WHEN LIFE BEGAN: an evolutionary viewpoint

PERHAPS there can be no more absorbing themes than the inquiry into, and the discussion of, the history of Life. We talk glibly of the aeons of the past, of the queer forms of life, now so dead and buried, that flourished unchecked in so-called ‘prehistoric ‘days, of missing links, and of ‘the Descent of Man.’ How is it that we can record the history of prehistoric times? How are we so confident of the life and habits of forms we only know as dead remains and which have left no descendants to these present days? By what devious and obscure paths do we follow the rambling pioneers of the world of Life, discern their footprints, feel the ashes of their long-abandoned camp-fires, and finally discover the ruins of their ancient dwelling-places?

This study and the elaborations of these themes are no time-laden branches of a mediaeval alchemy; indeed, the science, or if you like, the art, is scarcely two centuries old, at least in its purer and more informative aspect. Yet already it is established. Its legionaries are indeed legion; its truths no less than scientifically attested truths.

GOING BACK ALONG THE PATH OF TIME

How then shall we, the humble followers of a science that is new to us, approach the task? There are three main ways, but none is a carriage way, and certainly there is no royal road. Let us follow each at least a little distance and in a simple style, and see what we can make of it.

First, there is no one of us so poor in experience or intellect but can know a little of at least a few animals and plants, and for this purpose we can include Man (a term so used that it includes woman) among the animals. The most superficial observation will have convinced us that a great variety of forms exists. We can, for example, distinguish males and females, parents and offspring, and cousins of near or distant degree. We know that each has some sort of traceable ancestry or lineage, that some forms will interbreed and have offspring

like themselves, and that others cannot, or, at least, do not appear to have this habit or ability.

We see also that animals and plants do not interbreed and have no familiar relationship, though both are obviously living things, as living, and certainly as capable of death, as we ourselves are. We see, also, that every new-born animal or plant, however or in whatever form it may be produced, is directly related to some form of similar character or appearance. Search as we may we do not see that Life is of spontaneous occurrence, that it comes from inanimate nature, or that it flowers without some stem or root.

If we search more closely we find that superficial resemblance in animals and plants is often, though by no means always, correlated with an underlying structural similarity. We find, for example, that birds always have feathers and always lay eggs, while mammals never have feathers but always have hair and very seldom lay eggs. Reptiles, on the other hand, never have hair or feathers and may lay eggs or produce living young. Thus even a superficial examination is sufficient to show us that there are broad lines of demarcation between different groups of animals and plants, or, as we may say, we see the rough outlines of classification. We find later that these broad groups are divisible in other subgroups, into orders, families, genera and species.

FLEETING GLIMPSES OF THE DIM AGES: THE EMBRYOLOGIST’S VIEW EXAMINING all such groups or sub-groups or even individuals in great detail we see that there is sometimes very great disparity between them, and if we further study the similarities and disparities we can work out a certain relationship between living things. This does not give us, of course, the family tree, but it does give us the outline of its branches and the pattern of its foliage, leaving us to fill in the more solid and woody details of the trunk and inner branches. This method of approach, the easier and more obvious, is called, if we consider only animal life, Zoology and Comparative Anatomy.

!,The zoologist or anatomist is aware that when he studies the new-born young he is seeing the simplest form of the particular living animal. But if he can examine it before it is born, in what is called the embryonic or fcetal stage, he sees some remarkable things. At certain stages in the de-

velopment or embryology of the little animal it bears resemblances to forms very little akin to its ultimate condition. For instance, in the human embryo’s development, at one period it has gill clefts in the neck like those of a fish, and in another stage it has a well-developed tail. Examples of this sort could be multiplied a hundred-fold.

Embryology has now come to be almost an exact science, well documented and with a wide range of discovery. It is now known, perhaps widely, that the young individual in its pre-natal condition runs through the whole developmental history of its kind, though it does it very quickly, just like a cinematograph film that is speeded up. Thus we can learn that the ancestors of the human race once possessed tails, and before that belonged to a race with gills. It may not be a flattering observation, but it is interesting and, as we shall see, there is confirmatory evidence.

