Living Systems in Evolution
copyright © 1999 by Elisabet Sahtouris
If you look at a drop of pond water under a microscope, you will see a great variety of protists living together, creatures you would never have suspected were there, unless you've done this before. You may see, for instance, paramecia, tiny slipper-shaped algae rowed along by lineups of waving cilia oars. Then along will come a giant, blobby amoeba, changing shape before your eyes as it pushes out pseudopods, `false feet,' in its search for food. Perhaps it will get stuck in a tangle of rod-like algae, strings of cells sitting quite still and making energy from light with their green chloroplasts. Other green algae, such as euglena with their long whip-like tails, may also flit by.
If you look instead at a drop of seawater, you may see whirling dinoflagellates that glow in the dark by making their own light or diatoms and radiolaria, looking like beautiful crowns or glass ornaments. There seems to be no end to the fantastic patterns of these tiny single-celled creatures, whether they live alone or stuck together in colonies like the ball-shaped volvox.
The world of single-celled creatures in a drop of water is probably much as it was a billion years ago when there were no larger creatures. Yet these same protists went on to build and evolve all the larger multicelled creatures of Earth
On today's oceans great blankets of plankton, mostly made of protists, float about ever renewing the atmosphere with nitrogen, oxygen, methane, and other gases they release. Sulfur dioxide produced by plankton actually seeds the water droplets forming clouds. This means that the cloud cover over much of the planet, and thus the planet's warming and cooling system, is regulated in large measure by these tiny creatures. They also provide food for many heterotrophic species, from the almost microscopic shrimp that swim among them to the largest of whales. The wastes and dead bodies of those that eat plankton sink to the bottom where myriad saprotrophs decay them back into molecules that may come to the surface to nourish new plankton.
Plankton not only serve as part of a food cycle but play most important roles in balancing the chemistry of the atmosphere and the seas, as well as in the geobiology we talked about in the last chapter -- the transformation of the Earth's crust from rock to living matter and back to rock.
Among the creatures forming plankton are thousands of species of diatoms and radiolaria, those particularly beautiful protists whose shells look like fancy glass ornaments when seen through a microscope. In Gaia's dance there are more diatoms than any other kind of creature except bacteria and they are responsible for transforming a great deal of rock.
Land rock, we know, is dissolved by streams and rivers into salts and minerals, which they carry to the sea. Life in the sea needs these building materials, but like the gases of the air, they must be kept in balance. If too many of them pile up in the sea, living creatures will choke. One such mineral is silicon, in its silica form (silicon dioxide). A huge amount of silica is washed into the seas every year -- hundreds of millions of tons of it. But huge numbers of diatoms wait in the sea for these silica supplies, for silica is just what they need to build their sparkling shells. When they die, the diatoms sink to the bottom, leaving their silica shells to settle into rock -- three hundred million tons of silica rock every year!
The number of diatoms in the sea naturally adjusts itself to the supply of silica brought by the rivers. There is always just the right number of diatoms to use up the silica dissolved in the sea. This well-balanced system -- water dissolving rock, diatoms and other protists making it into lovely silica shells, then being eaten by others or dying so their shells sink down to the sea bottom to become new rock -- worked itself out in co-evolution as a kind of transport system within the great Gaian body. The shells of radiolaria are also part of this process. The famous white cliffs of Dover on the English coast are ancient deposits of the microscopic, chalky, snaillike shells of yet another marine protist -- foraminifera -- which were pushed out of the sea by the endless motions of the Earth's crust.
Many other geobiology cycles or systems are still to be discovered as we keep working on the puzzles of planet physiology. Unfortunately, our old view of nature made us see ourselves as just one of the many creatures competing for survival on a planet without enough for all to live. But now that we have microscopes, telescopes, spaceships, computers, and other instruments that show us so much more than we could see with our eyes alone, we are in a position to understand the pattern of life within life from the largest to the smallest holons.
We know the mitochondria cooperating in our cells long ago worked out a mutually consistent way of life with other cell parts. We know they and we have a mutually consistent arrangement as we provide their fuel and safety while they make our energy. We can see how all species, including our own, must work out their mutual consistency with one another as co-evolving parts of the great Gaian body.
