to an end. Natural death had not yet been invented to recycle materials, as bacteria do not necessarily or normally die and dissolve into reusable parts. Rather, parent bacteria split themselves to become their own offspring, and ever more offspring made ever greater demands on the food they needed in order to build themselves to full size.
Earth's crust had come alive by packaging ever more of its atoms and molecules into bacteria, many of which depended on ready-made food supplies that were now limited by the new atmosphere. The planetary process of coming alive was thus in danger of choking itself off by overcrowding and lack of food. It is not impossible that something like this happened to Mars -- that Mars once came alive and then died for lack of supplies, or lack of a means to recycle supplies efficiently enough to keep its early creatures healthy.
On Earth, the evolution of giant cell cooperatives probably began when tiny energetic breather bacteria began forcing their way through the walls of larger bubblers to get at their rich molecules -- not entirely unlike the way in which we humans invaded each other's kingdoms and countries to get at supplies and raw materials.
The problem with this approach was -- as it still is -- that eventually the invaders run out of supplies again, having eaten up their hosts. In the long run, invaders have little more to gain than their victims. But this crisis was a new challenge to life, and life proved as inventive in this situation as it had been in the face of previous problems.
Unable to get rid of, say, the invading breathers multiplying within them, the big bubblers seem to have negotiated an agreement with them that was of benefit to both parties. Perhaps in return for feeding on the bubblers' molecules, the breathers gave the bubblers some of the ATP energy they could make so much faster. This is not unlike the deals made between countries, when, for example, a rich country offers electrification in return for a Third World country's food products. As we have learned, it is not easy to make such deals truly beneficial to both sides. Such a bacterial agreement would have helped the big bubblers repair themselves or make extra molecules to feed the breathers in return for supplying them with plenty of energy.
Arrangements of this kind must have been worked out successfully, because the first protist fossils from about a billion and a half years ago show lots of breathers inside single big cells that were apparently healthy. This tells us that the breathers multiplied to large numbers within the host cell without damaging it. When such swelled cells divided, half the breathers went to one side, half to the other. And so the cooperative exchange of food for energy continued in ever more descendant cells.
Later such cells seem to have taken in other partners, such as bluegreen specialists in photosynthesis. The bluegreens were probably eaten by the big hungry bubblers, but they apparently resisted being digested. Eventually another cooperative agreement was reached: the bluegreens would make food molecules, which the bubblers and breathers needed, by photosynthesis. This cooperative arrangement allowed the giant cells to make ATP energy in all three ways within the same cell wall and thus to survive through all kinds of shortages.
As these new partnerships became more complicated, they also grew ever larger and heavier, no doubt finding it difficult to keep from sinking away from light or from moving too slowly to find food. Another cooperative solution -- another transformation of imperialism into mutual aid -- permitted them to move themselves farther and faster.
Remember that some breathers were shaped like twisting, lashing whips? Others had invented a proton motor. This was a spinning disk in a field of electrical potential, complete with microscopic ball bearings and an attached tail, or flagellum, so the bacterium could propel itself, literally motoring about! In the slime cities mentioned earlier, they can be seen along the canals among skyscrapers.
What might happen if these mobile bacteria stuck themselves onto larger cells in order to suck food from them? The big cells would find themselves moved about by lashing tails and propellers. This is just what seems to have happened. The evolving cooperatives must have found the pushy bacteria worth feeding in return for being driven to where they could find food or light. Eventually, the propellers and flagellae evolved into shorter, stiffer cilia arranged in rows so their rotating movements could be timed like the oars of our ancient ships. Cilia were so successful that almost every plant and animal cell living today has some part or parts that evolved from these ancient rowing hairs.
But before these happy cooperatives had actually evolved, there must have been a phase in which it was not clear whether cooperation would win out over competition, when the evolving protists were like a factory of workers without management, or a world of separate nations trying to take advantage of one another and all trying to give orders at once. After all, each kind of moneron had its own DNA plans, its own welfare requirements.
What was called for was serious organization. And monera, as we have seen, had already evolved a very effective communications system that could make such organization possible -- their own WorldWideWeb. They knew how to trade DNA to revise their own plans; they knew how to work from a common gene pool. And apparently they drew on this experience to set up a new kind of local gene pool, or information center.
