The conditions on the early Earth
were totally unlike the present. There were no seas and continents. The oxygen
in the air, so vital to life now, wasn't there. Like Mars and Venus today, the
early atmosphere was probably largely carbon dioxide. The present conditions
on Earth evolved over the billions of years since, together with life. Life
has not been a passive passenger on Earth, but has been a major shaper and maintainer
of the conditions it needed, as we shall see.
In its early stages, the Earth was hot. At its centre was the heavier core of iron and nickel, which made up most of its mass. Floating on the surface was a thin skin of lighter rock. Much of the surface was volcanic. There were no oceans because the surface was too hot for water to remain liquid.
The atmosphere was largely carbon
dioxide, but with some hydrogen, helium, hydrogen sulfide and water vapour.
Any rain that hit the surface of the Earth boiled away immediately.
The Hadean age
This period before life is called 'Hadean' or 'Hell-like' and fits well with the classical images of Hell.
Eventually, the Earth cooled until
its surface temperature dropped below the boiling point of water. The rains
fell for millions of years and the seas were formed. The earliest sedimentary
rocks date from this time.
When the seas had formed and the Earth was cool enough, conditions soon became suitable for the formation of the earliest forms of life. And life did form almost as soon as it became possible, within a few hundred million years.
Y: You call a few hundred million years 'soon'?M: On these time scales, yes.
Those early seas are what we call the 'primordial soup'. The seas and the atmosphere contained molecules which were simple combinations of hydrogen, oxygen, nitrogen and sulphur which later came to make up most of the components of life.
Local microenvironments formed where conditions were more stable and in which larger molecules could form. Simpler molecules were continually forming into more complex molecules and breaking up again to form simpler molecules, their links forged and destroyed by the energy of the Sun. For example, molecules of hydrogen cyanide (one hydrogen atom plus one carbon atom plus one nitrogen atom, written as HCN) could form a chain by connecting to itself five times. The result is adenine (H5C5N5), one of the main components of DNA.
Some of these molecules turned out to be catalysts: they acted as templates on which other molecules assembled to form larger, more complex molecules. Without a template, molecules assembled only by the chance combination of their ingredients. With a template, the same molecule could assemble much more readily. In effect, the template formed a map of the structure of the molecule it helped to form.
More and more of these reactions occurred, with simpler molecules forming larger ones, which later broke up again into their simpler components. Chains of reactions began to occur, where the results of some reactions were the starting materials for other reactions, whose results were the starting materials for still other reactions, and so on.
Eventually, some chains of reactions appeared which were closed: the results of the last reaction were the starting materials needed for the first one, so the chain could start again. By this point, life was not far away.
The other necessary component for life was the membrane: a molecular net with holes which allowed some materials through and blocked others. The earliest of these were simple repeated assemblies of molecules, formed by chains of molecules sticking to each other.
Put these all together&emdash;the molecules, including the catalysts or templates, taking part in closed chains of reactions, continually re-forming themselves inside a membrane which keeps all the materials together&emdash;and you have a rudimentary cell. This was the forerunner of today's bacteria, and the basis of all later life.
There is much more to the story before a fully bacteria-like cell appeared, but that is the start of it. The membrane had to allow in any raw materials not created by the cycle of reactions and allow out any by-products not used in them. The templates had to be able to re-form not only themselves but also the membrane. The whole had to be sufficiently robust so as to be able to re-form itself against damage caused by the continually changing micro-environment.
Y: So that is the secret of life! Are you sure about all this?M: That is the best I can do for this part of the Story, and no, I'm not sure about it. Almost none of it is based on direct evidence. It comes from a combination of laboratory experiments, mathematical and computer simulations, and a lot of theoretical speculation. It is all other people's work, and the details of it are way beyond my understanding. There are great gaps in it, but it seems plausible to me.
Far more important, and the reason I'm including this, is that the patterns and processes I've been describing here are similar to others which will appear later in the book. It is about the forming and re-forming of patterns from simpler components. This applies to higher forms of life. It also applies to social patterns, both constructive and destructive as we shall see. It all makes up a grand pattern, whose consistency on many levels will add to its overall plausibility.
