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 substances – nucleoproteins. This is also true of viruses, the simplest known living things that 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 this – and 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 salts 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: “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 cells – that 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 oxygen – in 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. Richard Dickerson hypothesised:

“One can visualise, 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.” …. “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 that 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 errors – mutations – may 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 polymers – combination of molecules – grouped 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]

  1. Engels, F. Dialectics of Nature, p. 282.

  2. Buchsbaum, R. Animals Without Backbones, Vol. 1, p. 12.

  3. Gould, S. The Panda’s Thumb, p. 181.

  4. Scientific American, 239, [1978].

  5. Quoted in Lewin, R. Complexity, Life at the Edge of Chaos, p. 51.