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 earth’s 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 earth’s history, and yet this convulsive period accounts for no less than 88 per cent 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 earth’s 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 per cent of nitrogen and roughly 20 per cent 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 that 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 earth’s 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 ultraviolet 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 ultraviolet radiation from the sun. However, changes in the composition of the atmosphere cut off the supply of ultraviolet 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 ultraviolet. 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 molecules – DNA 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 metabolism – absorbing 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 ultraviolet 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 millions 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 ultraviolet rays, which inactivate the life-inducing DNA molecule. The first primitive living organisms – the prokaryotic cells – were single-celled, but lacked a nucleus and were incapable of cell division. However, they were relatively resistant to the ultraviolet 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 organisms 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 and 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 mankind – the 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 oxygen – in fact they produce it in the sunlight – but 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.” (Bernal)[14]

  1. Bernal, J. op. cit., p. 26.