These primitive cells again show the whirlpool-like pattern: a form which is continually re-formed from a surrounding sea of its parts. With a cell, however, there is an important extra, the templates. The templates are the genetic material, the RNA and DNA which appears in every cell in all living creatures. They provide a description of the essentials of the structure of the cell. It is that description which gives the cell its coherence, enables it to maintain its wholeness.
We now have two ways in which a form can be maintained, the simple whirlpool-like form and the form which also contains its own description and uses it as a way of maintaining its wholeness. These two patterns will come up again several times later in the book, especially when we look at social patterns and the possibilities of coherent cultures.
These earliest forms of life developed
from primitive proto-bacteria to full bacteria over an extended period following
the formation of the seas, and it was still longer before any more complex life
evolved. This period, when bacteria were the only form of life, is called the
Archaen, meaning 'ancient' or 'beginning'.
Bacteria are much simpler cells than those which make up plants and animals. Plant and animal cells have a much more complex structure, including a separate nucleus within the cell membrane where most of the genetic material is contained, other internal membranes, and numerous small internal structures called 'organelles'. The reproduction of plant and animal cells is also much different from that of bacteria. To understand the development of these more complex cells, we first have to look more closely at the life of a bacteria.
Y: Just a minute! Surely bacteria are either plants or animals? Are you saying they are something different?M: Yes. The old basic division of life into two kingdoms: plants and animals is now obsolete. There are many creatures which do not fit comfortably into those two categories. The modern basic division is between cells with and without a nucleus. Those without a nucleus are the bacteria. They evolved first, in the Archaen, nearly 4 billion years ago. What we now call plants, animals and fungi came very much later, less than half a billion years ago.
A bacterium contains only a minimal set of genetic material, barely more than needed to re-form itself and reproduce. Yet, bacteria are extremely adaptable. They can survive in an amazing range of environments, from nearly boiling waters, airless conditions, and environments which would be poisonous to any other creatures. They adapt rapidly to changes in their environment, as is clear from the way they have developed resistance to many antibiotics. How does this happen?
Margulies and Sagan give a vivid description [3]:
"Its minimal number of genes leaving it deficient in metabolic abilities, a bacterium is necessarily a team player. A bacterium never functions as a single individual in nature. Instead, in any given ecological niche, teams of several kinds of bacteria live together, responding to and reforming the environment, aiding each other with complementary enzymes. The various kinds of bacteria in the team, each present in enormous numbers of copies, coordinate the release of their enzymes according to the stages in a task. Their life cycles interlock, the waste products of one kind becoming the food sources of the next. In huge and changing numbers, they perform tasks of which individually they are incapable."
The complementary metabolisms of the members of a colony of bacteria resemble, on the next larger scale up, the complementary nature of the reactions within each bacterium, with different reactions contributing materials needed by others and using the results of others.
In bacteria, sex and reproduction are completely separate. Reproduction generally occurs by a bacterium growing to twice its normal size whereupon its single strand of DNA duplicates itself and then the cell splits into two identical cells. The daughter cells are genetically identical to the single parent (which no longer exists!)
Sex, in bacteria, means the exchange of genetic material. Many mechanisms are available for this. Two bacteria may combine into one, which ends up with all the genetic material.
M: This might mean that for bacteria, sex and eating are not all that different!
Otherwise, separate bacteria may exchange genetic material through a small tube which forms temporarily to join them (called "conjugation"). Also, small bits of genetic material may get packaged up in various ways and travel between bacteria. The various forms of these packages include plasmids, phages and viruses. Viruses, which can be so deadly to creatures with complex nucleated cells like us, are not a separate form of life, but are just a normal part of the sexual repertoire of bacteria!
In plants and animals, the genetic material in one generation is very much like that in the previous generation. The form of successive generations changes very little within any species. Bacteria are much more fluid. They are continually changing their form and their genetic material, often very radically. This is how resistance to drugs can spread so rapidly from one form of bacteria to another.
Y: Why this prurient interest in the sex lives of bacteria?M: You may joke, but this is a crucial point. Do you remember that one of the aims of Part I of this book was to squash the idea that competition is the basic organising principle of nature? It should be clear from what I've said so far about bacteria that to explain their behaviour in terms of competition is to leave out most of what is significant.
