Popular Science Monthly/Volume 64/April 1904/Evolution of the Human Form
|EVOLUTION OF THE HUMAN FORM.|
THAT men, or thinking beings akin to man, exist only on that minute fragment of the universe we call the earth is a conception so highly improbable, in view of the vast multitude of planets which we may logically conceive to exist, that it seems as if no reasoning being could entertain it. It is true, indeed, that in our own solar system perhaps only two or three of the planets, perhaps only the earth, are in a condition suitable for human habitation, and that the earth has been so for a comparatively brief period. It may well be, therefore, that only a very small percentage of the planets of space are in a similar condition. But in view of the vast multitude of planets that presumably exist, the number of those that possess reasoning beings is probably great. If we deal with this question from the point of view of actual evidence, the fact that the only planet whose conditions we know is inhabited by man is a strong argument in favor of his wide-spread existence. On the other hand, the fact that man's existence upon the earth is dependent upon a certain limited range of temperature, of brief duration in the earth's total history, is an argument on the opposite side, and goes far to narrow the possible domain of life in the universe. Yet if we extend our view to embrace the past and the future as well as the present, we can not avoid the conclusion that the realm of life and thought in the universe is an immense one.
To this question of the existence of thinking beings appertains another, that of their form or physical character. Are we to suppose that these beings, wherever placed, resemble man, or that each planet develops a type of its own, and that, if we could bring together a collection of the men of different sections of the universe, we should have a diversified museum of animal forms, with but one characteristic in common, that of the faculty of abstract thought? This is the conception usually entertained by those who have indulged in speculation or fiction concerning the inhabitants of Venus, Mars and other planets of our system. Yet it is one that may be questioned. A study of the development of life upon the earth seems to lead to the opposite conclusion, and yields warrant for the theory that thinking beings, wherever they may dwell, resemble man in body as well as in mind In other words, we have reason to conclude that, if we were capable of traversing the universe, we should find beings akin to ourselves in many a remote corner of space.
On the earth, indeed, life exists under conditions which may be widely departed from in many other planets. Here the extreme range of favoring temperature is that between the freezing and the boiling points of water, the practical range being much smaller. Special conditions of surface material and formation, atmosphere, chemical action, etc., are also necessary. It is far from certain that the same conditions are necessary everywhere. Life may perhaps flourish on other planets under quite different conditions of temperature, gravitation and chemical action. It is true that, if all the spheres of space are made up of essentially the same chemical elements, as spectrum analysis seems to show, the range of life conditions can not greatly vary. Yet if the more abundant and active elements in any sphere differ from those of the earth, the consequent life conditions might vary accordingly and life exist under relations of temperature and chemical action unknown to us. The one thing essential, in every case, is an environment favoring organic chemism.
All this, however, is a side issue. It has no necessary bearing upon the question of animal form. If human beings could exist on some planets at 1000° instead of 100° F., and be made up of a protoplasm of quite different chemical composition, their forms and modes of action might still be closely the same. For the external forms of animals are due to physical, not to chemical, conditions. They are mainly results of the struggle for existence, and the effort to gain the most effective formation for the incessant battle of life. This must go on wherever life appears and develops, wherever the temperature or the active chemical elements may be. Much the same may be said of internal development. It seems to us that in any advanced stage of life the energy of animal motion must be a consequence of chemical change, due to something equivalent to oxidation of the tissues. There must also be an efficient agency for the supply of fresh nutriment to the wasting tissues, nerves for sensation and muscles for action, excretive and reproductive organs, etc., in short, organic conditions analogous to those which exist in our own bodies.
In truth, the minuteness of the earth as a planet, and the seeming insignificance of its life story as compared with that of all spheres and all periods, are apt to give us a false impression of the real significance of the development of life upon our place of abode. Though the process of organic evolution here may seem to us a minor one, a review of its history will serve to show that it has been a major one. And its final outcome in man can hardly be looked upon as a fortuitous result, but seems rather the inevitable consequence of an innumerable series of experimental variations. The life period upon the earth has been a very extended one, stretching through many millions of years, and living matter has passed through an extraordinary diversity of forms, from microscopic specks of primeval jelly to the highly organized form of man. Of any planet upon which thinking beings have appeared, doubtless much the same may be said. The beginning must have been at the same low level; the gradations must have been similar in general character; the ultimate may perhaps have been widely different, though there are what seem good reasons for believing that it was closely accordant.
