Essay on the Evolution of Life (With Diagrams)

Read this essay to learn about: 1. The evolution of life – Virus, Mycoplasma and Rickettsiae 2. Evolution of the plant kingdom 3. Fossils 4. The ages of the earth 5. Gondwana land and 6. Glossopteris flora.

The word evolution, arising from the Latin word evolvere (to unroll) means the deve­lopment of something by natural processes. Thus, we may talk of the evolution of the Earth, of the evolution of Man or of the evolution of a political dogma. When we talk of the Evolution Theory or about Organic Evolution we mean the evolution of life on this Earth, the greatest exponent of which idea was Charles Darwin.

If there can be a story based on facts there can be none more interesting than, that of the evolution of life on our Earth. The ideas of Evolution once began as poetic dreams, then as whispered theories, finally condensing into the bold assertions of Charles Darwin. Since then, evidences have piled up to such an extent that Evolution is no longer a mere theory but an established fact. Let us see how it took place.

In talking about the evolution of life we begin from the beginning. Astronomers and geologists have concluded that this Earth first evolved out of celestial bodies as a fiery spinning ball of extremely hot gases. Gradually, through hundreds of millions of years, the gases cooled down and a solid core was formed.

An atmosphere of air (possibly somewhat different in composition from what it is today) formed a covering round it and the hot steam poured down as water forming the first seas and lagoons that were rather shallow. Life first began on this shallow water when it cooled down a bit. How did it begin? We have not yet got the answer but, perhaps, someday we shall. In all proba­bilities its origin was merely accidental and was possible only in the peculiar conditions of that time.

Still there are some scientists who believe that life originated outside our Earth and then it got transferred here. Whatever may be the cause of its origin, life began on this water by the formation of a speck of that mysterious substance— protoplasm. But, possibly, before the appearance of protoplasm there was a long history of natural synthesis of the most interesting organic substance protein, which is the basis of all living matter.

Proteins are formed of a number of common elements—C, H, O and N. But, pro­tein molecules are very complicated and very large. As compared to the molecular weight 18 of water, that of insulin, a very simple protein and the only protein whose constitution is known (the structure was first established by Sanger in 1958 for which he was awarded the Nobel Prize), is about 5,000 while most protein molecules weight over 100,000 and some are known weighing about 10,000,000.

These complex mole­cules are formed of simpler amino acid molecules which have the peculiar characteristic of having both acidic (CO.OH) and basic (NH.H) Parts so that the acid part of one molecule can unite with the basic part of another and, thus, a number of amino acid molecules may form what is known as a peptide chain (Fig. 792).

These chains turn and twist to form the complex protein molecules (Fig. 793). The atmosphere on our very primitive Earth had not been the same through the ages. About 3500 million years ago, when ‘life’ was possibly being evolved, it was much warmer there was no free oxygen in the air which, on the other hand, was possibly composed of free hydrogen, ammonia, methane and water vapour. It has been shown by Miller and Urey in 1955 that such a mixture of gases in a closed chamber when subjected to electric discharges gives rise to many of the known amino acids by artificial synthesis.

Such synthesis of amino acids might have taken place naturally in those days as result of atmospheric electric dis­charges or natural radiation. As this type of synthesis was possibly only in the atmos­phere prevalent at that time, one may suppose that the origin of ‘life’ was possible only in those days. At least 21 types of amino acids are well-known today and they may combine in endless ways forming hundreds of protein types. Some of the proteins, like the egg albumin, have been obtained in crystalline forms.

We are now aware of a very interesting group of substances—the Viruses which are somewhat intermediate between the living and the non-living. The virus substances had been known since their discovery by Ivanovsky in 1892 as peculiar things which cause dreadful epidemic diseases in plants (e.g., tobacco mosaic virus) as well as animals (chicken-pox, smallpox, measles, influenza, poliomyelitis, rabies, etc.). The infective principles were known but they were not visible under the microscope and they could not be assigned to any type of living beings.

In 1953 Stanley obtained tobacco mosaic virus in the form of crystals which could infect other tobacco plants. Further researches have shown that these virus particles are nucleoproteins, i.e., compounds of nucleic acid and some protein. Each virus particle has a central core of ribonucleic acid (RNA) or, sometimes, deoxyribonucleic acid (DNA) and is encased by proteins.

Some virus particles are rod-like (e.g., tobacco mosaic virus— Fig. 794), others spherical (e.g., influenza, smallpox and polio virus—Figs. 795 and 796). Figure 797 shows the comparative sizes of some big protein molecules, some known genes, some virus particles, the X-chromosome of Drosophila, a bacterium and a red blood corpuscle.

