Popular Science Monthly/Volume 60/February 1902/The Descent of Man
|THE DESCENT OF MAN.|
By Professor LINDLEY M. KEASBEY,
BRYN MAWR COLLEGE.
GENERAL evolutionary evidence led anthropologists some time ago to postulate a pliocene precursor of man, but their surmise has only recently been substantiated by particular proof. The search for the so-called missing link was at first confined to the temperate zone. Many discoveries were made, but, as the skulls and implements unearthed were all taken from pleistocene deposits, there was nothing to indicate the existence of man on earth before quaternary times. It was not, indeed, until investigations were transferred to the tropics that earlier vestiges of human life were revealed. In the year 1894 Dr. Eugene Dubois discovered the upper portion of a stall and some skeletal parts of a distinctly human creature buried in the pliocene beds of East Java. These remains have since been subjected to the strictest scientific scrutiny and pronounced genuine. Taking the geological location of the discovery into account, there can be no further doubt, therefore, that in this tropical region the human species was already differentiated from the apes in tertiary times.
Judging from the size and form of the skull, and from such portions of the skeleton as remain, the variation of the human prototype, or to give him his scientific name. Pithecanthropus erectus, was evidently along both psychic and physical lines; the former showing a difference of degree, the latter exhibiting a distinction in kind.
There are two anatomical tests of intellectual superiority: cranial capacity and the convolutions of the brain, both of which can be applied in the present instance. It is a well-known fact that the frontal bone, which forms the vault of the anterior part of the cranium of young men and apes, is divided by a suture. So long as this line of growth remains open, the fore part of the cranium can expand; but if the anterior sutures of the skull consolidate early in life, the cranium cannot increase in capacity beyond the size reached in early infancy. In consequence of the early closure of these sutures in the anthropoid apes, the cranial capacity of these creatures is restricted, and the fore part of their brain rarely increases beyond the size attained at the end of the first year of life. With man, however, these sutures do not consolidate until a much later period, and, as the anterior lobes of the brain continue to develop, human cranial capacity increases accordingly. As a result, the average human being possesses four times as much brain surface as the anthropoid ape. Applying this test to the human prototype, we find that Pithecanthropus occupies an intermediate position between the higher apes and modern man. That is to say: if we set specimen anthropoid skulls in an ascending series according to cranial capacity, we shall discover that between the higher apes and Pithecanthropus there is a considerable gap; Pithecanthropus is followed shortly in the scale by the Neanderthal group of men, whose skulls were found in the quaternary deposits of Europe; these in turn are succeeded by the crania of the lowest living savages; and from this point the series runs on without interruption through the various races of man to the highest existing types. Thus on the human side there is practically no break in the gradual development of cranial capacity from tertiary to quaternary, and upwards through prehistoric to historic man. The only interval that occurs is on the animal side, between the lowest type of man and the highest type of ape; from which we may infer that the pliocene precursor stood considerably above his simian relatives in intellectual capacity, though the difference was merely one of degree.
The same result is reached by a comparative study of the convolutions of the brain. It has been demonstrated that the gyri of the brains of man and the anthropoid apes are similar, with the exception of the convolutions which enter into the formation of the frontal lobes. The superior and middle gyri of these lobes are much shorter in the brains of anthropoid apes than they are in the brains of man, and in the brains of anthropoid apes the inferior frontal gyri only exist in rudimentary form. These anterior lobes of the brain, or more exactly their cortical nerve elements, to a large extent control the higher intellectual faculties. The condition of these frontal convolutions is, therefore, like cranial capacity, an index of mental endowment. Applying this test to the human prototype, we find that with respect to his cerebral convolutions. Pithecanthropus stood considerably above the other anthropoids, and within the line of development leading to the higher human types. This is proved by the impressions on the interior of the Java skull, which show that the superficies of the convolution of the prototype's brain was double that of the largest brain of any anthropoid ape, and somewhat less than half that of the brain of modern man. The convolutions themselves are also well marked and distinctly human in form. Thus whether we judge from cranial capacity or from cerebral convolutions, it is evident that the pliocene precursor was psychically differentiated from the apes and endowed with the intellectual attributes of man.