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Of all the cooperative steps in Gaia's dance, we saw that one of the most important was the invention of sex -- the sharing of creature plans by uniting DNA from more than one source to create a new being. Because the exchange of genes among bacteria worldwide was and is so free and continual, biologists had to give up their attempts to classify species of bacteria -- recall that species are identified by their DNA. We can only identify strains of them that keep recognizable forms despite the free trade. Remember that this kind of original sex had nothing to do with reproduction, as Margulis pointed out in tracing the origins of sex. It is because of this sexual freedom, this efficient communications system lost forever in later kingdoms of life, that bacteria could remain streamlined creatures with tremendous flexibility, able to trade information worldwide and thus solve almost any emergency situation.
In the kingdom of protists, sex took some strange new twists, very likely quite by accident. These twists eventually linked and limited their sex to reproduction and to two partners within the same species. Thus the boundaries of sexual reproduction became our way of defining species boundaries. The within-species sex of the protist kingdom was passed on to multicelled creatures though sometimes different species co-evolved to help each other in their reproduction, as in the case of flowering plants cross-pollinated by birds, bats, moths, bees, or other insects. But before we get to larger creatures, let's see just how the kind of sex we know -- the production of offspring by the mating of two parents -- came about among protists.
A prokaryote without a nucleus, remember, reproduces by splitting or budding after its loose DNA loop unzips and copies itself. Even though this process gets a bit more complicated with the great quantity of DNA packed into a eukaryote's nucleus, most eukaryotes, including our own body cells, also divide in this way. The process begins with the unzipping and copying of each section of DNA, their neat coiling and recoiling into chromosome pairs buttoned together by centromeres or kinetochores, as we saw in Chapter 6. The nuclear wall dissolves to release them, and the cell constructs a fabric of microtubules on which the pairs of doubled chromosomes line up. These then unbutton themselves by dividing their kinetochores, which then button themselves to the tubes such that the two members of each pair of doubled chromosomes ride smoothly in opposite directions, pulled by the kinetochores to opposite ends of the cell before it divides into two with a full set of chromosomes each.
This usual way for cells to divide is called mitosis -- mito, as in mitochondria, meaning thread, such as humans invented for weaving. If you could watch cell mitosis through an electron microscope, the neat formation of microtubules called the mitotic spindle and the shuttling of chromosomes along its threads would indeed look like a weaving process.
Mitosis is a non-sexual, or asexual, way to reproduce. The offspring of mitosis are clones -- offspring of a single parent cell usually thought of as being exact copies of that parent. Of course we know that our whole bodies are cloned from the single fertilized egg we began as, and our cells are very varied. The idea that clones are all alike is linked to the idea that sexual reproduction is what brought variety into evolution. But the clones of rare asexually reproducing animal species, such as certain lizards, are quite as varied as the offspring of the more usual sexually reproducing species.
If sexual reproduction did not evolve by natural selection for the advantage of variety, as scientists thought for so long, then why did it evolve?
Again we owe the tracing of a story from the ancient microcosmos to Lynn Margulis and her co-workers, who combined clues from earlier biologists and from their own research. Margulis noted that sexual reproduction has three important aspects: the halving of chromosome numbers within each parent, the doubling of chromosome numbers by bringing two parent cells together, and the alternation between these two stages of halved and doubled numbers generation after generation.
How odd to halve chromosomes continually, only to double them again. Margulis investigated this mystery by looking for cases of halving and doubling chromosomes in the microcosm and for ways in which they might have become linked into a single reproduction system. The story that emerged is, like the evolution of eukaryote cells, one that begins with exploitation and ends in cooperative partnership, and once again starvation is the initial motive.
A desperately hungry protist, even today, may resort to cannibalism, and on occasion may fuse the swallowed victim's nucleus with its own. All nucleated cells will fuse with one another under the right conditions. Doubled chromosomes may also come about when a protist begins mitotic division and is then unable, for some reason, to finish the process, failing to divide after doubling the number of its chromosomes and fusing them back into a single nucleus. This, too, has been observed.
In either case, the extra chromosomes may work well in times of need but become unwanted extra baggage when things go well. So protists learned long ago-over a billion years ago, which was not so long after they had become protists -- to reduce the number of chromosomes again when this was to their advantage. The process of halving a cell's chromosomes is called meiosis, which means `lessening.' Some protists seem to have become experts at doubling and halving their chromosomes according to the demand of changing conditions from drought to plenty and back.