Instead of collecting one another's genes inside themselves, they streamlined themselves even further, giving up some of their DNA to a common gene pool of general cell plans, which became the cell nucleus. As time went on, the nucleus became a virtual library of information for producing proteins and, in ways we still understand poorly, probably took on the direction of the whole cell's affairs. The individual monera of the new cell became less independent but more secure, more inseparable parts of the new wholes.
This pattern of unity-> individuation-> conflict-> resolution-> cooperation-> unity-at-a-new-level-of-organization the ancient bacteria went through was mentioned in Chapter 2, and is a typically repeating cycle in evolution. We humans are engaged in today, as we learn to cooperate at a global level, thereby achieving a new planet-wide unity.
Nuclear DNA, originally donated by many generations of participants in each of the evolving cooperative cells, continues even in modern cells to contain a tremendous amount of duplication. Many theories have been proposed to explain the repetitive nuclear genes, but perhaps there is no more reason for them than there is for all the duplicated documents in our own government offices. In any case, the huge quantity of nuclear DNA could hardly be kept in bacterial style as loose loops floating about in the cell. It would have gotten tangled and broken, messing up plans. Besides, the DNA might well have been destroyed by all the oxygen-making going on in these cells. And so it seems that the DNA was collected from each part of the cell, wrapped tightly in complex close loops around proteins, and stored within a protective nuclear membrane. This nucleus became a kind of central cell office for keeping DNA plans in order, but accessible for use, thus making possible better organization of all the cell's work.
This nuclear central office and library evolved into a staggeringly complex yet elegant organization we are still working to understand. The DNA in each of our cells, if stretched out, would be about six inches (or 15 centimeters) long, though it is packed into the invisibly small nucleus together with proteins and water. A jet plane, as Jeremy Narby has pointed out, flying one thousand kilometers per hour would have to fly over two full centuries to reach the end of all the DNA contained in a single human body's several trillion cells strung end to end!
What is even more remarkable is that a single handful of rich, natural soil, a large part of which is masses of bacteria, contains even more DNA than that because of the closer packing of DNA throughout their tiny bodies. That means that our entire planet is coated in DNA -- just about the oldest surviving invention of all evolution -- the language of life itself. Only now do we recognize DNA as intelligent in its own right, when we see it rearranging itself appropriately within organisms under stress. How foolish, then, are we humans, to kill and sterilize our soils of all life with chemicals, thinking we know better than nature how to engineer and grow our food.
When it was time for the first nucleated cells to divide, each of several very long twisted DNA molecules unfurled itself, then split and replicated itself as it had in bacteria. Theses pairs stayed buttoned together at one point along their length, by a centromere, meaning central place (also called a kinetochore, or moving spot), while each member coiled neatly around itself, that coil coiling into a shorter one, and so on until it formed a compact chromosome thirty thousand times shorter than the DNA molecule was when it started out!
Later we will describe the next steps in cell division. Here we just want to indicate the remarkable feats performed within this nucleus -- this DNA-protein information center that evolved as the most important new feature of the giant cell cooperatives. Every living creature of Earth not a bacterium itself evolved from these nucleated cells, meaning that every living being of every kingdom of life beyond monera is made of the same basic kind of nucleated cell. Biologists call this superkingdom of cells, which includes the protist, fungus, plant, and animal kingdoms, eukaryotes (pronounced you-carry-oats, from the Greek meaning `with a karyon, or kernel -- the nucleus). To keep things straight, we call bacterial monera prokaryotes (pro-carry-oats, meaning `before a nucleus').
In evolving these eukaryotic protist cells as bacterial cooperatives, Gaia's creatures rediscovered some of the independence they had had before they became specialists depending on one another. The giant eukaryotes could now evolve new parts and ways of using them -- all sorts of special membrane walls and internal skeletons, gas-bubble vacuoles to control floating and sinking, other structures and chemical systems that helped do new jobs. A means was even found to circulate the jellylike cytoplasm in which all these structures are embedded, providing a transportation system for supplies and wastes.