Along physical lines the modification is even more marked. From the shape of the femur (discovered in close proximity to the skull), it is evident that Pithecanthropus stood erect and walked upright on the earth. If not already become, the prototype was, therefore, certainly in the course of becoming, a true biped, using his legs alone for locomotion and developing perfectly plantigrade feet. Freed from further service for locomotive purposes, his arms had, no doubt, become relatively short and his hands exclusively prehensile. In all these respects the reconstructed skeleton of Pithecanthropus shows a striking divergence from the characteristic anthropoid type. The apes as a group are stoop-shouldered tree-dwellers, with hands and feet primarily adapted to swinging and climbing. Even the few varieties that dwell habitually upon the ground, of which the baboon is the best example, have not lost their inherited ability to climb, while in walking these creatures have reverted to the quadruped habit. Some of the arboreal apes also come occasionally to the ground; but when they walk, these animals adopt a crouching attitude and use their arms as aids to locomotion. The gibbon is, in fact, the sole ape that walks erect, and this is only possible in his case by reason of the extraordinary length of his arms. When unconcerned this creature walks with his long arms held aloft like balancing poles and hands outstretched to grasp any overhanging support, but when frightened the gibbon likewise drops his arms to the ground and swings along between them as if on crutches. Taking structural characteristics into account, it is, therefore, possible to draw a sharp distinction between the short-legged, long-armed, stoop-shouldered, arboreal ape, and the long-legged, short-armed, erect, surface-dwelling ancestor of man.
Specific variation in the biological sense is the accomplishment of that which variability permits, environment requires, and selection directs. No stock or lineage breeds perfectly true; the line of every descent is marked by certain modifications. The general tendency of such modification is toward the preservation of the more useful and the extinction of the less useful or useless characters. Survival is thus the result of the selection of such variations as adapt the organism to its environment, the more plastic the organism the greater the possibility of variation; the more favorable the environment the higher the type of animal evolved. Attributing variability, then, to the anthropoid stock, from the fact of man's survival, we may presume that the modifications characteristic of the human species were favorable in this sense, that they adapted the pliocene precursor to his mundane environment. Judging from the further fact that the tree-dwelling apes have long since reached the stationary stage, while land-dwelling man has steadily continued to advance, we are also justified in considering the terrestrial habitat more advantageous than the arboreal abode. The descent of man appears, accordingly, to be marked by progress along the line of heredity and improvement in the way of environment, selection in this instance giving rise to a distinctly superior species.
Passing over to particulars, it is pertinent to enquire wherein this specific superiority consisted. In what respect was the erect ground-walking human prototype better fitted for advancement than his stoop-shouldered arboreal ancestors? Favorable structural and environmental variations are, generally speaking, along two lines: those that enable animals to escape more easily from their enemies, and those that place them in a better position to acquire sustenance for themselves. It is impossible to conceive that the pliocene precursor gained any advantage over his ape-like ancestors in the former direction by adopting an upright attitude and abandoning his arboreal abode; for in so doing he lost his earlier and easier means of escape through the trees, without acquiring by way of compensation any corresponding facility for taking flight on foot. The only alternative is, therefore, to suppose that the superiority of the human species was in connection with the food-quest. As far as quantity was concerned there was nothing, however, to be gained by coming down from the boughs, for the trees of the tertiary forest afforded a supply of nuts and fruits far in excess of the demand. The advantage must, therefore, have been qualitative, that is to say, in the way of a wider food-choice. As an erect, surface-dwelling creature man was evidently able to secure a greater variety of subsistence than his ape-like ancestors obtained from the trees. Being arboreal their food was confined to nuts and fruits, while by becoming terrestrial he was in a position to add roots and berries, and, in the course of time, also fish and flesh to his fare. Human progress appears, accordingly, to have been away from a strictly frugivorous in the direction of an omnivorous diet. A similar tendency is observable among the other anthropoids. The arboreal apes, for instance, are naturally frugivorous, but when taken from the trees and bred in captivity they readily become omnivorous. The semi-terrestrial types exhibit the same proclivities in their wild state. The gorilla, for example, usually lives on fruits, but also eats birds and their eggs, small mammals, reptiles and the like, and has even been observed to devour large animals when found dead. What is only an incipient tendency among the apes probably became an habitual practice with the ancestor of man. The superiority of the human being may thus be said to have consisted in the acquisition of qualities and the occupation of an environment which enabled him to widen the range of his food-choice.
