How Life Arose
Oparin and Engels
How Did Life Arise?
The Revolutionary Birth of Life
Early Life Forms
Photosythesis and Sexual Reproduction
The Cambrian Explosion
Plants and Animals
"What we do not know today we shall know tomorrow." This simple statement underlies the conclusion of a scientific paper on the Origin of Life written by the Russian biologist Aleksandr Ivanovich Oparin in 1924. It was the first time that a modern appreciation of the subject had been undertaken, and opened up a new chapter in the understanding of life. It was no accident that as a materialist and dialectician, Oparin approached this subject from an original perspective. This was a bold beginning, at the very dawn of biochemistry and molecular biology, and was backed up independently by the contribution of British biologist J. B. S. Haldaneagain a materialistin 1929. This work produced the Oparin-Haldane hypothesis, on which the subsequent understanding of the origin of life is based. "In it," writes Asimov, "the problems of lifes origin for the first time was dealt with in detail from a completely materialistic point of view. Since the Soviet Union is not inhibited by the religious scruples to which the Western nations feel bound, this, perhaps, is not surprising." (6)
Oparin always acknowledged his debt to Engels, and made no secret of his philosophical position:
"This problem (of lifes origins) has however always been the focus of a bitter conflict of ideas between two irreconcilable schools of philosophythe conflict between idealism and materialism," writes Oparin.
"A completely different prospect opens out before us if we try to approach a solution of the problem dialectically rather than metaphysically, on the basis of a study of the successive changes in matter which preceded the appearance of life and led to its emergence. Matter never remains at rest, it is constantly moving and developing and in this development it changes over from one form of motion to another and yet another, each more complicated and harmonious than the last. Life thus appears as a particular very complicated form of the motion of matter, arising as a new property at a definite stage in the general development of matter.
"As early as the end of the last century Frederick Engels indicated that a study of the history of the development of matter is by far the most hopeful line of approach to a solution of the problem of the origin of life. These ideas of Engels were not, however, reflected to a sufficient extent in the scientific thought of his time."
Engels was essentially correct when he described life as the mode of motion of proteins. However, today we can add that life is the function of the mutual reactions of nucleic acids and of proteins. As Oparin explained: "F. Engels, in common with biologists of his time, often used the terms protoplasm and albuminous bodies. The proteins of Engels must therefore not be identified with the chemically distinct substances which we have now gradually succeeded in isolating from living things, nor with purified protein preparations composed of mixtures of pure proteins. Nevertheless Engels was considerably in advance of the ideas of his time when, in speaking of proteins, he specially stressed the chemical aspects of the matter and emphasised the significance of proteins in metabolism, that form of the motion of matter which is characteristic of life."
"It is only now that we have begun to be able to appreciate the value of the remarkable scientific perspicacity of Engels. The advances in protein chemistry now going on enabled us to characterise proteins as individual chemical compounds, as polymers of amino acids having extremely specific structures." (7) J. D. Bernal offers an alternative to Engelss definition of life as "a partial, continuous, progressive, multiform and conditionally interactive, self-realisation of the potentialities of atomic electron states." (8)
Although the Oparin-Haldane hypothesis laid the basis for a study of life origins, as a branch of science it is more correct to ascribe it to the revolution in biology in the mid-20th century. Theories concerning the origin of life are largely speculative. There are no traces in the fossil record. We are dealing here with the simplest and most basic life-forms imaginable, transitional forms which were quite unlike the idea of living things we have today, but which nevertheless represented the decisive leap from inorganic to organic matter. Perhaps, as Bernal comments, it is more correct to say the origin not of life, but the origin of the processes of life.
Engels explains that the Darwinian revolution "reduced the gulf between inorganic and organic nature to a minimum but removed one of the most essential difficulties that had previously stood in the way of the theory of descent of organisms. The new conception of nature was complete in its main features; all rigidity was dissolved, all fixity dissipated, all particularity that had been regarded as eternal became transient, the whole of nature shown as moving in eternal flux and cyclical course." (9) The scientific discoveries since this was written have served to strengthen this revolutionary doctrine.
Oparin drew the conclusion that the original atmosphere of the earth was radically different from that of today. He suggested that instead of oxygen, the character of the atmosphere was reducing rather than oxidising. Oparin proposed that the organic chemicals on which life depends formed spontaneously in such an atmosphere under the influence of ultra violet radiation from the sun. Similar conclusions were arrived at independently by J. B. S. Haldane:
"The Sun was perhaps slightly brighter than now and as there was no oxygen in the atmosphere the chemically active ultra-violet rays from the Sun were not, as they now are, mainly stopped by ozone (a modified form of oxygen) in the upper atmosphere, and oxygen itself lower down. They penetrated to the surface of the land and sea, or at least to the clouds. Now, when ultra-violet acts on a mixture of water, carbon dioxide, and ammonia, a vast variety of organic substances are made, including sugars and apparently some of the materials from which proteins are built up." (10)
In a more generalised form Engels pointed in the right direction fifty years previously: "If, finally, the temperature becomes so far equalised that over a considerable portion of the surface at least it does not exceed the limits within which protein is capable of life, then, if other chemical conditions are favourable, living protoplasm is formed." He continued, "Thousands of years may have passed before the conditions arose in which the next advance could take place and this formless protein produce the first cell by formation of nucleus and cell membrane. But this first cell also provided the foundation for the morphological development of the whole organic world; the first to develop, as it is permissible to assume from the whole analogy of the palaeontological record, were innumerable species of non-cellular and cellular protista " (11) Although this process took place over a far longer time-span, this is a generally correct prognosis.
Just as Engels ideas were ignored at the time by the scientific community, so were those of Oparin and Haldane. Only recently are these theories getting the recognition they deserve. Richard Dickerson writes:
"Haldanes ideas appeared in Rationalist Annual in 1929, but they elicited almost no reaction. Five years earlier Oparin had published a small monograph proposing rather similar ideas about the origin of life, to equally little effect. Orthodox biochemists were too convinced that Louis Pasteur had disproved spontaneous generation once and for all to consider the origin of life a legitimate scientific question. They failed to appreciate that Haldane and Oparin were proposing something very special: not that life evolves from non-living matter today (the classical theory of spontaneous generation, which was untenable after Pasteur) but rather that life once evolved from non-living matter under the conditions prevailing on the primitive earth and in the absence of competition from other living organisms." (12)
How Did Life Arise?