Eukaryote cells, as we said, are on the average a thousand times bigger than prokaryotes, with a thousand times more DNA. They are in many ways as complex as human cities, or the bacterial colony cities described earlier. Until recently, scientists saw the nucleus as a computer behaving like an authoritarian dictatorship, containing all the information necessary to run the call and sending out `top-down' command and control orders for what is to be built, produced, carried about, or otherwise done. Now it seems that the governing of cells is more decentralized -- that the whole cell governs itself, using the nucleus as an information resource center.
Biology has made more progress in understanding the detailed composition of living things than in understanding their organization as a whole, but this trend is now shifting. In our analogy with cities, some cell parts are structures like roads and buildings; others are chemical messengers, carrying instructions to and from the nucleus; some are production centers like factories; others perform services, taking in and delivering food, collecting waste, making repairs.
With ever more powerful microscopes, moving picture microscopy and animation techniques, we begin to understand how busy and lively, how complicated and amazing, life is inside such cells. But what we see leaves much to be learned about how it is all organized to function so smoothly. Even structurally we are still discovering things of major importance in cells. For example, until very recently we thought cells were bags of jelly-like stuff with the nucleus and organelles suspended within the jelly. Then, to their surprise, microbiologist Don Ingber discovered that cells have an internal architecture not unlike our bone and muscle systems -- a tensegrity structure, with tension and compression components giving the cell integrity and the ability to move itself. Until his discovery, scientists had been dissolving this structure with the chemicals they used to prepare cells for study!
A huge new development is the understanding that nuclear DNA is reorganized in response to changes within and beyond the cell, that the entire cell, including its membrane or wall, is a creative autopoietic system.
All the cells of our own bodies (not counting the myriad bacteria living on and within us) are eukaryote cells, but we are just beginning to understand their evolution from ancient bacterial cooperatives. The story of this evolution has been simplified here. In actual fact it is one of the most fascinating and difficult puzzles ever to challenge biologists.
1993 Nobel Prize winning biologists Philip Sharp and Richard Roberts discovered that genes are broken into modules that can be reshuffled by spliceosomes referred to as a cell's `editors' because they snip out inappropriate DNA sequences occurring between meaningful `words.' Some of these modules are apparently shared by different genes, enabling evolution to proceed much faster than it could have if the old models held true. Especially interesting in light of this book's designation of a nucleus as a central library or information center is the new vocabulary of `editors,' `words,' etc. The picture emerging is consistent with the description given here of evolution as an intelligent process, rather than an accidental mechanical process. Sharp, in fact, speaks of a spliceosome as knowing where to cut and where to splice.
· · ·
The DNA plans and composition of our own cells are, of course, unique to humans and differ from those of frog and fern cells, bacterial or Bactrian camel cells. When it became possible to analyze DNA in detail, it also became possible to identify a species by its own particular DNA pattern. Many species look very much alike, yet can be distinguished by differences in their DNA patterns.
Among the parts of our cells outside their nuclei are large numbers of tiny things that produce ATP energy currency. These cell parts have long been understood as little mechanisms for burning food molecules with oxygen to produce the cell's energy. But with rising interest in and understanding of DNA, biologists made a very strange discovery: that these little machines have their own DNA, the coded plans of which are quite different from those of the nuclear DNA.
How could cell parts be of a different species than the creature made of those cells?
Clues soon turned up. However different this DNA is from the nuclear DNA, it was found to be rather like some other DNA that biologists knew about -- the DNA of bacteria quite like the breathers that evolved billions of years ago! At the time this was discovered, the story of bacterial cooperation in the evolution of eukaryotes was still unknown. Now that we know it, scientists are finding as many as a thousand different kinds of DNA outside the nucleus of a single cell.
The idea of cell symbiosis -- the origin of eukaryotes as prokaryotes living together in cooperatives -- had been proposed independently by a German, an American, and a Russian biologist around the turn of the century. All had noticed that the photosynthesizing chloroplasts -- meaning `green producers' -- in the cells of plants resembled bluegreen bacteria. The Russian, K. S. Mereschovsky, suggested that other ancient bacteria had evolved into other cell parts. But biologists, who were trained to see living things as put together from mechanical parts, could not see cell parts as creatures in themselves.