The lives of bacteria are totally interconnected and interdependent. They respond to changing conditions as a group. By modifying the mixture of metabolisms available they adapt as needed. Because of their genetic fluidity, there is a sense in which all the bacteria on Earth can be viewed as a single species. Because of their group interdependence, there is a sense in which they can even be viewed as a single, global super-organism[4]. To see individual bacteria as in competition with each other is to misunderstand the nature of their lives. An individual bacterium's 'fitness' to survive depends upon, at the very least, the adaptability of the local colony of bacteria around it.
The earliest bacterial colonies used as their inputs, their food, the most readily available molecules found in the primordial soup. These were simple sugars, carbohydrates (which are chains of sugars) and alcohols. This is the metabolism of a fermenter. These early forms of metabolism are still with us in the modern bacteria which make our cheese and wine and which live in the guts of most animals and form a vital part of the animals' digestive system.
There were many different forms of fermenter, with different metabolisms. In any colony, one fermenter's waste was another's food, so that the basic materials were recycled. Recycling and reuse of materials has been a basic principle of life from the start.
As the early baterial colonies grew and spread, more and more of the available materials became incorporated into their bodies. Over time, many new metabolic pathways were developed enabling spreading life to eat up more and more of the primordial soup.
There must have been many local crises, where no more soup was available and colonies died out. There must also have been many times, when under this pressure, a new metabolic pathway was discovered, and a new source of food allowed the colony to continue. Through the Archaen, nearly all the metabolic pathways used by bacteria today evolved, and these are the building blocks of the metabolisms of all the more complex forms of life.
One particularly important new metabolism to arise was photosynthesis, in which sunlight was the energy source. The earliest photosynthetic bacteria were sulfur breathers. They used sunlight as their energy source but they gave off hydrogen sulfide (the gas which gives rotten eggs their smell), not oxygen as do modern plants.
A later form of photosynthesizer took in carbon dioxide gas, which was then abundant in the air and dissolved in the surface of the early seas. Carbon dioxide and water itself provided most of the materials needed. These bacteria extracted carbon from the carbon dioxide, hydrogen from water and used them to build the molecules they needed, giving off oxygen as their waste product. Thus the major metabolic pathway used by all modern plants had arrived.
This was a key breakthrough for life. It had resolved its first major crisis. Life had eaten up most of the primordial soup, but could now continue to grow with carbon dioxide as its food.
Y: Are you saying that photosynthetic bacteria replaced all the others?M: No, not at all. Remember the interdependence, interconnectedness and recycling. All the others could continue too.
This was the beginning of the dramatic changes life was to make to the Earth. It had eaten up the soup, turning it into bacteria, and was now beginning to change the atmosphere. The long process of change had begun from an atmosphere composed mostly of carbon dioxide and no oxygen to today's atmosphere which contains only .03% carbon dioxide and about one fifth oxygen.
Today, we hear about carbon dioxide as a 'greenhouse gas', and that increases in it are leading to global warming. In the late Archaen, with levels of carbon dioxide falling, the world cooled. The first Ice Ages occurred.
We shall take this event as a rough
marker of the end of the Archaen age.
A snapshot of life in the late Archaen/early Proterozoic age"To a casual observer, the early Proterozoic world would have looked largely flat and damp, an alien yet familiar landscape, with volcanoes smoking in the background and shallow, brilliantly colored pools abounding and mysterious greenish and brownish patches of scum floating on the waters, stuck to the banks of rivers, tinting the damp soils like fine molds. A ruddy sheen would coat the stench-filled waters. Shrunk to microscopic perspective,
a fantastic landscape of bobbing purple, aquamarine, red, and yellow
spheres would come into view. Inside the violet spheres of Thiocapsa,
suspended yellow globules of sulfur would emit bubbles of skunky gas.
Colonies of ensheathed viscous organisms would stretch to the horizon.
One end stuck to rocks, the other ends of some bacteria would insinuate
themselves inside tiny cracks and begin to penetrate the rock itself.