The fusion of two sets of chromosomes into a single nucleus -- if they are from different protists, even if one protist has eaten the other -- is a sexual union. The halving of chromosomes in meiosis, as we just saw, was a solution to the unnecessary and troublesome burden of doubled or tripled chromosomes. What we call the haploid, or `half-set,' of chromosomes that seeds, pollen grains, eggs, and sperm contain, are half of our normally double, or diploid, number.
The chromosomes of all our body cells are paired, one of each pair from each of our parents. Far back in evolution this doubling must have occurred as described, and stuck as the normal number. When sexual nuclear fusion became linked to the reproductive formation of new generations of individuals, the double number had to be halved before each sexual-fusion and reproduction event to avoid doubling the chromosomes mercilessly in each generation, which would have been literally a dead end. Sperm, pollen, and egg cells are all produced by this meiotic halving process in such a way that the fusion of egg and sperm or egg and pollen results the normal diploid chromosome sets of animals and plants.
Cannibalism, fusing nuclei and then reducing chromosomes again, accidents of timing, and perhaps other events finally worked themselves out into a reliable system of sexual reproduction -- not the most elegant system nature ever devised, but one that has obviously worked well enough in the world of creatures less elegant and less sexually free than bacteria.
To carry out the work of a developing embryo, the DNA of haploid chromosomes from two parents must match stretch for stretch, gene for gene. As new species branch away from each others DNA sequences, their offspring may be infertile, as in the case of mules born to horses and donkeys, or tiglons born to lions and tigers. With further separation, mating become unproductive and ceases altogether. Branching species usually branch by occupying different ecological areas and so do not normally find each other to attempt mating, but humans have shown the possibility, though their hybrid offspring are sterile.
The change to the new way of reproducing did not happen all at once. Even today, in fact, some protists, such as paramecia, still reproduce both in the old way of mitotic cloning and in the newer way of sexual reproduction. Paramecia are a good example of nature's experiments with sex and reproduction, having as many as eight different sexes rather than only two, if we so want to label mating or gender types.
Many protists can reproduce with or without sex, that is, sexually or asexually. Sometimes one way serves their needs better, sometimes the other. Diatoms, for example -- those lovely tiny creatures with the fancy silica shells -- tend to reproduce just by mitotic splitting. They manage this by making their shells in two pieces that can come apart, one piece slightly larger, fitting neatly over the edge of the other to close it, just like a round pillbox with a lid.
The trouble is that while all the offspring must complete the half-shell box they inherit, diatoms have learned only how to make the smaller bottoms of box shells when given half a shell to start with. That means the split-off diatom that gets the bottom of the box has to use it as a top, making a smaller bottom than its parent diatom had. Over generations, then, lines of offspring inheriting bottom shells get smaller and smaller -- only those with direct inheritance of the original top shell maintaining the original size. This problem is eventually solved by sex!
Instead of continuing to split by mitosis, they resort to meiosis, producing little packages of single-set chromosomes called gametes, or sex cells, that have no shells at all and behave the way eggs and sperm do. When two of these gametes get together, it seems they have all the DNA plans for a new diatom, including plans for the top and bottom parts of a normal-size diatom shell. That way even the smallest diatoms can bring themselves, or at least their offspring, back to full size.
Sexually reproducing protists -- giants in a world of bacteria -- evolved into a new variety of complex patterns tailored to different lifestyles and environments. The co-evolution of monera and protists with these environments led to one improvisation after another, including the protists' formation of a new kind of cooperative that led to multicelled creatures, and thus to the remaining three kingdoms of life-fungi, plants, and animals -- in all their visibly fantastic variety.
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We have watched the Earth transform itself from a fiery ball to a crusty planet whose skin came ever more alive as giant molecules and enzymes formed, then packaged themselves as bacteria. We've seen living masses of microbes transform themselves as they discovered new lifestyles, rearranging and recycling minerals, creating the atmosphere, and so on. We saw how the monera collected themselves into the much larger protists, inventing nuclei and stumbling on sexual reproduction. Now our story continues with the evolution of ever larger and more complicated creatures.
Some protists began living together in colonies by sticking together after division rather than floating off on their own, though each protist in the colony remained as independent as if it had gone its own way. But just as protists evolved when various monera began working together as cooperatives, protists living together as colonies eventually took the next step of communicating with one another and developing joint plans.