It should be remembered that it is not possible to see anything smaller than 450 µµ with the optical microscope while the largest virus particle, the rod-like tobacco mosaic virus (Fig. 794), is only about 300 µµ long. If these virus particles are injected into the cells of plants or animals of the specific type, they multiply enormously. This multiplication is a characteristic of living substances. But virus particles as those of tobacco mosaic possess neither water nor enzymes without which no vital function can go on.

The virus particles do not multiply in the normal way. When these particles enter the living cell the protein casing is first destroyed. Then the protoplasm of the living cell forgers its normal way of metabolism and begins to produce innumerable new nucleic acid particles instead—the first virus particle somehow induces it to do it. The new nucleic acid particles get invested with new protein casings and then the virus particles increase rapidly.

The minute bacteria also have their virus enemies, the bacteriophages. These have been extensively used by doctors, because they are known to destroy bacteria causing dis­eases. The bacteriophages have tadpole-like structures (Fig. 797), their head part con­taining a nucleic acid (deoxyribonucleic acid or DNA) and the tail part containing some protein. It has been found that when a bacteriophage attacks a bacterium, it is only the DNA which enters the bacterial cells.

The whole structure at the time of infec­tion may be compared to that of a doctor’s hypodermic syringe from which the injection fluid (here, DNA) is pushed into the bacterium and the syringe proper is represented by the protein shell of the bacteriophage. Very soon after, the protoplasm inside begins to synthesise new bacteriophages until the cell is full of these and is destroyed. In the tobacco mosaic virus also, it has been found that the RNA alone (without the protein) can carry on the infection. So the nucleic acid is the more important and we shall again note its importance as we study chromosomes.

Further studies of virus types have shown that there are various degrees of comple­xity within them and, even though they cannot be considered as ‘living’, they surely possess many characteristics of the ‘living’. Recently, another group of pathogens have been detected and named Mycopalsma. Its structure (Fig. 798) is somewhat more complex than that of the virus. It contains DNA strands as well as RNA granules encased in a membrane of proteins, fatty matter and cholesterol.

It develops ‘baby’ particles inside it which bud off as new particles. Rickettsiae form another group of pathogenes (causing a type of typhus) which, like the mycoplasma come nearer to the bacteria. The peculiarity of these particles suggest that although what is known as ‘virus’ today was, perhaps, not evolved before the living protoplasm (it has been su­ggested that virus might have been evolved by the degeneration of in­tracellular parasites which lost their cell walls), there might have been substances intermediate be­tween living protoplasm and non-­living inorganic life. Whatever be it our definite conception of ‘life’ begins with the evolution of protoplasm with all its complexities of metabolism.

Only recently, Dr. Theodore O. Diener, a plant pathologist of the U.S.D.A., has discovered yet another virus-like particle about one-eightieth the size of the smallest virus. This also causes plant diseases but in its constitution there is merely a speck of nucleic acid without the protein coating which is the rule in viruses. This has been named a viroid.

The impact of the discovery of these sub-living objects may be seen in a new bio­logical classification of the living world proposed recently by Hu Hsen-Hsu (1965) which divides the living beings into (i) Protobiota (virus, etc.) and (ii) Cytobiota (cellular plants and animals).

Is there the existence of life anywhere outside our Earth? We have not yet got any definite evidence as to that. But, this Universe is big—much bigger than we can imagine. In it there are thousands of planetary bodies with exactly the same conditions as pre­vailing on our Earth. If that be so, and if the first origin of life was accidental, we can well imagine that there is a possibility of the co-existence of life on some other celestial bodies. It is also likely that such living bodies are different from what we are familiar with.

The first speck of protoplasm was different from all other inorganic matter hitherto known. It possessed ‘life’, i.e., it could grow, carry on metabolism, respire and reproduce. Reproduction was rapid and out of a single speck there were millions. The first formed living bodies were all alike and they depended solely on inorganic substances for their nutrition as the organic world was just born. At that time there was no differentiation into plants and animals.

But, through thousands of years, while the first formed speck of protoplasm was passing through millions of generations, very insignificant variations in the descendants were accumulating into large differences so that one could now distinguish different forms. Meanwhile, the world was changing and certain new variations were found to be better adapted to the newer conditions.