The final result of organic evolution depends largely upon external relations, the environment; largely upon the relations of organic matter to the chemical conditions of this environment. In certain particulars this has remained persistent throughout. The presence of water and air and the active oxidation of organic substance have been essential conditions of plant and animal existence through all the earth's life era. In other particulars the environment has varied immensely. At first physical only, it soon became in large measure vital. Organic beings, at first struggling for existence against adverse inorganic conditions, soon had to add to this a struggle against one another. As life grew more complex and diversified, so did the vital environment. The hurtful or helpful effects of heat and cold, storm and calm, poisonous and nutritious food, and other inorganic agencies, became of minor importance as agents in evolution in comparison with the intense competition for the food supply between living forms. The development of the carnivorous appetite in animals, with the subsequent necessity of methods of escape or defense in food forms, has been the most prominent selective agency in organic evolution, and the one to which we mainly owe the great diversity of advanced forms now existing. The struggle has been not alone between higher assailants and lower fugitives. It has also taken the form of the assault of lower upon higher forms. And it is of great interest to find that man, the highest of all, finds his most dangerous organic foes in the disease-producing microbes, among the lowest forms of life.
Life, then, in its progress upward, has moved in a somewhat narrow lane, whose borders it could not cross without encountering death. And in dealing with earthly evolution, we are in great measure dealing with evolution everywhere; since, whatever the organic conditions and the inorganic environment, the vital struggle for existence must have been much the same in all life-containing spheres. Nature may be held to have tried a great experiment upon the earth, carried on through a vast stretch of time, as if with intent to discover what ultimate result would arise from this long-continued action of inorganic and organic forces upon living forms.
This experiment has not lacked a sufficiency of material. During unknown millions of years it has had to do with forms innumerable, a great battle going on in which myriads of unlike combatants were pitted against one another, each species being produced in such multitudes as to give it the fullest opportunity to sustain itself if capable. At every stage of the conflict the best adapted forms crowded down or annihilated their inferior competitors; themselves to be similarly dealt with when some new and superior combatant appeared. One needs only to look down the long record of paleontology, and consider that this represents only unit survivors of untold myriads, to recognize that nature has dealt with a superabundance of material, and to conceive that the final result may have been inevitable rather than fortuitous.
If we attempt to review the course of organic evolution upon the earth, we find ourselves confronted with so many types of life, so great a diversity of forms, such varied methods of motion and degrees of activity, that it is quite out of the question to deal with the subject adequately in a brief space. We can simply glance at it here, as an attempt to follow the whole line of progress would lead us too far afield.
Taking organic evolution as a process of colloid cell development—in distinction to the inorganic crystal development—we meet with a probably very long period of pristine evolution in which a single cell composed the whole organism. From this period examples indicating perhaps nearly the whole process of evolution still survive. The possibilities of progress in this direction were apparently very fully tried before organisms composed of a number of cells appeared. But when these came they quickly showed their superiority to the single-celled type alike in size and in complexity of organization.
From the basic generalized condition of living substance two great organic kingdoms arose, the fixed and the moving forms, plants and animals, the one living upon inorganic, the other upon organic material. Between these two inevitable resultants of the nutrient conditions the question of comparative rank is self-evident, the animal takes precedence of the plant. But the development of the latter was only in a minor degree due to inorganic influences. In water, where the animal assault on plants is not great nor varied, their evolution has been small. On land, where it has been severe and diversified, plant evolution has been large. But in no instance has it advanced from the purely physical to the conscious stage.
It is to the metazoa that we must go for the higher stages of evolution. Of the varied phases of this type of life, we can refer only to those of general character. No matter upon what planet life may have originated, we can not well avoid the conclusion that it must have had the organic cell as its unit, and that everywhere in its upward progress the many-celled self-moving form, feeding upon organic nutriment. must have been reached, as a stage superior to the minute single-celled animal, or the immobile plant, fed with inorganic nutriment.