After all, communication had always played a major role in Gaian life, from the bacterial Web to the DNA-RNA-protein communication systems between the nucleus and cytoplasm of eukaryote cells, and the communications systems between cell membranes or walls and the outside world. Now it was time to develop communications among eukaryotes, and one way in which they did it was by sending chemical messages to each other.
This chemical communication made it possible for the individual protist cells to harmonize the things they did -- such as beating their cilia oars together in rhythms that moved the whole colony smoothly along in one direction. The ability to communicate soon became useful in many new ways of cooperating, especially in divisions of labor among different cells in protist colonies, thus beginning the evolution of multicelled creatures.
A most peculiar and fascinating in-between step in the evolution of multicelled creatures from single cells is the appearance of slime molds. Scientists have devoted whole careers to studying these strange creatures that seem to be part protist, part fungus, part animal. Slime molds start out as separate amoeba-like creatures, but when their food source runs out, they emit chemical messages that attract them to one another until they gather together into a visible slug-like community. You can see their jellylike mass sometimes on the underside of a rotting log or leaf. Some varieties form a large sheet of jelly.
The slug-like community can actually move itself about like a brainless worm, but eventually it stops and begins sprouting stalks that form fruiting bodies on their tips. These then release spores -- tiny dried-up packets of DNA and other cell materials -- the way any self-respecting mold would. The spores blow through the air and, after settling in a new moist place, form new amoeba-like creatures to start the cycle all over again.
Slime molds thus are capable of specialization and cooperation under hunger conditions, if not otherwise. Note that we have now found hunger as the prod behind the cooperative evolution of nucleated cells, the invention of cooperative sexual reproduction, and the evolution of multicelled-creature cooperatives -- all creative responses very different from the competitive struggle Darwin attributed to food shortages.
In other ancient protist colonies that did not lead the double lives of slime molds, some of the cells became specialists at making food, others at catching it, still others at breaking down and digesting food. In some colonies there were specialized cells to move the whole thing about; others contained specialized cells for sticking it tight onto rocks. The first multicelled creatures were cooperative colonies of protists, just as the first protists had been cooperative teams of monera -- multi-creatured cells. Our present human process of globalizing seems to be forming us into a new planet-sized multi-creatured cell, in what we might call a fractal biology of repeating evolutionary patterns. But let us go on with the story of the first multi-celled creatures for now.
In some colonial creatures, certain cells began specializing in the sexual reproduction of the colony as a whole. That meant a few cells could reproduce the whole colony, instead of each cell in the colony reproducing itself. This was a big step in the evolution of colonies into creatures and in the evolution of embryos as a way of starting new generations. For the continued life of the species, all the other cells now depended on the specialized cells that produced gametes -- those haploid chromosome packages we saw diatoms making. We call them gametes because they were not yet either the specialized eggs and pollen of plants or the eggs and sperm of animals. Early gametes from different parent looked alike, though they had to pair to make a new being.
The development of multicelled creatures that reduce themselves back to single-celled creatures in each generation to carry out their reproduction, brought inevitable death by aging into Gaia's dance -- a new way to ensure recycling. Bacterial progenitors, remember, do not die except by unfortunate accidents, such as being burned by ultraviolet. These in fact happen often enough to keep bacterial populations within bounds, but they do not die of old age. Instead, they phase out their physical and genetic identities over generations of gene-trading offspring.
Multicelled plants and animals, however, leave their bodies behind as their genes continue on in new generations. DNA is the oldest living survivor in all nature. From a microbial point of view, the large multicelled bodies cloned from eggs and seeds have no further value as new generations emerge, except as excellent sources of food.
The fact that death is necessary for multicelled life to continue virtually without end has been hard for us humans to grasp and accept. If new creatures kept coming to life without others giving up their lives, the supplies in the Earth's crust would soon be used up and the mass of creatures would all die together of crowding and starvation, as we humans are rapidly learning from our successful efforts to increase food supplies and delay death.
With the death of creatures so that others can recycle the materials of their bodies, life can go on and on. In fact, the way living things die to make way for new life in Gaia's dance is very much the way things happen in any dance. Every dancer knows that each dancer can only perform one step at a time -- that old steps must be abandoned so that the dancer's body will be free to perform new ones, which may then repeat or change the pattern of old steps.