These two facts of variation and adaptation were gradually combining into a great moulding force causing the evolution of new forms of life. Very soon it was found that life was evolving in two directions; firstly, towards, the development or animals which is beyond the field of Botany and secondly, to develop plants where the most striking achievement is the development of chlorophyll and, later, of the erect terrestrial plant habit with vascular bundles.

There is no reason to suppose that evolution went on only along one or two particular lines. Plant and animal forms were changing in all direction. Most of the lines were being abandoned later on as the forms failed to survive the rigour of their environ­ments while a few lines survived and again evolved. Evolution went on in this way.

The course of evolution may be traced with reference to the different plant groups that exist today and those which are definitely known to have existed in the ancient days. The picture of the first days of evolution is rather hazy. There was some form of primordial life out of which arose the bacteria (Schizomycetes), the peculiar slime moulds (Myxomycetes), the blue-green algae (Cyanophyceae) and above all, the Flagellatae—an interesting group of unicellular naked organisms which move about in water and out of which the true algae arose. Out of the different types of flagellates arose the diatoms (Bacillariophyta), the brown algae (Phaeophyta), the red algae (Rhodophyta) and the green algae (Chlorophyta).

Out of these, all the groups ended blindly evolving no farther, but the Chlorophyta line which, possibly, gave rise to the land plants. The fungi (Eumycetes) arose by degeneration at some early and hazy stage of evolution. The Chlorophyta possibly gave rise to the Bryophyta on one hand and the Pteridophyta (sometimes called Vascular Cryptogams, beginning with such simple forms as the extinct Psilophytopsida—the so-called ‘Rhynia stock’) on the other.

Out of the ancestral Pteridophyte as simple as the Psilophytes arose four types of Pteridophytes—(i) the Psilotopsida; (ii) the Lycopods, Selaginella and Isoetes (Lycopsida); (iii) the Equisetums (Sphenopsida) and (iv) the ferns (Filicopsida). It is the Pterophyta line represented by the Filicopsida among the Pteridophytes, or probably, the Progymnosperms, a specialised group of Pteridophytes, which developed further giving rise to the seed plants (Spermaphyta). The Spermaphytes developed into the Gymnosperms at an early stage and the Angiosperms at comparatively recent times. This probable course may be traced in a plan as shown in Fig. 799.

The above hypothesis on evolution is not purely imaginary but is substantiated by the invisible writing on rocks. Let us see how.

When the first seas and lagoons were formed, the torrential rain was causing strong river currents to bring down debris of rock and earth which deposited on the seabed forming thick layers of sediments. In course of time these layers were being consolidated into what are known as sedimentary rocks. Such sedimentary rocks are being continually formed and are being deposited one layer above another.

These sedimentary rocks contain not only mineral matter but also remains of plants and animals that were present in that particular period of time. Such remains are known as fossils. Geologists can ascertain the ages of the different strata of sedimentary rocks.

Thus, if a fossil is dis­covered, it is possible to know at what time that type of plant or animal flourished. This foot has been of the greatest help in tracing the evolution of the plant and animal kingdoms through ages as, it is found, rocks of different ages contain remains of different types of organisms.

It is almost impossible to make a correct estimate of the ages of the rocks in terms of years. Yet, different estimates have been made, and, naturally, they vary widely. One such estimate puts the total age of all the rocks under survey as about 4500 million years.

The first 3000 million years of this is covered by crystalline rocks which formed the original surface of the Earth when it first cooled down. These rocks scarcely contain any living remains and are called Azoic (lifeless) or Archaeozoic (primitive life) rocks. The latter name is because evidences are accumulating that life did exist even in these very ancient days.

Evidences are accumulating that algal life existed 2500 million years ago and bacterial life started even before them (3400 million years old fossils are claimed from South Africa). During the next 1000 million years we get the Proterozoic (beginning of life) rocks which also show very little evidence of life. But, on the whole, there is evidence that the thallophytes and the protozoans had already developed and so plant and animal kingdoms had differentiated.

Evolution of life gained a new momemtum after the Proterozoic days. The next 330 million years are covered by the Palaeozoic (ancient life) Crocks. As the lowermost Palaezoic rocks are called Cambrian all the rocks before that (Atchaeozoic and Pro­terozoic) are sometimes called Pre-Cambrian during which life developed rather slowly and fossils are practically unknown. During the first half of the Palaezoic (Early Palaezoic—Cambrian and Ordovician) probably all life was confined to the seas and lagoons as only the Thallophytes are known. A dreadful bleakness and stillness reigned on land.