There is no subject of such tremendous import for us as the question of how living, feeling, thinking creatures arose out of inorganic matter. This riddle has occupied the human mind from the earliest times, and has been answered in various ways. We can broadly identify three trends:
1st theory God created all life, including humans.
2nd theory life arose from inorganic matter, by spontaneous generation, as maggots from decaying flesh, or beetles from a dunghill (Aristotle).
3rd theory life came from outer-space in a meteorite, which fell on the earth, and then developed.
This transformation from inorganic to organic is a comparatively recent view. In contrast, the theory of spontaneous generationthat life originated from nothinghas a long history. From ancient Egypt, China, India and Babylon came the belief in spontaneous generation. It is contained in the writing of the ancient Greeks. "Here maggots arise from dung and rotting meat, here lice form themselves from human sweat, here fireflies are born from the sparks of a funeral pyre, and finally, frogs and mice originate from dew and damp earth For them spontaneous generation was simply an obvious, empirically established fact the theoretical basis of which was of secondary importance," states Oparin. (13) Much of this was bound up with religious legends and myths. By contrast, the approach of the early Greek philosophers was materialist in character.
It was the idealist view of Plato (expressed also by Aristotle), that invested spontaneous generation with a supernatural quality and later formed the basis of mediaeval scientific culture and dominated peoples minds for centuries. Matter does not contain life but is infused with it. Through Greek and Roman philosophical schools, it was borrowed and elaborated by the early Christian church to develop their mystical conception of the origin of life. St. Augustine saw in spontaneous generation a manifestation of divine willthe animation of inert matter by the "life-creating spirit." As Lenin points out, the scholastics and clerics seized upon that which was dead in Aristotle and not upon that which was alive. It was later developed by Thomas Aquinas in according with the teachings of the Catholic church. A similar standpoint is held by the Eastern churches. The Bishop of Rostov, Dimitrii, in 1708 explained that Noah did not take in his ark those animals capable of spontaneous generation: "These all perished in the Flood and after the Flood they arose anew from such beginnings." This was the dominant belief in Western society up until the mid-19th century.
The great T. H. Huxley in his Edinburgh lecture in 1868 first clearly explained that life had one common physical basis: protoplasm. He stressed it was functionally, formally and substantially the same over the whole range of living things. In function, all organisms reveal movement, growth, metabolism and reproduction. In their form they are composed of nucleated cells; and in substance, they are all made up of proteins, a chemical compound of carbon, hydrogen, oxygen and nitrogen. This graphically reveals the underlying unity of life.
The French scientist Louis Pasteur, the father of microbiology, in a series of experiments finally discredited the theory of spontaneous generation. "Life could only come from life," said Pasteur. The discoveries of Pasteur dealt a crushing blow to the orthodox conception of spontaneous generation. The further triumph of Darwins theory of evolution forced the vitalists (the idea of the "life force") to look at the origin of life in a new way. From now on their defence of idealism came in the argument of the impossibility of understanding this phenomenon on the basis of materialism.
As early as 1907, in a book called Worlds in the Making, the Swedish chemist Svente Arrhenius put forward the theory of panspermia, which concluded that if life could not occur spontaneously on the earth, then it must have been introduced from other planets. He described spores travelling through space to "seed" life in other planets. Any life spores entering our atmosphere, as with meteorites, would burn up. To counter these criticisms, Arrhenius argued that life was therefore eternal, and had no origin. But the evidence contradicted his theory. It was shown that the existence of ultra-violet rays in space would quickly destroy any bacterial spores. For example, microorganisms selected for their toughness, were put on the space-capsule Gemini 9 in 1966, and exposed to radiation from space. They lasted six hours. More recently, Fred Hoyle thought that life had been brought to earth in the tails of comets. This idea has been revamped by Francis Crick and Leslie Orgel who suggested that earth may have been deliberately seeded by intelligent life from outer space! But such theories really solve nothing. Even if we accept that life came to earth from another planet, that still does not answer the question of how life arises, but merely puts it back another stageto the hypothetical planet of origin.
It is not necessary to travel to outer space for a rational explanation of the origins of life. The origins of life can be found in the processes at work in nature on our own planet over three and a half billion years ago, under very special conditions. This process can no longer be repeated, because any such organisms would be at the mercy of existing life-forms which would make short work of them. It could only arise on a planet where no life existed, and also when there was little oxygen, since oxygen would combine with the chemicals needed to form life and break them down. The earths atmosphere at that time was mainly made up of methane, ammonia and water vapour. Experiments in laboratories have shown that a mixture of water, ammonia, methane and hydrogen, subject to ultra violet radiation produced two simple amino acids, and traces of more complicated ones. In the late 1960s, complex molecules were found to be present in gas clouds in space. It is therefore possible that, even at a very early stage in the earths formation, the elements for the emergence of life, or near-life, were already present in the form of amino-acids. More recent experiments have proven beyond all doubt that the proteins and nucleic acids which are the basis of all life could have emerged from the normal chemical and physical changes taking place in the primordial "soup."
According to Bernal, the unity of life is part of the history of life and, consequently, is involved in its origin. All biological phenomena are born, develop and die in accordance with physical laws. Biochemistry has demonstrated that all life on earth was the same at a chemical level. Despite the enormous variation between species, the basic mechanism of enzymes, coenzymes, and nucleic acids appear everywhere. At the same time, it forms a set of identical particles which hold themselves together by the principles of self-assembly in the most elaborate structures.
