During the Cambrian and Ordovician period – 570-440 million years ago – there 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-continent – Pangaea. 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 northwest corner of Gondwanaland to impinge on North America, creating a semi-continuous landmass, 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 ultimatum – either…or. A thing is either living or dead; an organism is either a plant or an animal, and so on. In reality, things are not 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 mother’s 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 plant – if 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 animals – they 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’ movement – but 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 can’t 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 making – conceived 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 chlorophyll – then 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 family – for 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. 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 else – they 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 fish’s fin ever become a terrestrial limb? Most fishes build their fins from slender parallel rays that could not support an animal’s weight on land. But one peculiar group of freshwater bottom-dwelling fishes – our ancestors – evolved 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 water – presumably 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.” (Gould)[26]

The Eusthenopteron had muscular fins, and lungs as well as gills. During dry periods, these fishes ventured from the pools to breathe 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 mammals – in 1843 I saw the eggs of the duck-bill in Manchester and with arrogant narrow-mindedness mocked at such stupidity – as if a mammal could lay eggs – and now it has been proved![27]

  1. Engels, F. Anti-Dühring, pp. 26-7.

  2. Rhodes, F. op. cit., pp. 138-9.

  3. Gould, S. Ever Since Darwin, pp. 107-8.

  4. MESC, Engels to Schmidt, 12 March 1895.