The second avenue of approach is, therefore, through Embryology, which gives us in the developmental stages of the embryo a fleeting glimpse of the past, like the leaves of the family album slipped through the ringers.

A SLOW-MOTION FILM OF THE PAST: THE FOSSIL HUNTER’S STORY THE third and last avenue of approach is through the study of fossils, and it is in some ways a supplement to the method of comparative anatomy, and in others a reversal of the method of embryology. It is a slow-motion film, not, unfortunately, a complete film, and one much the worse for wear; but we have full liberty and plenty of time in it to examine each piece of the evidence. This study of fossils, which we shall see is called Palaeontology, gives us direct confirmation of the observed embryological facts, it amplifies the structure and findings of comparative anatomy, and gives, above all, actual facts and details of the history of the things that have lived in the past, which are the relatives and ancestors of all the living things we know now.

EARLY MISTAKES ABOUT THE FOSSILS

IT is this third method of approach that we must examine more closely now. What are fossils? What is this science of Palaeontology? How can we proceed to investigate it?

The term Palaeontology is derived from three Greek words meaning ‘the science of ancient life ‘(palaios, ancient;

ontos, life or being; logos, science). It is concerned with the evidences of animals and plants that existed in former times and which are found occurring naturally in geological deposits. Fossils are so called from the Latin word fossilia, meaning ‘things dug up,’ because most of them have to be excavated from their rocky, sandy, or clay covering.

Fossils have been known for centuries, but the early observers did not attach any great importance to them. Some regarded them merely as curiosities, others recognised that they had once lived but had become petrified or had been left derelict by the retreat of the waters. At the time of Aristotle it was commonly believed that living things could be generated from mud and slime and their subsequent petrifaction was, therefore, not surprising. General opinion, therefore, was content to regard them merely as curiosities, a belief that persisted for many centuries. But a small number of more scientifically-minded persons, including Leonardo da Vinci the painter, in later days came to realise that they were the preserved remains of once living creatures, and ultimately, at the beginning of the eighteenth century, this was the accepted view. To-day the study of fossils is the whole-time occupation of quite a number of persons in universities, museums, and other educational and even commercial institutions, besides being the hobby of hundreds of people. We will now proceed to examine the subject and see what it can tell us.

THE MYSTERY OF HOW LIFE BEGAN

MOST people to-day realise that the world is a very old structure, so old that the approximate figure of two thousand million years which may be given as its age is beyond the comprehension of most of us who are not mathematicians and astronomers. For at least half of this immense period the record is so inscrutable that we have absolutely no knowledge of the forms of life that were flourishing then. How life itself originated is a complete mystery. It has been suggested that the original ‘germ ‘of living substance was conveyed to this earth by a meteorite which came from another world. This theory seems, however, rather to beg the question. Another suggestion is that the germ or spore of life might be borne from some planet to this world without a meteoric conveyance merely by the action of light and its pressure. Demonstration of this ‘light pressure ‘is easy, but neither

of these theories helps us very much for they both simply transfer the problem from our door to that of another world. Perhaps the most satisfactory theory so far advanced is that known as the Protobion (Greek, protos, first; bios, life). We may assume that in the earlier days the atmospheric and climatic conditions of the world were rather different from now. Temperatures may have been more or less stable; night and day much alike. The humid atmosphere charged with carbon dioxide lay heavily on the land where the muds and oozes of the waters’ edge may have been rich in such elements as carbon, nitrogen, and phosphorus. These substances are relatively unstable, and it is not impossible that under these conditions and in the presence of some additional substance which could work a change in them while remaining unaffected itself, the combination of these chemicals may have taken place and the first life-jelly may have been formed. This simple chemical compound, endowed with the purely physical power of absorbing food chemicals (I.e. endowed with the power of eating), and able to reject waste products, with limited powers of jelly-like movement, may have formed the first life, the Protobion, the first link of a chain whose last segment we ourselves shall never see.