The Revolutionary Birth of Life
It is now becoming clear that the earth in its early stages did not function in the same manner as today. Atmospheric composition, climate, and life itself, developed through a process of convulsive changes, involving sudden leaps, and all kinds of transformations, including retrogressions. Far from being a straight line, the evolution of the earth and of life itself is full of contradictions. The first period of the earths history, known as Archaean, lasted until 1.8 billion years ago. In the beginning, the atmosphere consisted mainly of carbon dioxide, ammonia, water and nitrogen, but there was no free oxygen. Before this point, the earth was lifeless. So how did life arise?
As we have seen, up to the beginning of the 20th century, geologists believed that the earth had a very limited history. Only gradually did it become clear that the planet had a far older history, and moreover, one that was characterised by constant and sometimes cataclysmic change. We see a similar phenomenon in relation to the supposed age of the solar system, which turns out to be considerably older than what was previously believed. Suffice to say that the advances of technology after the Second World War, especially the discovery of nuclear clocks, provided the basis for far more accurate measurements, which gave rise to a giant leap forward in our understanding of the evolution of our planet.
Today we can say that the earth became a solid planet more than 4.5 billion years ago. For everyday thinking, this seems an unimaginably long time. Yet, when dealing with geological time, we enter an entirely different order of magnitudes. Geologists are accustomed to dealing with millions and billions of years, as we think of hours, days and weeks. It became necessary to create a different time-scale, capable of embracing such periods of time. This closes the "early" stages of the earths history, and yet this convulsive period accounts for no less than 88% of the total history of the planet. Compared to this, the entire history of the human race so far is no more than a fleeting moment. Unfortunately, the paucity of evidence from this period prevents us from obtaining a more detailed picture of the processes.
To understand the origin of life, it is necessary to know the composition of the earths early environment and atmosphere. Given the likely scenario that the planet was formed from a dust cloud, its composition would have been largely hydrogen and helium. Today the earth contains large amounts of heavier elements, like oxygen and iron. In fact, it contains roughly 80% of nitrogen and roughly 20% of oxygen. The reason for this is that the lighter hydrogen and helium escaped from the earth's atmosphere as the gravitational pull was insufficient to hold them. The larger planets with greater gravitation, like Jupiter and Saturn, have retained their dense atmosphere of hydrogen and helium. By contrast, our much smaller moon, with its low gravity, has lost all its atmosphere.
The volcanic gases which formed the primitive atmosphere must have contained water, along with methane and ammonia. We presume these were released from the interior of the earth. This served to saturate the atmosphere and produce rain. With the cooling of the earths surface, lakes and seas began to form. It is believed that these seas constituted a prebiotic (pre-life) "soup," where the chemical elements present, under the impact of ultra-violet light from the sun, synthesised to produce complex nitrogenous organic compounds, such as amino acids. This effect of ultraviolet was made possible by the absence of ozone in the atmosphere. This constitutes the basis of the Oparin-Haldane hypothesis.
All life is organised into cells, except for viruses. Even the simplest cell is an extremely complex phenomenon. The standard theory is that the heat from the earth itself would have been sufficient for complex compounds to form out of simple ones. The early life forms were able to store energy derived from the ultra violet radiation from the sun. However, changes in the composition of the atmosphere cut off the supply of ultra violet rays. Certain aggregates, which had developed the substance known as chlorophyll, were able to make use of the visible light that penetrated the ozone layer that filtered out the ultra violet. These primitive algae consumed carbon dioxide and emitted oxygen, leading to the creation of the present atmosphere.
Throughout the whole course of geological time, we can observe the dialectical interdependence of atmospheric and biospheric activity. On the one hand, most of the free oxygen in the atmosphere resulted from biological activity (through the process of photosynthesis in plants). On the other hand, changes in the composition of the atmosphere, in particular the increase in the amounts of molecular oxygen present, triggered off major biological innovations, which enabled new forms of life to emerge and diversify.
How did the first living cell arise out of the primordial soup of amino-acids and other simple molecules some four billion years ago? The standard theory, as expressed in 1953 by the Nobel prize winning chemist Harold Urey and his student Stanley Miller, was that life arose spontaneously in an early atmosphere of methane, ammonia, and other chemicals, activated by lightning. Further chemical reactions would permit the simple compounds of life to develop into increasingly complex molecules, eventually producing the DNA double helix, or the single stranded RNA, both of which possess the power of reproduction.
The odds against this occurring by accident are truly staggering, as the Creationists love to point out. If the origin of life were a truly random event, then the Creationists would have a powerful case. It would really be a miracle! The basic structures of life and genetic activity in general depend upon incredibly complex and sophisticated moleculesDNA and RNA. In order to make a single protein molecule it would be necessary to combine several hundred amino-acid building blocks in a precise order. This is a formidable task, even in a laboratory with the latest equipment. The odds against such a thing occurring by accident in some warm little pool would be astronomical.
This question has recently been approached from the point of view of complexity, an offshoot of chaos theory. Stuart Kauffman, in his work on genetics and complexity, raised the possibility that a kind of life arose as a result of the spontaneous emergence of order out of molecular chaos, through the natural workings of the laws of physics and chemistry. If the primordial soup was sufficiently rich in amino-acids, it would not be necessary to wait for random reactions. A coherent, self-reinforcing web of reactions could be formed out of the compounds in the soup.
By means of catalysts different molecules could interact and fuse with each other to form what Kauffman calls an "autocatalytic set." In this way, order emerging from molecular chaos would manifest itself as a system that grows. This is not yet life as we know it today. It would have no DNA, no genetic code, and no cell membrane. Yet it would exhibit certain lifelike properties. For instance it could grow. It would possess a kind of metabolismabsorbing in a steady supply of "food" molecules in the form of amino-acids and other simple compounds, adding them to itself. It would even have a primitive kind of reproduction, extending itself to spread over a wider area. This idea, which represents a qualitative leap, or "phase transition" in the language of complexity would mean that life had not arisen as a random event, but as a result of the inherent tendency of nature towards organisation.