If we may then accept it as inevitable that organic evolution everywhere, if sufficiently advanced, must have reached the stage of the metazoon animal, this may be taken as the necessary basis of higher progress in any life-bearing planet. In the metazoon we have a creature consuming organic food, which it is necessary to seek, and thus needing powers of self-motion, either of the body as a whole or of its members. And in any planet in which beings equivalent to man appeared the faculty of consciousness must have been equally necessary at an early stage, as a highly advantageous aid in the struggle for existence.
This type of life once attained, it formed a fertile field for the operation of the principle of natural selection. Upon the earth, and presumably everywhere, it developed into innumerable forms, each adapted to some passing or permanent condition of the environment. Assuming that the agencies of internal organic activity were everywhere much the same—including active chemical change, due to oxidation or something similar, vascular organs for the conveyance of nutriment to the wasting tissues, apparatus for sensation and motion, and the like—and that these led to the development of specialized organs equivalent to the lungs, the heart, the brain, etc., we shall confine ourselves here to the subject of variation in outward form and condition.
Even in this there are a multitude of relations to consider, and we can deal here only with those of general character. A main one is that of activity as contrasted with inactivity. Many of the new forms became sessile animals, their only active parts being tentacles or other organs of offense and defense. Others became free-moving animals. Of the two types the latter was evidently the best adapted to high development, both physical and mental, its free motion greatly diversifying its environment and bringing it into much more varied relations than could be enjoyed by the plant-like sessile forms. The more active the animal, the more diversified its powers of motion, the more acute and varied its organs of sense, the more alert its powers of consciousness, the higher seemingly would be its position in the ranks of life and the superior its opportunities for upward progress. And this rule must have prevailed not only on the earth, but throughout the universe.
This being the case, not alone the sessile, but the sluggish, forms were at a disadvantage as compared with the active. Anything, then, likely to prevent rapidity and diversity of motion must have acted as a check to progress. Activity is essential to the most effective offensive powers, and upon these the higher stages of development depend; but various types of animals became defensive rather than offensive in habit. These include the armored classes, of which the mollusks are the most marked example. To these may be added the forms that seek concealment, either by burrowing or otherwise. These creatures are necessarily sluggish, either from the weight of their armor or their lurking habits. They live upon inactive food, their environment is limited, their contact with nature narrow, their powers of sensation and consciousness little developed. The conditions of their life definitely take them out of the line of the higher progress, in which they can not compete with the more active forms.
In considering then the classes of animals adapted to advanced development, it seems necessary to confine ourselves to the free-moving, agile forms. And among the inhabitants of the ocean—in which life had its origin and its lower stages of development—these are not to be sought among the crawling and burrowing, but among the swimming species. With these the highest activity is dependent upon the most suitable formation of body and the most capable organs of motion.
If we may pursue our fable of nature's experiment in evolution, it can be said that very numerous trials in form were made. There seem possible to colloid substance only two general types of form, the circular or radial and the elongated. Both these were produced in numerous varieties, the circular type embracing two large classes of animals, the cœlenterata and the echinodermata, all of them sluggish, many of them sessile, their general shape and radiated limbs being very ill adapted to active motion. In this respect they were at a great disadvantage as compared with the bilateral, elongated type.
We thus seem to find the experiment of organic evolution, after millions of years of incessant effort, reaching the type which in its simpler stages is popularly designated as the worm, as the form best adapted for advanced evolution. The pristine worm was not in itself a promising creature. Its organs of motion were inefficient and its movements sluggish. Probably several worm-like types appeared, simply organized elongated animals of varied formation, to which we owe, in their final development, the three classes of animals known as the mollusca, the arthropoda, and the vertebrata. This development of an elongated, bilateral animal would seem to have been an inevitable stage in the evolution of animal life, sure to appear in any planet where life had sufficiently progressed, and capable of unfolding into a number of different types. In addition to the great types named, several of minor importance appeared upon the earth, and different ones may well have arisen elsewhere.