LIFE A THOUSAND MILLION YEARS AGO

HOWEVER this primitive germ may have arisen we have no possible means of learning its composition or characters. The earliest of the rocks1 are grouped together as pre-Cambrian, a grouping which includes the oldest and most diversified rock deposits in geological history. So crushed and worn are those rocks that no definite knowledge can be obtained of any forms of life that may have then existed. Their record is truly inscrutable and perhaps irretrievably lost. We know that masses of phosphatic material occasionally occur and these probably indicate the ultimate remains of once living things, but beyond that we know nothing of pre-Cambrian life.

In the succeeding period, known as the Cambrian, however, we find that fossils are widely scattered and that diverse and complicated forms are not unplentiful. Down through the long corridor of time, over one thousand million years long, there comes this evidence of the flowering of life. Many of these fossils are beautifully preserved and capable of the

most minute examination. So here is evidence that during that first half of the world’s history, of which we can catch no satisfying glimpse, the simple original ‘germ ‘must have been developing in complexity and giving rise to a great series of very different sorts of animals, as if the tiny seed had produced at last a great tree with many branches but with each of the leaves of different appearance.

At any rate, one of the most striking features in the geological history of the world is that, first, we have this great pre-Cambrian period like an unillumined stage, and then, suddenly, as if the floodlights had been switched on, we see a new scene with many well-developed types of life, set to play their part as the opening chorus of this great drama of living things.

On the other hand, complex as the developing forms undoubtedly were, they must also have been without shells or any hard skeleton and they were certainly water-living forms. This helps to explain the absence of fossils in the pre-Cambrian rocks, for fossilisation is difficult where there are only soft structures in the body. For this reason the pre-Cambrian period is occasionally known as Collozoic (Greek, kolla, glue; zoon, an animal).

HOW TIME’S DEEPEST ARCHIVES WERE FORMED IT should be explained here that for an organism to be preserved certain definite conditions must be fulfilled, and their fulfilment is largely a matter of chance. The animal dies on land or in the water, and if it is a soft-bodied and shell-less form, as the earliest were, decay is usually so speedy a process that no trace is ever left. Occasionally, however, where the ‘body ‘is left in comparatively fine mud in quiet waters, a faint impression may be made and the subsequent consolidation of the material may preserve for ever this trace of the long-vanished form.

Naturally, shelled or bony animals are by far the more common among fossils. This is due to the resistant nature of the material which will withstand the transportation of the dead creature by river or flood, and will also endure the geological or physical changes to which the containing deposit is tater subjected.

The animal dies, and its body may lie at the place of death or may be carried by some agency to new surroundings, so that the position in which a fossil is found is not always the

place of death. If the body or specimen lies on land it may disintegrate and leave no particular trace, though it may be covered by wind-blown sand or volcanic ash or have preservation aided by chemical means, such as percolating waters. Where the dead animal is left or borne into water the chances of fossilisation are much greater. Silt, mud, and sand may be gently laid upon it, and as the years roll by the deposit may harden and become the rock we see to-day. Thus, so very many years afterwards, and again by chance, we may break into that rock and discover its long-hidden history.

These processes of fossilisation and this building up of sedimentary deposits has always been going on so that really the historically blank core is surrounded by layer after layer of younger and younger age, most of the deposits bearing within them the secrets of their life.

Thus there might be a continuous record from the first preservable things up to the present day, if it were not that geological processes are always going on. Rivers, rain, snow, frost and ice all do their work of breaking down the tardily-built cliffs and of rotting hills. Earthquake and flood, volcanic action and the slower hand of time exert their influence so that the record becomes discontinuous and jumbled. Thus, nowhere do we have revealed the unbroken sequence of geological events. The complete story is obtained only by collating and adding up the fragments that are visible in cliffs and quarries, in river-banks, canyons, and railway cuttings. With this evidence from the very numerous fossils that have there been obtained, have been marshalled the groups of facts that have gone to make the story of Palaeontology.

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