The first animal organisms were cells able to absorb the energy built up by the plant cells. The changed atmosphere, the disappearance of ultra violet radiation, and the presence of already existing life-forms rules out the creation of new life at the present time, unless it is achieved by artificial means under laboratory conditions. In the absence of any rivals or predators in the oceans, the earliest compounds would have spread rapidly. At a certain stage, there would be the qualitative leap with the formation of a nucleic acid molecule capable of reproducing itself: a living organism. In this way, organic matter arises out of inorganic matter. Life itself is the product of inorganic matter organised in a certain way. Gradually, over a long period of million of years, mutation would begin to appear, eventually giving rise to new forms of life.
Thus we can arrive at a minimum age for life on earth. One of the main obstacles to the evolution of life as we know it was the absence of an ozone screen in the upper atmosphere in Archaean times. This allowed the penetration of the surface layers of the oceans by universal radiation, including ultra-violet rays, which inactivate the life-inducing DNA molecule. The first primitive living organismsthe prokaryotic cellswere single-celled, but lacked a nucleus and were incapable of cell-division. However, they were relatively resistant to the ultra-violet radiation, or even, according to one theory, dependent upon it. These organisms were the predominant form of life on earth for a period of some 2.4 billion years.
The prokaryotic unicellular creatures reproduced asexually through budding and fission. Generally, asexual reproduction creates identical copies unless a mutation develops, which is very infrequent. That explains the slowness of evolutionary change at this time. However, the emergence of the nucleated cell (eukaryotes) gave rise to the possibility of greater complexity. It seems likely that the evolution of the eukaryotes arose from a colony of prokaryotes. For instance, some modern prokaryotes can invade and live as components within eukaryotic cells. Some organelles (organs) of eukaryotes, have their own DNA, which must be a remnant of their formally independent existence. Life itself has certain principal features, including metabolism (the total of the chemical changes that go on in the organism) and reproduction. If we accept the continuity of nature, the simplest organism that exist today must have evolved from simpler and simpler processes. Moreover, the material bases of life are the commonest of all the elements of the Universe: hydrogen, carbon, oxygen, nitrogen.
Once life has appeared, it itself constitutes a barrier which prevents the re-emergence of life in the future. Molecular oxygen, a by-product of life, arises from the process of photosynthesis (where light is transformed into energy). "The life that we have on Earth today is, in fact, divided into two great categories long recognised by mankindthe oxygen breathing animals and the photosynthetic or light-growing plants", states Bernal. "Animals can live in the dark, but they need air to breathe, either free air or oxygen dissolved in water. Plants do not need oxygenin fact they produce it in the sun-lightbut they cannot live and grow for long in the dark. Which, therefore, came first? Or did some other form of life precede them? This alternative now seems almost certain. Detailed studies of the life histories, the internal cellular anatomy and the metabolism both of plants and animals show them to be divergently specialised dependants of some zoo-phyte. These must have been like some of the bacteria of today that can at the same time carry on the functions of animals and plants, and act both as oxidising and as photosynthetic agents." (14)
Early Life Forms
It is a striking fact that the chromosomes of all living organisms, from bacteria to humans, are similar in composition. All genes are made of the same kind of chemical substancesnucleoproteins. This is also true of viruses, the simplest known living things which stand on the threshold of organic and non-living matter. The chemical composition of the nucleoproteins permits a molecular entity to reproduce itself, the basic characteristic of life, both in genes and viruses.
Engels points out that the evolution of life cannot be understood without all kinds of transitional forms:
"Hard and fast lines are incompatible with the theory of evolution. Even the border-line between vertebrates and invertebrates is now no longer rigid, just as little is that between fishes and amphibians, while that between birds and reptiles dwindles more and more every day. Between Compsognathus and Archaeopteryx only a few intermediate links are wanting, and birds beaks with teeth crop up in both hemispheres. Either or becomes more and more inadequate. Among lower animals the concept of the individual cannot be established at all sharply. Not only as to whether a particular animal is an individual or a colony, but also where in development one individual ceases and the other begins.
"For a stage in the outlook on nature where all differences become merged in intermediate steps, and all opposites pass into one another through intermediate links, the old metaphysical method of thought no longer suffices. Dialectics, which likewise knows no hard and fast lines, no unconditional, universally valid either or which bridges the fixed metaphysical differences, and besides either or recognises also in the right place both thisand that and reconciles the opposites, is the sole method of thought appropriate in the highest degree to this stage. Of course, for everyday use, for the small change of science, the metaphysical categories retain their validity." (15)
The boundary-lines between living and non-living matter, between plants and animals, reptiles and mammals, are not so clearly drawn as one might suppose. Viruses, for example, form a class which cannot be said to be life as we generally understand it, and yet they clearly possess some of the attributes of life. As Ralph Buchsbaum states:
"The viruses are among the largest proteins known, and several different ones have already been prepared in pure crystalline form. Even after repeated crystallisations, a treatment no obviously living substance has ever been able to survive, viruses resume their activities and multiply when returned to favourable conditions. While no one has yet succeeded in growing them in the absence of living matter, it is clear that viruses help to bridge the gap that was formerly thought to exist between nonliving and living things. No longer can it be said that there is some sharp and mysterious distinction between the nonliving and the living, but rather there seems to be a gradual transition in complexity.
"If we imagine that the earliest self-propagating substances were something like viruses, it is not difficult to suppose that an aggregation of virus-like proteins could lead to the development of larger bacteria-like organisms, independent, creating their own food from simple substances, and using energy from the sun.
"Such a level of organisation may be compared to present-day forms like the independent bacteria, some of which conduct photosynthesis without chlorophyll, using, instead, various green or purple pigments. Others utilise the energy derived from the oxidation of simple slats of nitrogen, sulphur, or iron. These, for instance, can oxidise ammonia to nitrates, or hydrogen sulphide to sulphates, with the release of energy which is utilised in forming carbohydrates." (16)
The relatively brief interval between the formation of the planet and the cooling of its surface crust, meant that the emergence of life occurred in an amazingly short space of time. Stephen J. Gould explains that "life, for all its intricacy, probably arose rapidly about as soon as it could." (17) The microfossils of 3.5 billion years are, as expected, prokaryotic cellsthat is without a nucleus (methanogens, bacteria, and blue-green algae). They are regarded as the simplest forms of life on earth, although even by this time there was diversity. Which means that between 3.5 and 3.8 billion years our common ancestor emerged, together with other forms that became extinct.