Yet if we seek for the highest class of form likely to arise from the worm-like unit, our field of search is restricted. If activity and flexibility of body are advantageous, we must seek these in the swimming rather than in the crawling forms; in the naked rather than in the armored; in those of simple rather than in those of multiple organization—like the arthropods; in those with lateral rather than in those with oral limbs—like the mollusks; and finally in those with the smallest available number and most efficient character of limbs and other organs. This leads us to the vertebrates for the highest type that appeared in the waters of the earth, as the outcome of forms almost numberless in variety. In this we have an oval-shaped elongated animal, its organs of motion much the most effective of the many that had appeared in the progress of life, its vital organs unified and simplified to the greatest extent possible, its skeleton internal instead of external, used solely as a support, in no sense as an armor.
If we consider the fish in its most primitive varieties, we certainly seem led to the conclusion that it is the form to which the evolution of life would lead in any planet, as the basis of the higher development. In Amphioxus, for instance, we find the elongated bilateral animal simplified to an extraordinary degree; without external armor of any sort, with the simplest vital organs, with only the beginning of an internal skeleton, and with merely the suspicion of a fin, virtually a flattening of the skin. In this form we have the vertebrate reduced to its lowest terms, or the worm advanced to its highest. In the hag we find again a boneless and scaleless creature, with a sheath of cartilage to represent the backbone and with no organs of motion other than a fin-like flattening around the tail. Much the same may be said of the lamprey. From forms like these the fish seems to have developed, with all its subsequent variations.
The fish remains the highest form of water-developed life. It has made comparatively slight steps of progress during the immense interval since the paleozoic age. The limitations of its habitat seem to have checked the development of organic form at this stage. It can not be said that the evolution of life in the water has been in any sense restricted by deficiency of time or narrowness of variation. The variety of forms that have appeared is surprising when we consider the uniformity of conditions in the water, and are only to be accounted for as the result of a very active vital struggle. We find its simplest and least specialized higher result in Amphioxus, of which the ultimate result is the fish, beyond which, during millions of years, no progress has been made. And a full consideration of what has taken place on the earth strongly suggests that the oceanic evolution of life in any planet must have ended at some not dissimilar stage. Mentally it stands at a low level; and the whale, a mammal which has returned to the fishform, is as low as the fish in mental powers.
Life in water was the basis of life on the land. It could not originate there de novo, land conditions being unadapted to the early life stages. Land life was therefore handicapped by its origin. It had to start with what the ocean had to offer, and to begin with physical conditions which afterward could, at the best, only be modified. These conditions have left ineradicable traces even in man, the most removed of all from the original types. Of water animals only the elongated forms sent representatives to people the land—the worm, mollusk, arthropod and vertebrate. And of these, the latter two alone seemed well adapted to their new habitat, the arthropod developing into an extraordinary multitude of species, though its inferiority of organization removed it, at the start, from any competition with the vertebrate as a basis for the higher evolution. We find in the bee and the ant the ultimate development of the insect intellect, and the insect form is decidedly restricted by its characteristic condition.
Despite the immense variation that has taken place in land vertebrates, their marked departures from the fish type have not been numerous. One of the most important of these was the development of the fin into the limb, yielding the quadruped. Another was the replacement of the gill by the lung. But varieties of partially air-breathing and four-paddled fishes still exist, as if to serve us as object lessons in these stages of development.
Land animals were exposed to much more varied natural conditions than water animals and the struggle for existence between different forms was quite as acute. Yet, though a vast number of differing forms appeared, they were all built on the original lines of structure, the type of organization of the fish strictly limiting that of the land vertebrate. A development takes place, but it is on the lines already laid down. The internal organs vary and become more effective in action, the cold-blooded is succeeded by the warm-blooded, the egg-bearing by the young-bearing, etc. There is much change in external form. Animals became adapted to running, to flying, to swimming, to crawling, to burrowing. There are many variations in the feet and limbs, and in some cases these vanish, as in the serpent and the whale. Some animals are clothed in scales or bony armor, some in hair or feathers. But no new type appears, and though the mental powers increase, ages pass with little indication of the coming of any animal possessed of advanced powers of thought. Mentally the higher land vertebrate progresses far beyond its highest water kindred, but its powers of thought, after gaining a certain development, remain in great measure dormant, and there is nothing to indicate that the quadruped could ever progress in thought beyond a certain low level. If so on the earth, probably so everywhere: the influences acting on the quadruped do not seem calculated to produce any advanced thinking powers.