Long skinny filaments would leave the pack of their brethren, gliding
by slowly, searching for a better place in the sun. Squiggling bacterial
whips shaped like corkscrews or fusili pasta would dart by. Multicellular
filaments and tacky, textilelike crowds of bacterial cells would wave
with the currents, coating pebbles with brilliant shades of red, pink,
yellow, and green. Showers of spores, blown by breezes, would splash
and crash against the vast frontier of low-lying muds and waters." Margulies and Sagan, op. sit. p.97 |
Oygen was not present in significant quantities in the early atmosphere. It is too reactive. It combines so readily with many other substances that it does not persist naturally as free oxygen. A water molecule (H2O) is simply two atoms of hydrogen combined with one of oxygen. A carbon dioxide molecule (CO2) is one atom of carbon combined with two of oxygen. Rust is iron oxide, or iron combined with oxygen.
For a long time, the oxygen produced
by the early photosynthesizers was rapidly absorbed by the rocks. Some ancient
banded iron rocks containing layers of iron oxide have been found, which are
evidence for this process. Much of our present iron ore dates from this time.
Early banded iron rocks are evidence
for oxygen
produced by early photosynthetic bacteria.
Eventually, the oxygen given off by bacteria was more than could be absorbed by the rocks. It then began to build up in the atmosphere. This was a major change to the Earth. While to us oxygen is vital and a key to life, to the bacteria in the late Archaen it was a deadly poison. The reactivity of oxygen destroyed cells. The build-up of oxygen in the late Archaen was the greatest pollution crisis life the Earth has ever faced.
M: The next bit is my favourite part of the Five Billion Year Story. The details are not very clear, and much of it is quite speculative, but here is what I have been able to make of it.
Major crises require radical solutions. The outcome of the oxygen crisis was the development of the eukaryotic cell: a new compound cell with a nucleus. Those cells now form the basis of all animals and plants. Their development marked the major division in the forms of life. How and why did it happen?
As oxygen began to accumulate in the Earth's atmosphere, and also dissolved in the sea waters, certain bacteria evolved which, rather than being killed by it, made good use of it. Some forms of cyanobacteria, one of the early photosynthesizers, learned the trick of using oxygen in a form of internal, controlled combustion as a source of energy. This was the beginning of respiration. These bacteria could take in oxygen and give off carbon dioxide. This trick not only protected cyanobacteria from the ravages of oxygen, it was also a very efficient form of metabolism, compared with that of the earlier fermenters. Cyanobacteria thrived. Many new oxygen-using forms developed from them. They quickly replaced the oxygen-sensitive bacteria on the oxygen-rich surface, while other bacteria survived underneath them in the lower levels of mud and water.
The stage was also now set for a new kind of development, the compound cell. We know of cases of small bacteria living independent lives inside larger cells in a symbiotic relationship. The smaller bacteria find plentiful food inside the host, and their metabolism contributes to that of the host. In some cases, the invader might have started as a predator, killing their host. But then, there were some of the hosts who did not die. They developed an ability to tolerate their internal partners, and moreover, a need for this new bit of internal metabolism. This was a new form of symbiosis.
In the midst of the oxygen-crisis, this symbiotic arrangement, with oxygen-using bacteria inside, proved very attractive to some. It was the safe place to be in the new, oxygen-rich world. Symbiotic colonies of bacteria began to evolve which could use oxygen and which combined the strengths of their various members.
In a modern eurkaryotic cell, the oxygen-using part is an organelle called a mitochondrian. Mitochondria today retain many of the characteristics of free-living bacteria. They have their own DNA, separate from that in the nucleus of the cell. They reproduce independently of the cells they are within. Their internal structure and chemistry is very similar to that of some bacteria.
Other cell structures are like this too. Plant cells contain choloroplasts in which the photosynthesis takes place. These too have their own DNA, reproduce independently, and are nearly identical to a bacteria called prochloron.
The image of the evolution of the eukaryotic cell is that of a colony of bacteria, living a symbiotic life to their mutual benefit. Over time, the internal structure changed, with much of the genetic material coming together into a central nucleus. The process by which the DNA in the nucleus of a eukaryotic cell divides is much more complex than in a bacterium. The details of this division are aided by structures which also might have had bacterial origins.
The new large nucleus now contained
most of the cell's genes. That is, it contained most of the templates needed
to build the molecules from which the parts of the cell were made. Again, this
provided a description of the cell which was the key to the maintenance of its
wholeness.