There was little, if any molecular oxygen atmosphere at this time. The organisms that existed at the time did not require oxygenin fact it would have killed them. They grew by oxidising hydrogen and reducing carbon dioxide to methane. It has been suggested that these organisms must have been similar to eocyte cells that inhabit the very hot environment of volcanic vents. They obtain their energy not from oxygen but through converting sulphur to hydrogen sulphide.
"One can visualise," writes Richard Dickerson, "that before living cells evolved the primitive ocean was teeming with droplets possessing special chemistries that survived for a long time and then disappeared again." He continues:
"Those droplets that by sheer chance contained catalysts able to induce useful polymerisations would survive longer than others; the probability of survival would be directly linked to the complexity and effectiveness of their metabolism. Over the aeons there would be a strong chemical selection for the types of droplets that contained within themselves the ability to take molecules and energy from their surroundings and incorporate them into substances that would promote the survival not only of the parent droplets but also of the daughter droplets into which the parents were dispersed when they became too big. This is not life, but it is getting close to it."
(18) Given the lack of fossil evidence, it is necessary to examine the organisation of modern cells in order to cast light on their origins. For the simplest life forms to reproduce, a genetic apparatus containing nucleic acids must be present. If cells are the basic unit of life, we can be almost certain that the original organisms contained nucleic acids or closely related polymers. Bacteria, for example, are composed of a single cell and are likely to be the prototype of all living cells.
The bacterium Escherichia coli (E. coli) is so small that a million million of its cells could be enclosed into a volume of one cubic centimetre. It contains a cell wall, a membrane, which keeps essential molecules enclosed; it also selects and draws in useful molecules from outside the cell. It maintains the balance between the cell and its environment. The main metabolism of the cell takes place in the membrane, where hundreds of chemical reactions take place that use the nutrients in the environment for growth and development. The bacterium, E. coli, reproduces every twenty minutes. This unique transformation within the cell is made possible by a group of molecules called enzymes. These are catalysts which speed up the chemical reactions without being altered in the process. They work repeatedly, continuously transforming nutrients into products.
Reproduction is an essential element of life. When cell division occurs, a set of identical daughter cells is created. The mechanism for duplication, for making new protein molecules with exactly the same sequence as the parent cell, is encoded in the nucleic acids. They are unique in that they alone, with the assistance of certain enzymes, are able to reproduce themselves directly. The DNA (deoxyribonucleic acid) carries all the information needed to direct the synthesis of new proteins. However, the DNA cannot do this directly, but acts as a "master copy" from which messenger RNA (ribonucleic acid) copies are made, which carry the information of the sequence to the synthesising system. This is known as the genetic code. Nucleic acids cannot replicate without enzymes, and enzymes cannot be made without nucleic acid. They must have developed in parallel. It is likely that in the original "soup" of elements there existed RNA molecules that were also enzymes, which developed on the basis of natural selection. Such RNA enzymes came together to form a helix, and become the basis for self-replicating RNA. The genetic replication is, however, not without occasional errors. In the bacterium E. coli the error rate is one in every 10 million base copies. In the course of millions of generations such errorsmutationsmay have little effect, but alternatively, they may lead to profound changes in the organism, and on the basis of natural selection, lead to the formation of new species.
The next stage in organic evolution was the development of other polymerscombination of moleculesgrouped together into whole families. A structure was needed to enclose the molecules: a semipermeable cell membrane. Cell membranes are complex structures, barely poised between a solid and liquid state. Small changes in the composition of the membrane can produce a qualitative change, as Chris Langton explains: "Twitch it ever so slightly, change the cholesterol composition a bit, change the fatty acid composition just a bit, let a single protein molecule bind with a receptor on the membrane, and you can produce big changes, biologically useful changes." (19)
Photosynthesis and Sexual Reproduction
As can be seen from what has already happened, the evolution of the cell is a relatively advanced stage of organic evolution. As the abundant components of the biotic soup became exhausted, it became necessary to evolve water-soluble organic materials from the atmosphere. From fermentation, the simpler but less efficient form of metabolism, photosynthesis was the next step. The special chlorophyll molecule had evolved. This allowed living organisms to capture solar energy for the synthesis of organic molecules. The first photosynthesizers removed themselves from the competition for dwindling natural energy-rich molecules and set themselves up as primary producers. Once the photosynthetic process was achieved, the future of life was assured. As soon as it emerges and produces enough oxygen, respiration becomes possible. In accordance with the laws of natural selection, once photosynthesis started it made its mark on all subsequent living things, and was undoubtedly so successful that it wiped out earlier forms of life.
This development represents a qualitative leap. The subsequent evolution to more complex forms is a drawn out process eventually leading to a new branch of life, the nucleated cell. At the top of the eukaryotic tree, several branches appear simultaneously, such as plants, animals and fungi. According to the American molecular biologist Mitchell Sogin the amount of oxygen affected the pace of evolution. The chemical composition of ancient rocks suggests that atmospheric oxygen increased in relatively distinct steps separated by long periods of stability. Some biologists believe that the explosion of life could have been triggered by oxygen reaching a certain level.
The nucleated cellthe eukaryotescompletely adapted to oxygen and showed little variation. The emergence of this revolutionary new life form allowed the existence of advanced sexual reproduction, which in turn, accelerated the pace of evolution. Whereas the prokaryotes consisted of only two groups of organisms, the bacteria and the blue-green algae (the latter produced oxygen through photosynthesis), the eukaryotes consist of all green plants, all animals and fungi. Sexual reproduction represent another qualitative leap forward. This requires the genetic material to be packaged inside the nucleus. Sexual reproduction allows the mixing of genes between two cells, the chances of variation being far greater. In reproduction, the chromosomes of the eukaryotic cells fuse to produce new cells. Natural selection serves to preserve favourable genetic variants in the gene pool.