Then, gradually, a change occurred and life moved from water to land. This acquisition of the terrestrial habit was a big event in both the plant and the animal kingdoms. It is certainly difficult to tell today when exactly the migration of plants from water to land took place. Recently, some woody elements (tracheides) and spores of land plants have been discovered from Indian Cambrian rocks as also in some other countries.

It is then justifiable to say that the first land plant (Pteridophytes) existed during Cambrian, i.e., in the Early Palaeozoic and the migration might have taken place even immediately before the Cambrian. During the Middle Palaeozoic (Silurian and Devonian) we get evidence of the existence of Bryophytes (Hepaticites devonicus) and of well-developed Pteridophytes (Psilophytales, Protolepidodendron—allied to Lyco- podium, Hyenia—allied to Equisetum and some Primofilices—primitive ferns).

In the Late Palaeozoic (Carboniferous and Permian) we get the maximum development of the Pteridophytes. The Permo-Carboniferous is known as the Coal Age as at this period the first big forests were formed on the edges of lagoons at river estuaries and these forests have carbonised and have given rise to the present-day coal.

These forests were formed by giant trees which were allied to the Lycopods and the Equisetums and are now extinct. There were also some ancient, ferns and the first Gymnosperms—the Cycadofilicales or Pteridosperms and the Cordaitales. Perhaps the atmosphere differed in its constitution and was warmer at that time.

At the end of the Palaeozoic (that is, after the Permian) there was some climatic change on this Earth- There was a period of desolation. All the older forms of life died and when climatic condition again became favourable, newer plants were evolved. This period is covered by the Mesozoic (middle life—Triassic, Jurassic and Creta­ceous) rocks covering about 120 million years. This was the age of gigantic reptiles like the Dinosaurs and of the Gymnosperms. The ferns also attained the maximum height of their development in this age.

Towards the end of the Mesozoic (Cretaceous) another great change came over this Earth. Perhaps there was some catastrophic change and the older forms of life be­came unfit for survive. Most of the older forms vanished, the Angiosperms among the plants and the Mammals among the animals raised their heads.

The rocks since the Mesozoic to the present day (about 60 million years) form the Cainozoic or Cenozoic (recent life). The world during this period has not varied much although there have been periods of cooling down of the Earth involving the approach of glaciers. It is during this period that the present-day flora (predominated by the Angiosperms) and the present-day fauna (predominated by the Mammals) evolved.

Evolution during these ages was by no means uniform. It was always being moulded by the frequent climatic and geographic variations of this Earth. During all this time there had been alternating periods of extreme cold when glaciers advanced to areas which are very warm today, and of warm spells when a luxuriant vegetation covered the face of the Earth including the poles which are frozen today.

There were also alter­nating phases of humid and arid climates. Changes were sometimes catastrophic caus­ing sudden extinction of whole races of plants and animals. The geography also was always changing. Geologists think that during the late Palaeozoic Coal Age (and per­sisting in the Mesozoic) there was a vast continent in the south covering what are India, Africa, Australia, South America and Antarctica today. This continent is named Gondwana land (Figs. 803 and 804) after the Gond tribe of Madhya Pradesh.

To the north of it were two continents—Angaraland (modern Siberia and North China) and the European-North American land mass. In between was the sea of Tethys. This geography explains the fact that at this age the same types of plants and animals flourished in places like India, Africa, Australia and South America, which ate so distant today. The particular Gondwana flora of the Permo-carboniferous has been named the Glossopteris Flora from the preponderance of a leaf-type called Glossopteris (Fig. 805) which was probably a pteridosperm. (seed-bearing fern).

The Gondwana fossils are represented in our country in two groups of rocks called Lower Gondwana (Permo- Carboniferous, i.e., Late Palaeozoic—predominated by Glossopteris), and Upper Gond­wana (Mesozoic—predominated by Plilophyllum leaves of the Cycadeoidales group of Gymnosperms). The former can be seen in the Talchir-Barakar Raniganj-PanChet coal mine areas and the latter is Panchmarhi, Rajmahal and Jabalpur. The Gondwana land was possibly formed during the Carboniferous and began to split up during the Cretaceous. The Gondwana Flora is treated more elaborately in College Botany Vol. II.

This is the story how on a hot fiery mass life first came into being and then how that speck of protoplasm developed into human beings and lovely flowers of today.

But, scientists cannot take any theory for granted unless solid evidences are advanced prov­ing the theory. The above is a statement of the phenomenon of Evolution.