Here we have the story of perhaps the greatest step in life's evolution. It is certainly clear that those new organisms which were not killed by oxygen, and in fact, could use it, survived much better than those which could not. The descendents of those which could not use oxygen now remain mostly in specialised environments, including our guts. Symbiosis&emdash;living together for mutual benefit&emdash;was the key to the emergence of a new level of the organisation of life.
These new, more complex cells had some striking advantages over bacteria. Some were much more mobile. They contained undulating hair-like protrusions to propel them along. These are likely to have evolved from spirochetes, whip-like bacteria with similar properties. This mobility and their larger size helped in food gathering. Their extra complexity enabled them to cope with a wider range of conditions.
These new creatures flourished. For
about two billion years, until the animals, plants and fungi developed, they
and the bacteria were the only forms of life. Those creatures which have cells
with nuclei, but are not animals, plants or fungi are called protists.
Many of the protists are single celled creatures, like amoeba and paramecium. But some are also multi-cellular, like slime molds, seaweeds, kelp and sponges. (In fact, there are multi-cellular forms of bacteria too.) These multi-cellular forms generally develop as clones of a single cell. There is very little specialisation of cells, unlike the plants and animals which were still to come.
Y: And isn't a multicellular creature also a form of collaboration rather than competition?M: Exactly! It is just one of many cases where what is actually obvious is not often noticed.
Life in the Proterozoic
Multi-cellular forms too had many advantages, with their size contributing to the stability of their local environment. For a cell which is part of a multi-cellular form, much of what is of importance to it in its local environment consists of its sibling cells. By enhancing the survival of its local environment, the cell enhances its own survival. It is an example of life co-evolving with its environment so that the life form and local environment become closely matched.
Our story moves on to the final part, the last 570 million years out of 5 billion, during which time most of the standard evolutionary story took place. It is the story of life as told mainly by the fossil record. Most of the earlier parts, which I have just told, have been discovered only within the last few decades. Earlier life left no fossils, and awaited the development of more subtle techniques before it was discovered by us.
As the Proterozoic age drew to its close, levels of oxygen began to build up towards modern levels. High in the stratosphere, the ultraviolet radiation turned some of that oxygen into ozone. (Oxygen molecules normally consist of two oxygen atoms. Ozone molecules are made up of three oxygen atoms.) This layer of ozone then absorbed most of the ultraviolet, creating a shield which protected life, and made possible its spread onto land.
As oxygen built up, carbon dioxide
levels fell still further. The Earth cooled and another period of ice ages occurred.
A major change in the geological record marks this point. After it, fossils
appeared, and life proliferated as never before. This period is called the Phanerozoic.
Biologists now divide life into 5 kingdoms: the bacteria, the protists, the plants, the animals and the fungi. By the beginning of the Phanerozoic, the first two of these were well established. The protists, as we have seen, evolved from symbiotic colonies of bacteria. Multi-cellular forms of these evolved into the plants, animals and fungi. All of them (and that includes all of us) are really symbiotic colonies of cells, which in turn are symbiotic colonies of bacteria.
The three new kingdoms each developed as expressions of a new and specialised life strategy. The plants use sunlight as an energy source to enable them to build themselves mostly out of carbon dioxide and water. They give off oxygen as a by-product.
The animals take in oxygen and use it in a controlled combustion process as an energy source, with carbon dioxide as a by-product. Animals generally need fairly complex chemicals as the starting points for forming and re-forming themselves: carbohydrates, proteins, fats, sugars. To get these chemicals, some of them eat the plants, some eat other animal's bodies and some eat other animal's waste products. Their waste products and dead bodies also feed the plants, fungi, bacteria and protists.
The fungi specialise in external digestion. They give off chemicals which transform some of what is around them into the chemicals they need as food, which they then absorb. They often live in a symbiotic relationship with plants, supplying the nitrogen and phosphorus the plants need.
The parts go round and round. Oxygen and carbon dioxide are cycled between plants and animals. Some creatures build up bigger bodies from simpler parts. Some creatures turn the bodies of other creatures back into their simpler parts. Bacteria, protists, plants, animals and fungi are all intricately enmeshed in this. It is a grand whirlpool pattern made up of smaller whirlpool patterns made up of still smaller whirlpool patterns.