One of the key aspects of life is reproduction. All animal and plant cells have the same basic internal structures. Reproduction and the passing on of parental characteristics (heredity) takes place through the union of sex cells, the egg and sperm. The genetic material DNA through which the characteristics of life forms are transmitted from one generation to the next is contained in the nucleus of all cells. The cell structure which is made up of cytoplasm also contains a number of miniature organs called organelles. The internal structure of the organelles is identical to different types of bacteria, which seems to indicate that the composition of the animal and plant cell is the result of these once independent organs, with their own DNA, combining to form a co-operative whole. In the 1970s microtubules were discovered. These are protein rods which fill every cell in the body like an internal scaffolding. This internal "skeleton" gives shape to the cell and appears to play a role in the circulation of protein and plasma products. The advent of the eukaryotic or nucleated cell constituted a biological revolution some 1,500 million years ago.
From asexual budding and fission emerged sexual reproduction. Such an advance served to mix up the hereditary material of two individuals, so that the offspring would differ from the parents. This provided the variation on which natural selection could work. In every animal and plant cell the DNA is arranged in pairs of chromosomes in the nucleus. These chromosomes carry the genes which determine individual characteristics. The new offspring, while combining the characteristics of its parents, is nevertheless different from them. It appears that the origin of sexual reproduction is connected with primitive organisms ingesting one another. The genetic material of two individuals were fused producing an organism with two sets of chromosomes. The larger organism then split into two parts with the correct amount of chromosomes. Single and paired chromosomes existed, but through time the paired condition became the normal mode of existence of plants and animals. This laid the basis for the evolution of multicellular organisms.
By about 700-680 million years ago, the first metazoa appeared. These were complex multicellular organisms that require oxygen for their growth. During that period the oxygen content of the atmosphere increased constantly, reaching its present level only 140 million years ago. The processes at work in evolution have a markedly dialectical character in which long periods of gradual quantitative change are interrupted by sudden explosions. Such a period occurred about 570 million years ago.
The Cambrian Explosion
It requires an effort of the imagination to recall just how recent a phenomenon complex forms of life on earth are. Picture a world in which the earth consisted of barren windswept rocks, in which the most complex forms of life were mats of algae and pond scum. This was the situation for the great majority of the earths history. For thousands of millions of years the development of life was virtually static. Then suddenly, this stagnant world suddenly erupted in one of the most dramatic explosions in the history of life. The fossil record now reveals an extraordinary proliferation of different forms of life. The emergence of animals with shells and skeletons preserves this progress in tablets of stone. The explosion of new forms of life in the oceans was paralleled by the mass extinction of the older stromatolites, which had been the dominant life-form in the Proterozoic period. The appearance of a vast multitude of many-celled creatures transformed the face of the earth for all time.
"Perhaps the most remarkable (and also the most perplexing) thing about the fossil record is its beginning," F. H. T. Rhodes writes. "Fossils first appear in appreciable numbers in rocks of the Lower Cambrian age, deposited about 600 million years ago. Rocks of older (Pre-Cambrian) age are almost completely unfossiliferous, although a few traces of ancient organisms have been recorded from them. The difference between the two groups of rocks is every bit as great as this suggests: a palaeontologist may search promising-looking Pre-Cambrian strata for a lifetime and find nothing (and many have done just this); but once he rises up into the Cambrian, in come the fossilsa great variety of forms, well-preserved, world-wide in extent, and relatively common. This is the first feature of the oldest common fossils, and it comes as a shock to the evolutionist. For instead of appearing gradually, with demonstrably orderly development and sequencethey come in with what amounts to a geological bang." (20)
In spite of his genius, Darwin was unable to come to terms with the Cambrian explosion. Clinging to his gradualist conception of evolution, he assumed that this sudden leap was only apparent, and due to the incompleteness of the fossil record. In recent years, new and startling discoveries in palaeontology have led to a major revision in the interpretation of evolution. The old idea of evolution as an uninterrupted process of gradual change has been challenged in particular by Stephen Jay Gould, whose investigations into the fossil record of the Burgess Shale (an important fossil location in British Columbia) have transformed palaeontology.
Life developed, not in a straight line of uninterrupted evolutionary progress, but through a process aptly described by Stephen Jay Gould as punctuated equilibria in which long periods of apparent stability are interrupted by periods of sudden and cataclysmic change characterised by mass extinctions of species. For 500 million years the border-lines of geological periods are marked by such sudden upheavals in which the disappearance of some species clears the way for the proliferation of others. This is the biological equivalent of the geological processes of mountain formation and continental drift. It has nothing in common with the vulgar caricature of evolution understood as a simple process of gradual change and adaptation.
According to the classical theory of Darwin the emergence of the first complex multicellular forms of life must have been preceded by a long period of slow progressive change, which culminated in the "Cambrian explosion" 500 million years ago. However, the most recent discoveries show that this is not the case. The investigations of Gould and others show that for two-thirds of the history of life on earthnearly 2.5 billion yearslife remained confined to the lowest recorded level of complexity, prokaryotic cells, and nothing else.
"Another 700 million years of the larger and much more intricate eukaryotic cells, but no aggregation to multicellular animal life. Then, in the 100-million year wink of a geological eye, three outstandingly different faunasfrom Ediacara to Tommotian, to Burgess. Since then, more than 500 million years of wonderful stories, triumphs, and tragedies, but not a single new phylum, or basic anatomical design, added to the Burgess complement."
In other words, the emergence of complex multicellular organisms, the basis of all life as we know it today, did not arise out of a slow, gradual "evolutionary" accumulation of adaptive changes, but in a sudden, qualitative leap. This was a veritable biological revolution, in which, "in a geological moment near the beginning of the Cambrian, nearly all modern phyla made their first appearance, along with an even greater array, of anatomical experiments that did not survive very long thereafter." During the Cambrian period, nine phyla (the basic unit of differentiation within the animal kingdom) of marine invertebrates appeared for the first time, including protozoa, coelenterata (jellyfish, sea-anemones), sponges, molluscs and trilobites. It took about 120 million years for the complete range of invertebrate phyla to evolve. On the other hand, we had the rapid demise of the stromatolites, which had been the dominant life-form for 2 billion years.