Thus the symbiosis theory was ignored until Lynn Margulis an American microbiologist who became James Lovelock's partner in developing the Gaia hypothesis, revived it and produced a great deal of evidence to support it.
After much work, Margulis and others have shown that these energy-producing cell parts really are descendants of the ancient breather bacteria that came to live inside larger prokaryote cells, cooperating in building the first eukaryote cells. Luckily, teams of biologists working to unravel the ancient mysteries of cell symbiosis have found many clues in the behavior of today's bacteria. Rather vicious breathers can still be found drilling their way into other bacteria to reproduce there and eat the host bacteria from the inside. In the Tennessee laboratory of Kwang Jeon, protist hosts so invaded learned to tolerate and then to cooperate with their invaders in a mutually dependent relationship that brought about a new kind of creature. Surprisingly, this replay of the ancient evolutionary shift from outright aggression to full cooperation happened in only a few years' time.
Today, we find the descendants of the ancient breathers living and multiplying in the cells of every kind of protist, fungus, plant, and animal. It's high time we knew them by name. They are mitochondria -- pronounced, mite-o-KON-dree-a -- a word that comes from the Greek meaning `thread grains,' because under a microscope they look like tiny grain hulls packed full of thread.
Using the oxygen we breathe, mitochondria make all the energy our bodies need to keep going and to repair themselves. Without our mitochondria we could not lift a finger. In fact, it is these swarms of ancestral bacteria, working night and day in all our cells, that keep us alive.
Or are we working for them? Lewis Thomas, in another of his perceptive and poetic insights, suggested that if anything in nature is a machine, perhaps it is us -- maybe we are giant taxis which mitochondria built to travel around in safely and comfortably. Certainly mitochondria have done very well spreading themselves all over this planet, inside every other living thing, almost since the Earth came alive. There are so many of them swarming in our own cells that it's hard to guess at their actual numbers, but all together it is estimated they would weigh almost as much as the bodies they live in -- that is, mitochondria make up much of our weight, and the weight of elephants and insects, clams and monkeys, toadstools and lizards, fish and worms.
In plants, from seaweed to sunflowers, potatoes, and palm trees, mitochondria live together with their relatives, the chloroplasts, which give plants their green color. You will easily recognize them as descendants of the ancient bluegreens and know that as they make energy from sunlight, water, and carbon dioxide, they also make the oxygen their mitochondrian cousins need for making their energy.
Mitochondria and chloroplasts, together with their still free-living monera cousins, the bacteria, are by far Gaia's most numerous and important creatures, though we are very late in recognizing our complete dependence on them. Quite to the contrary, we discovered bacteria in the context of medicine and treated them only as enemies. They are so hidden and tiny that, for years, we paid no attention to their good works, but we now know that their cooperation is the essence of the entire Gaian life system.
Besides making the vital gases oxygen and carbon dioxide for each other, the chloroplasts and mitochondria of eukaryotes, together with their prokaryote cousins, form other cooperative cycles, for example, the food chains mentioned earlier. While plants make their organic bodies from simple minerals, water, and carbon dioxide, animals can only make their bodies from the ready-made organic molecules of plants and other creatures. It is primarily bacteria that cause the decay of dead plants and animals, reducing them to the simple substances on which new plants can live.
Margulis' discovery, that eukaryote protists evolved cooperative internal schemes to overcome the problems caused by competition among prokaryote bacteria, was almost as much a shock to the world of science as was the Gaia hypothesis itself. Besides showing that cell `mechanisms' such as mitochondria are creatures in their own right, she was suggesting that harmonious cooperation played a big role in evolution. This ran counter to the beliefs stemming from Darwin's work, adopted by scientists in western countries, that evolution was just a survival race driven by competition.
The theory of evolution through competition has played a big role in our world, not only in science but in shaping our whole human outlook and way of life. Only by understanding its origin and its widespread effect on our lives can we understand how to change our view of ourselves and our role within our larger Gaian planet. Let's look, then, into the way our Darwinian view of evolution came about.