Recycling is the essence of the pattern of life. It started at a local level with the bacteria. The possibilities expanded with new forms of bacteria and then protists. With the development of plants, animals and fungi, many new pathways opened along which the parts could be recycled. As a result, the total mass of life on Earth expanded tremendously: the explosion of life.
Early in their evolution, plants developed a form of sexual reproduction more familiar to us. Some of their cells divided in half without first doubling the genes within them, so that each of the new cells had only half the complement of genes. These then combined with similar cells from another individual. The result was a cell with the full amount of genetic material, half from one parent and half from the other.
This sexual reproduction from specialised
cells was part of a general pattern, where plants came to be made up of parts
whose cells were specialised for different functions: roots, stems, seeds, leaves,
and ultimately: flowers. Each individual now was a symbiotic colony of parts,
each of which supported the others in its own way. This specialisation allowed
greater variety in life strategies: leaves could catch the light while roots
absorbed water from under the ground.
Maintaining wholenessThe first cells, the bacteria, had a wholeness which was more than a whirlpool-like unity because they had genetic material which acted as templates for the molecules of which they were made. In the more complex cells, the eukaryotic cells, the templates, the genetic material, are mostly concentrated in the nucleus. Again, they provide the description which maintains the wholeness of the eukaryotic cell. With multi-celled creatures with specialised cells, another level of description comes into play. The specialised cells within one creature can be very different. In plants, the difference between pollen, petal, leaf, bark, and root is striking. Yet, it is nothing compared to the difference between cells in animals: white blood cells are similar to amoeba while bone cells are more like a tree trunk. Nerve cells, skin cells, muscle cells, light sensitive cells in the eye are so different that in isolation they might be thought to be totally unrelated. All the cells in a multi-cellular creature contain identical genetic material, but only some is used in each type of cell. Only the templates for the parts needed for that cell are active. It is as though the genes for many different creatures were combined in one place, but with only some of them switched on at any one time. The next level of description of a multi- cellular creature is that which determines which genes are switched on and off in a particular cell. The switching on and off of genes is an essential part of the functioning of an organism, beyond just the growth and specialisation of the cells. Switching on a gene means that the chemical for which it is a template gets produced. It is largely under the control of hormones, which are a set of chemicals which circulate through an organism, as a general control system. At any of these levels of wholeness, bacteria, eukaryotic cell, or multi-cellular organism, coordination at an overall level replaces any competition between the parts. |
The earliest plants were the mosses and liverworts. They were followed by ferns with seeds. These were the first land plants. They formed the first forests between 345 and 225 million years ago. Our modern coal fields are the remains of some of those early forests, showing that the recycling was not complete. By burning that coal today we are returning carbon dioxide to the atmosphere which was removed from it by those forests when they were alive.
The first conifers appeared about 225 million years ago. They relied on the weather to distribute their seeds. The first flowering plants appeared about 123 million years ago. They evolved together with the insects in a symbiosis in which flowers attract the insects to the nectar within, and in the process of travelling from plant to plant, the insects pollinate the plants.
Animals, like plants, are complex symbiotic colonies of specialised cells. The distinction which is now made between animal-like multi-celled protists and what are now considered true animals was that the latter developed from an embryo.
Another key feature of true animals is sophisticated communication between cells, particularly through a nervous system. The combination of nerve cells and muscle cells allowed synchronised contraction of the muscle cells to produce coordinated movement. The combination of nerve cells and cells which responded to light or to chemicals in the water or air gave sight, smell and taste.
The earliest animals were some primitive worms, dating from about 700 million years ago, actually before the start of the Phanerozoic age. From the earliest worms came the later segmented worms. From these came the other semented creatures, like those with external skeletons (trilobites, crabs, shrimp, insects and spiders).
From some of the segmented worms developed creatures with a chord of nerves running through its length. When this became encased in bone it formed the first spinal chord, and from this came the fish, then the amphibians (345 million years ago), then the reptiles (245 million years ago), and finally the mammals (210 million years ago) and the birds.
Over time the mixture of creatures changed. The parts were rearranged in different ways. New pathways along which the parts were cycled arose. Old ones dissappeared.