"Modern multicellular animals make their first uncontested appearance in the fossil record some 570 million years agoand with a bang, not a protracted crescendo. This Cambrian explosion marks the advent (at least into direct evidence) of virtually all major groups of modern animalsand all within the minuscule span, geologically speaking, of a few million years." (21)
For S. J. Gould, "We find no story of stately progress, but a world punctuated with periods of mass extinction and rapid origination among long stretches of relative tranquillity." (22) And again: "The history of life is not a continuum of development, but a record punctuated by brief, sometimes geologically instantaneous, episodes of mass extinction and subsequent diversification. The geological time scale maps this history, for fossils provide our chief criterion in fixing the temporal order of rocks. The divisions of the time scale are set at these major punctuations because extinctions and rapid diversifications leave such clear signatures in the fossil record." (23)
Plants and Animals
During the Cambrian and Ordovician period570-440 million years agothere was an impressive rise of graptolites and trilobites, and a major growth of diversity in marine species all over the world, including the emergence of the first fish. This was the result of the extensive spreading of the sea floor, especially of the Iapetus Ocean. During the Silurian period (440-400 million years ago) the melting of the ice-sheets caused an important rise in the sea-level. The shallow seas that covered much of Asia, Europe and North America were not a serious barrier to the migration of species, and, not accidentally, this was the period when marine transgression reached its maximum extent.
By this time there was a somewhat odd distribution of the continents. The southern continents were loosely clustered together to form a proto-Gondwanaland (Africa, South America, Antarctica, Australia, India), but North America, Europe, and Asia were separate. There was a small proto-Atlantic Ocean (Iapetus) between Europe and North America, and the South Pole lay somewhere in North-West Africa. Subsequently, the continents drifted together to form one, single super-continentPangaea. This process began 380 million years ago, when the Iapetus Ocean disappeared, giving rise to the creation of the Caledonian-Appalachian mountain belt. This event resulted in the collision of the Baltic with Canada, uniting Europe with North America. By that time, continuing convergence caused the north-west corner of Gondwanaland to impinge on North America, creating a semi-continuous land mass, in which all continents were united.
Such a massive increase in land area in turn produced a revolutionary leap in the evolution of life itself. For the first time, a form of life attempted to move from the sea to the land, at its coastal margins. The first amphibians and land plants appeared. This was the starting-point for an explosive growth of animal and plant life. That period was marked by the elimination of the shallow seas environment, and, as a consequence, the mass extinction or sharp decline of many marine species. Evidently, the changing environment forced some species to move from the coastal areas to the land, or die. Some were successful, others not. The great majority of marine organisms adapted to life in the shelves and the reefs of the shallow seas became extinct. Amphibians eventually gave rise to reptiles. The first land plants underwent an explosive growth, creating huge forests with trees reaching heights of 30 metres. Many of the coal deposits now being exploited have their origin in this remote period, the products of the accumulated debris of millions of years, rotting on the floor of prehistoric forests.
Formal logic approaches the natural world with an ultimatumeither or. A thing is either living or dead; an organism is either a plant or an animal, and so on. In reality, things are no so simple. In Anti-Dühring, Engels writes: "For everyday purposes we know and can definitely say, e.g., whether an animal is alive or not. But, upon closer inquiry, we find that this is sometimes a very complex question, as the jurists very well know. They have cudgelled their brains in vain to discover a rational limit beyond which the killing of the child in its mothers womb is murder. It is just as impossible to determine the moment of death, for physiology proves that death is not a sudden instantaneous phenomenon, but a very protracted process." (24)
We have already pointed out the difficulty in classifying very primitive organisms, such as viruses which stand on the borderline between organic and inorganic matter. The same difficulty arises in distinguishing between plants and animals. Plants fall into three major divisions. The first (Thallophyta) includes the most primitive forms, either single-celled organisms, or loosely organised groups of cells. Are these plants or animals? It may be argued that they are plants because they contain chlorophyll. They "live" like plants.
Rhodes has this to say on the subject:
"But this simple answer does not solve our problem of recognising a plantif anything, it makes it more confusing, for instead of providing a convenient clear-cut dividing line between plants and animals it points us to the hazy overlapping zone between the two kingdoms. And just as the viruses carried us back to the threshold of life, so these lowly thallophytes carry us to the ill-defined threshold that separates the plant world from the animal.
"Now many of the protozoans are, as we have seen, clearly animalsthey move, grow, assimilate food, and excrete waste products very much as undoubted animals do. But there are some tantalising exceptions. Let us look for a moment at the tiny unicellular organism Euglena, a common inhabitant of ponds and ditches. It has a more or less oval body which is moved through the water by movements of the flagellum; the creature can also crawl and perform worm-like movement: in other words it is capable of typically animal movementbut it contains chlorophyll and obtains nutrition by photosynthesis!
"Euglena is really a living contradiction to most of our ideas about the differences between animals and plants, and the contradiction arises, not because we cant decide which of the two it is, but because it appears to be both. Other forms which are very closely related lack chlorophyll and behave as any other animal, using the long thread-like lash to swim, taking in and digesting food, and so on. The implication of this is clear. Plants and animals are abstract categories of our own makingconceived and formulated purely as a matter of convenience. Because of this, it by no means follows that all organisms must fit into one group or the other. Perhaps Euglena is a living remnant of the ancient and primitive group of minute aquatic organisms which were the ancestors of both animals and plants. But can we not resolve the conflict by considering chlorophyll as distinctive? Can we suppose that if chlorophyllthen a plant will give us a sage rule? Unfortunately this too will not do, for some of these thallophytes (the fungi) which in other respects are very plant-like, do not possess chlorophyll. In fact, these fungi represent a problem familyfor in various members within it, almost all the typical plant characters (need for sunlight, absence of movement, and so on) break down. And yet, on balance, its members seem to be plants." (25)
The diversity of multicellular life represents a further qualitative leap in the evolution of life. The change from soft-bodied organism to ones with mineralised hard parts, as recorded in the Burgess Shale, represents the development of higher organisms. Certain substances like salt and calcium soak into the cell structure and tissues of sea creatures, which need to secrete them. Within the cell, the organelles which deals with metabolism or energy, mitochondria, absorb calcium and phosphate and ejects it as calcium phosphate. This mineral can be deposited within cells or can be used to build an internal or external skeleton.