It is evident enough that the variation of environment was favorable in this respect, for the terrestrial habitat certainly offered boundless opportunities for the development of an omnivorous taste; but it is not so easy to see how the characters we have described as human rendered these opportunities available. Berries and roots were plenty in the woods, the streams were alive with fish, and the tertiary forest abounded in game of all sorts; but the superficial food-ground was already preempted by other animals, which were not likely to allow man to encroach upon their subsistence without a struggle. Adaptation to the new surroundings must consequently have been effected at the expense of a conflict with the former lords of the forest. But the human prototype does not appear to have become fitted for such a contest either through heredity or environment. From his ape-like ancestors the pliocene precursor merely inherited a large cranial capacity and the ordinary anthropoid characters; while in adapting him to the terrestrial habitat, selection simply set him upright on his feet and accorded him the free use of his arms and hands. Leaving aside his inherited endowment for the moment, the structural modifications that occurred during the period of specific differentiation seem at first sight to have set man at a positive disadvantage over against the frugivorous apes, on the one hand, from whom he descended, and the land-dwelling carnivora, on the other, with whom he had henceforth to contend. The food of frugivorous creatures remains fixed in its place and only requires to be plucked. These animals have, therefore, no need of powerful prehensile organs in attack, while for defense they usually rely upon their locomotive organs in flight. The prey of carnivorous creatures has, on the contrary, to be caught and killed, and on this account these animals are supplied with vigorous attacking organs, which in times of necessity may readily be employed for defense. The apparent anomaly in man's case is that in becoming terrestrial, he lost his former facility for climbing and making his escape through the trees, without by way of compensation acquiring sufficient strength or agility to cope with the land-dwelling carnivora. Cut off from escape above and surrounded with animal enemies below, physically unfitted to lie in wait and spring, having neither claws nor talons wherewith to grasp and hold, and not being fleet enough either to take flight or to follow fast on foot, how then was it possible for man to gain his acknowledged ascendancy over the beasts?
As a group the anthropoidea are structurally adapted to two sets of physical exercises: swinging and climbing, and striking and throwing. In a more restricted sense, however, the two practices are incompatible, for skill in one direction can only be acquired at the expense of proficiency in the other. For the former exercises, moreover, instinct alone is sufficient; while for the latter a certain amount of ingenuity is required. Being arboreal, swinging and climbing are essential to the frugivorous apes, both for food-getting and for flight, and on this account their instincts are set and their organs especially adapted to this purpose. Those that come occasionally to the ground in search of other aliment are, however, also able after a fashion to strike and throw. Both the gorilla and the chimpanzee, for example, are ingenious enough to swing sticks, and the orang will break off branches and fling them at his tormentors or hurl the thick husks of the durian fruit. Nevertheless, striking and throwing are exceptional even with the semi-terrestrial apes, or at most only occasional exercises with such as have sometimes to defend themselves upon the ground. But for man the conditions were reversed. After the human prototype had parted company with his arboreal fellows to become a land-dwelling creature, swinging and climbing were no longer essential to his success. Henceforth he had to win a place for himself on the ground, and lacking natural means of attack and defense, in the course of his contest with the carnivora, he was compelled to exercise ingenuity in the choice of artificial implements and develop his incipient capacity to strike and throw. It is not so strange, therefore, as it at first sight appeared, that in adapting the human prototype to his earthly environment selection should have simply set him upright on his feet and accorded him the free use of his arms and hands; for, with his inherited mental endowment, these slight structural modifications were just such as were necessary to make him a weapon-wielding animal and so set him above his enemies.