The changes in the mixture of life forms was not usually smooth. The pattern we have already observed in the Archaen and Proterozoic ages, of crises followed by the emergence of new forms appears to be the normal pattern. The Phanerozoic too is marked by massive crises, sudden great extinctions when a large proportion of the life forms vanished, followed by the development of new forms.
About 245 million years ago there was a massive extinction which killed off 52% of all families of life. The reptiles emerged after that one. This was more severe than the extinction 65 million years ago when 11% of all families of life were killed, including those famous reptiles, the dinosaurs. This ended the period when the reptiles were dominant and allowed the mammals to take over.
Y: Doesn't this mean that at a time of crisis the fittest survive? And you keep writing about symbiosis, but you barely mention competition and survival of the fittest. Surely you aren't just going to ignore them?M: I think I've spelled out enough of the history so that I can tackle this issue of competition and fitness directly now. Just for starters, I'll answer your first question. At a time of crisis the meaning of fitness changes, so different creatures survive than survived before it.
I think by now the picture of the intricate interconnectedness of life should be very clear. All creatures rely on many others to provide their food, shelter, decompose the accumulated rubbish all create, and generally provide the rest of all the cycles of which they are part. The environment for each is all of the rest. The life cycle of any creature cannot be understood without understanding its environment. All creatures are simultaneously separate individuals and parts of a larger whole.
From this perspective, 'fitness' means fitting in the sense that a piece in a jigsaw puzzle fits into the hole left for it. That hole is the particular way of life, the niche, of that individual. Fitness certainly does not mean a general superiority of one creature over another.
A niche can also be seen as an opportunity, a possible way of living for something. An organism is formed and finds its place in its conversation with its environment. The niche is the other side of that conversation.
Under normal conditions, when there is no crisis in progress, virtually all the niches available get filled. In natural grassland there will be a mixture of grazing animals, each with somewhat different teeth and digestive systems so they can eat somewhat different mixtures of the vegetation. There will be different birds, each with different shaped beaks specialised to eating different sizes of insects, or those that live at different depths under the surface. There will be some animals specialised to feed on virtually every one of the plants and some predators and parasites that eat virtually all of the animals.
Of course, the jigsaw puzzle analogy is somewhat limited. The edges of the biological holes are not sharp as in the puzzle. There is a certain amount of overlap between the niches of one creature and another. It is in these areas of overlap that competition comes into play. It provides a jostling for position which clarifies this overlap and leaves the niches more distinct.
Y: You are giving competition a very marginal role then?M: No, it's a very important role. The overall shape of the pattern, the mixture of creatures and how they live is determined by the interconnections and symbiosis. The clarity with which the parts of this pattern fit together, the 'goodness of the fit' in that sense comes from competition. That is what I understand by 'survival of the fittest'.
Another limitation of the jigsaw puzzle analogy is that the shapes in the puzzle are fixed, while biological niches are continually changing. Under normal, non-crisis conditions, the changes are relatively small and slow. Creatures become highly specialised, very closely adaptedto the conditions around them. This means they are very closely linked to the other creatures around them, their food species, symbiotic species, predators and so on.
Then, along comes a crisis: The primordial soup runs out, oxygen begins to appear, an ice age begins or ends. The leading theory for some of the major extinctions in the Phanerozoic, including that of the dinosaurs, is that they were triggered by a massive meteorite hitting the Earth. A crisis may also occur on a much smaller and more local scale: a pond dries up, a forest is cut down, a new and more virulent parasite appears.
When the crisis hits, the shape of all the holes changes. The niches are no longer what they were. The intricate web of support is rendered. Suddenly 65 million years ago, it was no good being a mighty dinosaur, capable of killing any large animal around, and especially those puny little mammals. If the food of your food goes, you go too. In a crisis, the fitness of an individual animal or species is no longer the major determinant of survival. It is the set of relationships in the whole ecosystem around it which counts.
After a crisis there is no longer as close a matching of life forms. New pathways become available. New strategies for living can emerge. A crisis favours generalists, creatures which can feed off a wide range of others or who can live in varied conditions. There is a rapid development of new forms of life.
Under the new conditions, some creatures find they have some very useful abilities, perhaps developed for another purpose. Fish living in shallow shore waters who learned to breathe air a little and use their fins to push along the bottom might find they could survive on land a little. They were pre-adapted to the new conditions.