The development of a skeleton usually takes place through the seeding of mineral crystals onto fibrous protein, called collagen. Collagen, which makes up around a third of all protein of vertebrates, can only be formed in the presence of free oxygen. The first move towards vertebrates seems to be the Pikaia of the Burgess Shale, a fish-like animal. The sea-squirts also appear to be an evolutionary link between those animals that were fixed to the sea floor and obtained their food from filtered nutrients, and free-swimming fish. These fishes (ostracoderms) were covered with shell-like scales, with no teeth or jaws. This revolutionary leap in the Silurian period produced the first vertebrates.
It was in this period (410 million years ago) that the jaws evolved from the front gill, which allowed the hunting of other animals instead of sucking nutrition from the sea floor. "The first fishes did not have jaws," says Gould. "How could such an intricate device, consisting of several interlocking bones, ever evolve from scratch? From scratch turns out to be a red herring. The bones were present in ancestors, but they were doing something elsethey were supporting a gill arch located just behind the mouth. They were well designed for their respiratory role; they had been selected for this alone and knew nothing of any future function. In hindsight, the bones were admirably preadapted to become jaws. The intricate device was already assembled, but it was being used for breathing, not eating." This was clearly a case, in Marxist terms, of elements of the new within the old. The first jawed fish, the acanthodians, or spiny sharks, gave rise to many kinds of bony fish. From these fishes evolved the first land vertebrates, the amphibians.
Gould continues: "Similarly, how could a fishs fin ever become a terrestrial limb? Most fishes build their fins from slender parallel rays that could not support an animals weight on land. But one peculiar group of freshwater bottom-dwelling fishesour ancestorsevolved a fin with a strong central axis and only a few radiating projections. It was admirably preadapted to become a terrestrial leg, but it had evolved purely for its own purposes in waterpresumably for scuttling along the bottom by sharp rotation of the central axis against the substrate.
"In short, the principle of preadaption simply asserts that a structure can change its function radically without altering its form as much. We can bridge the limbo of intermediate stages by arguing for a retention of old functions while new ones are developing." (26)
The Eusthenopteron had muscular fins, and lungs as well as gills. During dry periods, these fishes ventured from the pools to breath air through their lungs. Many of the Carboniferous amphibians spent much of their time on land, but returned to water to lay their eggs. From there, the evolutionary leap was in the direction of reptiles, which spent all their time on land and laid fewer eggs enclosed in a calcium carbonate shell. Commenting on these leaps in evolution, Engels writes: "From the moment we accept the theory of evolution all our concepts of organic life correspond only approximately to reality. Otherwise there would be no change. On the day when concepts and reality completely coincide in the organic world development comes to an end. The concept fish includes life in water and breathing through gills: how are you going to get from fish to amphibian without breaking through this concept? And it has been broken through, for we know a whole series of fish which have developed their air bladders further, into lungs, and can breathe air. How, without bringing one or both concepts into conflict with reality, are you going to get from egg-laying reptile to the mammal, which gives birth to living young? And in reality we have in the monotremata a whole sub-class of egg laying mammalsin 1843 I saw the eggs of the duck-bill in Manchester and with arrogant narrow-mindedness mocked at such stupidityas if a mammal could lay eggsand now it has been proved!" (27)
The Palaeozoic-Mesozoic boundary (250 million years ago) represents the greatest period of extinction in the entire fossil record. Marine invertebrates were especially affected. Whole groups became extinct, including the trilobites which had dominated the oceans for millions of years. Plant life was not seriously affected but 75% of amphibians and over 80% of reptile families disappeared. At present, it is estimated that four or five families disappear every million years. But at the end of the Palaeozoic, we had the disappearance of 75-90% of all species. By such catastrophic events did the evolution of the species unfold. Yet this process of mass extinctions did not represent a step back in the evolution of life. On the contrary, precisely this period prepared a mighty step forward in the development of life on earth. The gaps left in the environment by the disappearance of some species gave an opportunity to others to rise, flourish and dominate the earth.
The factors which influence the distribution, diversity and extinctions of life forms are endlessly varied. Furthermore, they are dialectically interrelated. Continental drift itself causes changes of latitude, and therefore climatological conditions. Variations in climate will create environments that are more or less favourable for different organisms. Tolerance to temperature fluctuations and climatic conditions are key factors in this process, giving rise to diversification. We see that diversity usually increases as we get closer to the equator.
The break-up of continents, their separation and collisions, all these factors change the conditions under which the species develop, cutting off one group from another. Physical isolation produces new adaptive variations, reflecting changes in the environment. Continental fragmentation thus tends to increase the diversity of life-forms. Kangaroos survived only because Australia was isolated from the other continents very early, before the explosive rise of the mammals which caused the disappearance of large marsupials in all the other continents. Similarly, the destruction of oceans produces mass extinctions of marine species, yet at the same time creates the conditions for the development of new land plants and animals, as was the case at the inception of the Pangaean land mass. Death and birth are thus inseparably linked in the chain of evolutionary development, where the mass extinction of one species is the prior condition for the emergence and development of new ones, better equipped to cope with changed conditions.
The evolution of the species cannot be regarded as an isolated self-contained fact, but must be seen as the result of a constant and complex interaction of different elementsnot only the infinitely large numbers of genetic mutations within living organisms, but also the continual changes in the environment: fluctuations in sea-level, water salinity, the circulation of oceanic currents, the supply of nutrients to the oceans, and, possibly, even factors like the reversal of the earths magnetic field, or the impact of large meteorites on the earths surface. The dialectical interplay of these diverse tendencies is what conditions the process of natural selection, which has produced forms of life far richer, more varied and more wonderful than the most fantastic inventions of poetry.
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