But besides becoming psychically and physically fitted for striking and throwing, man's faculties had also to be trained before he could acquire proficiency in the art of weapon-wielding. To deal a straight blow with a club, or hit a distant mark with a missile, it is necessary to take accurate aim; and this involves the development of a good eye. Heredity favored the human prototype in this respect, for his ape-like ancestors had for centuries been accustomed to rely upon their sense of sight in their search for subsistence. In developing his incipient capacity to strike and throw, the pliocene precursor had, therefore, simply to turn his inherited acquisitive sense to the additional service of distance-determining and range-finding. Long practice and hard training must, nevertheless, have been necessary before primeval man acquired the knack of aiming accurately. The semi-terrestrial apes, whose instincts are still set and whose organs are primarily adapted to swinging and climbing, have never acquired any special facility in this direction. With the exception, indeed, of the land-dwelling, tree-climbing baboon, who is apparently able to hurl branches and hard clods with considerable dexterity, they all exhibit a ludicrous lack of skill in striking and throwing. On the other hand, by dint of observation and imitation, the small boy of our day soon learns to take accurate aim, and savages without exception show surprising skill in this direction. Considering the necessity of the case, and judging from the fact of human survival, it is extremely probable, therefore, that in the course of his contest with the carnivora the prototype acquired the knack of aiming accurately and eventually became an adept in the art of weapon-wielding.
The keen eye that the human prototype inherited from his ape-like precursors was also useful to him in other ways, for food-getting, pathfinding and perceptive purposes in general. In some respects, indeed, sight may be regarded as the most serviceable of all the senses. Touch and taste, upon which the lower orders of life rely, require actual contact with the objects to be distinguished, and, consequently, only afford a concept of the immediate environment. Hearing and smell predominate among the vertebrates, have a broader range and are especially useful in this, that they allow the mind to distinguish particular sounds and odors from surrounding conditions, and so afford a perception of the local environment. The sense of sight offers still further advantages in that it conveys a concept of the special as well as of the universal environment and allows the mind not only to distinguish particular objects, but also to compare them with each other and so form a general conception of the outer world. Sight has this disadvantage, however: it only gives instantaneous information from one quarter, and necessitates a turning of the head or body to take in surrounding conditions, because light travels only along straight lines; whereas hearing and smell accord instantaneous information from all quarters, because sounds and odors disseminate in every direction from the center of disturbance. Thus though primeval man might well enough have relied upon his sense of sight exclusively for acquisitive purposes, defense demanded that he develop a sentinel sense besides. Heredity also determined this choice. Like the other anthropoids, man continued to rely upon his hearing to warn him of danger approaching from behind. Thus beside becoming adapted to walking and weapon-wielding, we may imagine the human prototype developing during the process of differentiation which fitted him for his mundane career, the keen eye and acute ear that had been handed down to him through heredity from his ape-like ancestors.
Working upon the biological principle of variability and following the interaction between heredity and environment, by a process of reconstruction we have been able to form a tolerably complete conception of the original condition of man. He was evidently a land-dwelling, omnivorous, weapon-wielding animal. For this manner of life he was psychically and physically fitted during the process of specific differentiation. From his ape-like ancestors he inherited his prehensile hand, his keen eye and his acute ear; in adapting him to the earthly environment selection increased his cranial capacity, developed the convolutions of his brain, set him upright on his feet and accorded him the free use of his arms for acquisitive purposes. Thus endowed, primeval man was evidently enabled to cope successfully with the carnivora, and eventually make himself master of the forest.
Taking the geological location of the remains of Pithecanthropus into consideration, we concluded that the differentiation of the human species took place during tertiary times. Reasoning in like manner from the geographical situation of the discovery, we may suppose Indo-Malaysia to have been the cradle-land of mankind. The assumption is further substantiated by the fact that this tropical region is still the home of many of the higher apes, and was probably the point of departure for the dispersion of the other anthropoids. If we turn, now, to the evidences of quaternary culture we shall find a multitude of human relics buried in the pleistocene or post-pleistocene deposits of every continent of the globe. The widespread diffusion of these remains proves beyond peradventure the existence of man in every quarter of the earth at the beginning of the prehistoric epoch. Obviously, then, the descendants of the pliocene precursor must have wandered far and wide from the original abode during the long geological era that elapsed between the pliocene and post-pleistocene periods. It is incumbent on us, therefore, to determine how this migratory movement was effected. Close upon the problem of the differentiation of the human species comes, in other words, that of the dispersion of mankind over the face of the earth.