At first the new forms don't have to be very well adapted to the conditions. The first land animals couldn't walk very well. The first birds couldn't fly very well. Competitive pressures soon sort out a new set of niches with specialised, well adapted creatures. As walking or flying predators appeared, their prey learned to run and fly fast. As the new set of niches becomes filled, change slows down and a 'normal' period arrives.
Y: I think I'm beginning to get the idea. You aren't saying that competition isn't important, just that this symbiosis business and being part of the web of life is actually the major part of what counts.M: Yes, that's right. Basically, I think the image of nature as a war is a projection onto nature of a market economic system! It is very recent, only appearing since Darwin in the last century. It replaced a view of nature as 'God's harmony' with everything in its place in a hierarchy. That was a projection of feudalism onto nature.
Y: And aren't you trying to create a projection onto nature of your view of society? What is nature really like?
M: Since you put it that way, I suppose that is what I am doing (and I don't think it is just my view). I will certainly make the parallels very explicit later in the book. What is nature really like? I suppose a sage would say that nature 'just is'.
Y: I'm not completely satisfied yet. Can we go back to competition? What about the competition between male animals for dominance or access to females? Surely that is a major part of their life?
M: OK. I'd better say more about what happens within one species. The same principles apply, and we can sometimes even begin to glimpse another level of organisation.
Within one species the overlap between niches is especially strong. All the foxes in a wood eat the same prey. All the rabbits like the same plants. What generally happens is that the niches for creatures of the same species are geographic. Individuals or groups have their own territories within which there is sufficient to meet their needs. Competition again appears at the boundaries of the territories. Birds sing in large part to say "This is my territory!"
For many creatures, and especially for animals, other members of their own species are of major importance in their environment. At the very least, they need them to mate. Thus it is not surprising that when competition arises between members of a species, it is usually minimised. Often there is a ritual or rule which determines which creature wins. With certain butterflies, the first one on a particular leaf has priority. With many animals, threatening postures are enough to see off a rival.
The issue between two rival animals is to see which one gets to mate, or gets the territory. This is an issue which is a matter of communication between creatures who have very limited means of communication. If the appropriate criterion is the size and showiness of your tail feathers, then a display will settle the issue. If it is a matter of strength, then a fight is needed. However, fights rarely end in death or serious injury. They generally end when it is clear who will win.
I'll round off this chapter by looking at social behaviour in animals. It is striking how many different animals live in groups: flocks, herds, schools, prides, or whatever. For these animals, the benefits of being in a group clearly outweigh competitive pressures from sharing a niche. Being part of a group can give protection from predators (grazing animals), help with care of offspring (lions), cooperation in hunting (dogs).
Some of the social insects have very
specialised forms. An individual ant or termite is hardly a separate creature.
It may be able to gather food, or produce eggs or defend the colony, but not
any of the other tasks. It is more like a cell in the super-organism which is
the colony.
Metaphor 2 - The starlingsImagine that you are watching a flock of starlings near some fields or gardens. It is a large flock of several hundred birds and very dramatic to see. As you watch, you notice that there is a main group and various smaller groups, constantly changing. A few birds circle high in the air watching for predators. One or two take off to the next field to search for food. A few more peel off from the main group to join them. If they are successful and settle onto that field, others will see them and join them and soon the bulk of the flock will leave its present location and join them. If the watchers signal danger (you perhaps), the whole lot will suddenly take off and fly away. Birds are the ultimate symbol of freedom. None of those starlings tells any of the others what to do. They live in a flock, choosing their roles from moment to moment in support of the flock, synchronising their behaviour with each other, because that is what it is to be a starling. Freedom and collaboration are in no sense in conflict with one another. |
With that, I'll bring the Five Billion Year Story to a close, although with a chapter still to come on human evolution. The main points I hope you are left with are:
3
Lynn Margulis and Dorian Sagan, Microcosmos, Four Billion Years of Microbial
Evolution,
Allen and Unwin, 1987. p. 71. I recommend
this book highly and have drawn heavily from it.
4
Sorin Sonea and Maurice Panisset, The New Bacteriology, Boston, Jones
and Bartlett,
1983, p.22