We have, in the foregoing pages, followed in its general features the evolution of animal life upon the earth. In view of the immense period over which this evolution has extended, the extraordinary variety of forms which have appeared, and the strict limitations of the problem by natural influences, chemical, meteorological and vital, it is not easy to perceive how the final result could have deviated widely from that which we have before us. If the process were gone over again upon the earth, the great probability is that it would end once more in the mammalian quadruped. On other planets different chemical and physical conditions might affect the result, though the general principles of vital action could not greatly deviate and the evolution of the organs would doubtless pursue much the same course. As regards external form, the struggle for existence must operate in the same way and probably to the same effect.
Let us, for example, take the head of the quadruped, with its facility of motion, its apparatus for mastication, its sense organs, its nerve center. Can any one suggest an improvement upon the general arrangement of these organs, the ultimate outcome from a myriad of experimental efforts? The nasal openings stand above the mouth, in the best position to give warning of dangerous odors from food. The eyes are placed at the highest altitude and in the frontal position, the best location for their special duty. The ears are situated to catch sounds from the rear and the front, but preferably the latter. The brain is situated in the immediate vicinity of these organs of special sense, as if to favor quickness of sensation. All the organs of the head, indeed, seem remarkably well placed and adapted to their particular duty, and when we consider the varied positions which these organs have occupied in lower forms of life, we may justly look upon those in the quadruped as the final 'posts of vantage' resulting from a multitude of trials. Similar deductions might be made from other sections of the body, internal and external.
But we have not yet reached the evolution of a thinking being, an animal dependent much more upon its mental than upon its physical powers. In each advanced type of animal some mental progress was made; largest of all in the quadruped mammal; yet even in the latter it ended at a low stage. This is evident if we compare the quadruped with man, the former dependent very largely upon its physical, the latter mainly upon his mental powers. Evidently, in any planet, some step of progress beyond the quadruped was necessary for this result. On the earth this step was towards the form of man, the only true biped. May it not have been different in other planets, yielding human beings widely diverse in form and general bodily relations?
The answer to this query depends upon that special characteristic to which man—wherever found—owes his superiority. His physical difference from the lower animals is by no means great. It consists principally in an adoption of the upright attitude, a reduction of the organs of locomotion from four to two and the development of a hand with grasping powers. But in this physical deviation lies the secret of the whole mental deviation. The species of animals below man are obliged to depend upon their personal organs, being incapable of availing themselves of natural objects. The elephant, with its grasping trunk, and the apes, with their partly freed hands, are nearly the sole exceptions to this rule. Man, on the contrary, by the freeing of his fore limbs from duty in locomotion and the grasping power of his hands, became able to add to his own powers those of nature, to employ weapons and tools fashioned from non-living matter, and thus to initiate a new cycle of evolution that was scarcely touched upon in the world below him.
The employment of tools and weapons separate from those provided by nature in the body is a condition demanding the active exercise of the mental powers, and at once gave man an incitement to the development of the mind which did not exist in the lower animals. We need not pursue this subject farther. The new process of evolution thus begun,—that of the exercise of the faculty of thought in making use of the powers of nature—it went on until it yielded man as he now exists, a being in whom the intellect controls not only his own bodily actions but largely all nature below him.
If now, we may justly conjecture that animals resembling our quadrupeds in general organization appeared on other planets, and if a thinking being analogous to man also appeared on any of these planets, it is very difficult to conceive how he could have arisen in any widely different way. It certainly seems as if the evolution of the higher intelligence in any planet must have depended upon some means of making use of the forces of nature, and the first step towards this, starting from the quadruped, would seem necessarily to be in the direction of the biped, with free arms and grasping hands.
There is thus considerable reason to believe that the beings which answer to man upon any of the planets of the universe must at least approach man somewhat closely in physical configuration. They may differ in minor details of organization, in many cases they may have escaped the special organic weaknesses of man, but it certainly seems as if a human traveler, if he could make a tour of the universe, would find beings whom he could hail as kindred upon a thousand spheres.