Subjectively speaking, there were evidently no difficulties in the way. The human prototype was, as we know, structurally fitted to walk, and his omnivorous manner of life must in itself have led him further and further forth in search of subsistence. In so far as physical capacity and psychic motive are concerned we may, therefore, think of the pliocene precursor as an ambulatory, omnivagant animal. It is only when objective conditions are taken into account that obstacles appear to arise. Granting primeval man's ability and desire to wander, how are we to imagine he endured the vicissitudes of climate that met him on the march? and how are we to suppose he crossed the seas that separate the several continents? Before the great antiquity of the human race was assured, it was necessary to assume an almost miraculous power of adaptation on man's part, and furthermore to endow him, somewhat inconsequently, with an innate knowledge of navigation; but now that science has succeeded in tracing man's ancestry back to tertiary times, we may more logically accept the explanation that geology affords. Instead of proceeding upon the presumption that the climate and configuration of the earth was then as it is now, we must reckon with the geological changes that have since occurred and work out our conclusions accordingly. In so doing we shall find that during these early ages of his existence on earth, environmental influences opposed no insuperable barriers before the dispersive propensities of man.
If, as we have supposed, the prototype was differentiated from the apes in Indo-Malaysia during the pliocene period, and arrived in remote regions of the earth before the prehistoric epoch, the dispersion of the human race must have been coincident with the ice age. It is with the climatic and topographic condition of the glacial period, therefore, that we have to do in determining the original routes of migration. According to the astronomical explanation of glacial phenomena, which best accords with the geological facts as far as they are known, the Northern and Southern Hemispheres were alternately subjected to frigid conditions. Owing, however, to the disposition of the land-masses of the globe, glacial influences were more widespread in the north than in the south. When the glaciers proceeded from the arctic regions, the climate of the northern continents grew cold, the thermal equator moved somewhat south of the geographical equator and the southern peninsulas became predominately tropical. When, on the other hand, the ice advanced from the antarctic regions, the highlands of the southern peninsulas were glaciated, and, as the thermal equator moved north of the geographical equator, the northern continents enjoyed an equable climate, ranging from tropical to temperate conditions and devoid of great seasonal variations. Several such glacial cycles appear to have elapsed during quaternary times. After the third advance of the ice from the north, however, the glaciation of the hemispheres became less severe and the genial conditions more permanent, until, towards the end of the great ice-age, the glaciers were confined to the arctic and antarctic regions, and the globe became divided as at present into temperature zones.
As the ice advanced for the first time from the arctic regions, the temperature of the Northern Hemisphere became gradually colder, until during the early part of the pleistocene period, continental glaciers spread down over central Europe and North America. The increasing frigidity of the Eurasian continent at this time was doubtless sufficient to deter dispersion from the Indo-Malaysian cradle-land towards the north. Mountain ranges also hindered progress in this direction, for the Himalayas must have acted as a barrier towards Asia and tended to deflect the lines of migration east and west along the central latitudes. Immediately preceding and during the first glacial epoch, therefore, climatic and topographic influences combined to confine the original course of dispersion within the Indo-Mediterranean-Malaysian belt and the lower peninsulas of the Old World. This southern portion of the Eastern Hemisphere, it should be noticed, is separated from the northern continental area by a broken mountain range, running from the Himalayas to the Pyrenees. These mural masses protected the low-lying lands along the southerly slopes of the mountains from the increasing cold of the glaciated north, so that in spite of the fact that the thermal equator ran somewhat south of the geographical equator at this time, tolerable climatic conditions prevailed everywhere below the Himalayan line. In these early days, moreover, the portion of the Old World constituted a practically continuous land-mass. Toward the west India was connected with Africa, and Africa was joined to Europe by two or more isthmuses. In the opposite direction, the Malaysian peninsula was extended toward Australia through what are now the separate islands of Sumatra, Borneo and Java; Australia was likewise connected with New Guinea, and the immense island continent thus constituted was to all intents and purposes coterminous with the southeasterly extensions of Malaysia. It must have been possible, therefore, at this time for primeval man to proceed from the Indo-Malaysian abode along the central latitudes, to the Atlantic on the west, and far out across the Pacific on the east, without having to cross the open seas.