Gaia, our living Earth, has lived for billions of years and has billions more years of life ahead. Our individual bodies will die and be replaced by others, much as the cells of our own bodies are constantly dying and being replaced by new cells. Every seven years or so we are in fact a wholly new person through such replacement of cells, yet we only see the changes in our bodies as aging, not as endless newness. In the same way, from Gaia's point of view, there is no death -- just endless replacement of old cells in her body with new ones.
Every atom that is now, ever was, or ever will be a part of us will live on somewhere in Gaia's ever-evolving dance for billions of Earth years yet to come. Even after the Earth dies, those atoms will live on as part of our galactic dance, some perhaps finding their way into new living bodies of new planets.
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Animals, as part of our own planet, were a marvelous evolutionary development in the face of yet another problem. Some protists, after running out of food, were unable to make their own food from light because they contained no bluegreen bacteria, or their chloroplast descendants. We are not sure whether our own protist forebears never took in any bluegreens or whether they took some in and later lost them. We do know that plants have always had both chloroplasts and mitochondria, which allow them to make food using sunlight and to burn food using oxygen. Animals, including ourselves, can only burn ready-made food.
This means that plants could -- and still do -- live their whole lives sitting in one place, making their own food, while animals had to evolve ways of going after their food. Animals, as we will see, evolved all sorts of equipment, from eyes and ears to feet and wings, to heating and cooling systems, to nervous systems with brains for organizing all this complexity, just to help them chase after food -- and all because they had no chloroplasts!
Nature is, of course, never quite as orderly as we would have her, so she managed to leave around some puzzles such as the giant green clam, an animal that does have chloroplasts and uses them to make emergency energy from sunlight, though it is clearly an animal in every other way.
Among the earliest multicelled animals to evolve from protist colonies were polyps. Luckily, there are still many living polyp species that match ancient fossils and so give us clues to their early evolution. Actually, polyps look more like plants than the animals they are. Sea anemones, which look like flowers, are polyps; forests of coral are huge polyp colonies.
The polyp animal is shaped like a tube with a flowerlike circle of tentacles at one end around its mouth. The other end of the tube is stuck to a rock or to the body of another polyp in its colony. And there it stays. It is a simple animal with a body organized to catch its prey in its tentacles and stuff the food into its mouth.
Still, many polyps have rather amazingly complicated cells along their tentacles. These cells have a special name; we call them nematocysts -- meaning `thread bags' -- because they evolved from ingrown cilia that grew into extremely long, thin, hollow threads and became very specialized in their job. When prey touches one of these surface cells, the long coiled thread shoots out under the pressure of liquid filling it, tangling the victim and paralyzing it with poison barbs. Nematocysts are a wonderful example of the amazing patterns of organization that nature has worked out even within the cells of the smallest and simplest of creatures. Nematocysts are such good self-contained weapons that other creatures, after eating polyps, may not digest the nematocysts but may, instead, keep them for their own use in catching prey.
Polyps reproduce by budding like bacterial forbears. This job is sometimes assigned to certain members of a polyp colony, which are fed by the others so they can concentrate on their important work. In some species the polyp buds grow up stuck to the parent, but in others something much more interesting happens. The newly budded polyp breaks off, flips over so its tentacles hang down, and floats off into the sea. As it grows, it becomes a glassy bell or umbrella with a softly fringed edge of trailing streamers -- a jellyfish, as we call it, though it is not a fish at all. Its proper name is medusa -- a name taken from the ancient myth of a woman who had snakes on her head instead of hair.
Medusae are a much more adventurous stage of polyp life that learned to reproduce sexually. Some species tried having both sexes in the same individual -- as flowers and earthworms have them -- while other species began making separate males and females. In any case, all medusae produce female eggs and male sperm, which fuse to make baby medusae. The baby medusa is so different from its parents that it, too, gets its own name. We call it a planula. The planula is a long, flattish blob that rows itself about freely for a while using a fringe of cilia. Then it settles onto a rock and sticks itself tight to grow into a polyp.
The life of a polyp is thus a matter of metamorphosis -- changing form from planula to polyp to medusa. Such metamorphosis was later repeated in evolution, in butterflies and moths, for example, like so many other earlier step patterns that are woven again and again into the later dance. Polyps in countless variety still abound in the seas, looking much as did their ancient forebears. Yet sometime, somewhere in the dim past, some of them became discontented with this three-stage metamorphosis, which always came back to a sedentary phase, and went on to invent more adventurous lives.