During the course of the pleistocene period the ice disappeared from the north and glaciation set in from the south, the glaciers in this instance proceeding from the antarctic regions and from the highlands of the southern peninsulas. The thermal equator also moved north of the geographical equator at this time, and during the long interglacial epoch that followed, lasting at least ten thousand years, the whole Northern Hemisphere enjoyed an equable climate, ranging from tropical to temperate conditions and devoid of great seasonal variations. The effect of this change must have been to limit the lines of migration toward the far south, and greatly to extend the course of dispersion towards the northeast and northwest. Or to put it more exactly: entry into South Africa was probably barred by the increasing cold, but as the influence of the southern glaciers did not extend as far north as the Indo-Mediterranean-Malaysian belt, the inhabitants of this region were doubtless free to wander east and west as before along the central latitudes. Nor did climatic conditions any longer prevent men from penetrating into the continental area beyond. Mountain masses still barred the way in the center, to be sure; but passages were open on either side; from the Mediterranean region into Europe, and from the Malaysian region into Asia. Above the Himalayan line the Tibetan plateau interposed itself between these Mediterranean and Malaysian emigrants and probably kept them for long ages apart. For this reason, and doubtless also because the maritime regions offered greater attractions to primeval man than the inland areas, the earliest lines of migration appear to have followed the Atlantic and Pacific coasts of the Eastern Hemisphere into the northern latitudes, where the shore lines of the Eurasian continent stretch out toward America. During the first and second interglacial epochs the two hemispheres were, indeed, practically coterminous in these parts. This was due to the fact that the level of the northern oceans was lowered at these times, leaving land-bridges exposed to view which have since become covered by the sea. As a result, the Atlantic and Pacific routes of migration were continued along the northern latitudes into the Western Hemisphere. Toward the northwest, the British Isles then formed an integral part of the European continent, and from this peninsula, land-connections were in all probability extended through Iceland and Greenland to America. In the far northeast, Asia was likewise joined with America by what geologists call the Miocene bridge, which probably lasted into quaternary times; and after the Behring strait finally broke through, the Aleutian island chain still linked the two continents together along the Pacific. As far as climate and topography were concerned, during these interglacial epochs there was nothing, therefore, to prevent the Mediterranean and Malaysian emigrants from pushing northwestward through Europe and northeastward though Asia into America.
As the glaciers advanced successively from the arctic and antarctic regions, the climate and topography of the Northern and Southern Hemispheres varied in this way at least three times. After the third glacial epoch, however, the changes were less marked, until towards the close of the ice age, the configuration of the earth gradually assumed its historic form and the globe became divided as at present into temperature zones. The possibilities of dispersion during the glacial, interglacial and post-glacial periods may, accordingly, be generalized as follows: Each time the Northern Hemisphere was glaciated, the migrations of men must have been confined for the most part within the Indo-Mediterranean-Malaysian belt and the southern peninsulas of the Old World. During the genial epochs that intervened between the three great glacial movements from the north, the continental area was open to incursion on either side, the climate of the Northern Hemisphere was everywhere equable, and the topographic conditions were such as to encourage migration along the Atlantic and Pacific shores of the Eurasian continent into the arctic peninsulas of the New World. As the arctic glaciers became more and more restricted to the northern regions, primeval men were probably able to hold their own in the continental area and migrate east and west across Eurasia. But after the third glacial epoch, if not before, the Atlantic land-bridge was broken; so that henceforth access to America was only possible along the Pacific, by means of the Aleutian island chain.
Cave deposits, kitchen-middens and fossil remains mark the course of the dispersion of mankind in these different directions. The aboriginal inhabitants of the now separated continents and isolated islands of the globe also preserve certain distinguishing characteristics by which the lines of their respective ancestries can be traced back along these several routes to more or less definite points of departure about the Indo-Malaysian abode. There is archeological and ethnological evidence, therefore, to show that primeval men migrated originally along the lines laid down by the climate and topography of the glacial periods. But these indications of the actual courses of migration are beyond the range of the immediate enquiry, and should, therefore, be reserved for separate consideration. It is sufficient for the present to have pointed out the possibilities of dispersion, which may be summarized in conclusion as follows: From the skeletal parts of Pithecanthropus we are assured that the pliocene precursor could walk; the nature of his food-quest affords the wandering motive; the widespread diffusion of quaternary culture convinces us of the fact of dispersion; and the data of geology define the routes provided by nature for the original migrations of mankind.