Creation by Evolution/Connecting and Missing Links in the Ascent to Man
CONNECTING AND MISSING LINKS IN THE ASCENT TO MAN
By Richard Swann Lull
Professor of Palaeontology, Yale University
Two truths impress themselves strongly upon the mind of the student of animals, first, the continuity of life, and second, the immensity of time during which that continuity has endured. Our conviction of the continuity of life is justified by an overwhelming host of facts, obtained in part from observation of existing creatures, in part from the study of the geologic record. The evidence of the continuous succession of living forms is conclusive to modern scientists, having in mind, as they do, the classic experiments of Spallanzani, Redi, Tyndal, and Pasteur, which proved, apparently beyond the possibility of dispute, that all life is derived from preëxisting life. The evidence afforded by the fossil record, on the other hand, does not at first sight seem so convincing, for the absence of certain “missing links” in the chain of life is striking. Were the tale a short one, the apparent gaps in the record would be of greater relative moment, but, in view of the great length of the revealed story they become comparatively insignificant.
These gaps are in reality comparable to missing pages in an ancient and partly mutilated volume, which, although their absence surely mars the perfection of the whole, it nevertheless cannot destroy our understanding of the story that is told if we possess the imagination of the historian or the prehistorian. When this imagination restores the links in the chain of life we call them hypothetical; the links thus restored are not random guesses, but carefully predicted stages, and many such predictions have been shown to be approximately correct by fortunate subsequent discoveries. It is therefore not too much to expect that more such predictions may yet be verified through the intensive systematic exploration of to-day, which is very different from the often unorganized search of the past. Indeed, the memorable finding of the planet Neptune, after the astronomer's mathematical calculation had shown that it must be in a certain part of the heavens at a certain time has had its parallel in more than one recorded verification of a like prediction in palaeontology. To trace complete continuity in the succession of living forms that have inhabited the earth would necessitate the finding of a representative of each generation, a thing that is manifestly impossible. There must always be breaks in the record, but we are disposed to insist that the breaks should not be very great, and that they should not occur at highly critical stages in evolutionary advance. To find serene security in our faith in the continuity of life we must still await discoveries that will bridge certain breaks.
The origin of living matter—of organic matter—from the lifeless material of the inorganic world was a most momentous step, for it led ultimately to the peopling of the globe with its countless hosts of animals and plants. When, where, and how life began, however, we do not know, although much purely academic discussion has been waged about the question. As students of the origin of the earth must assume the preexistence of matter and energy, so students of organic evolution must assume the existence of something organic to evolve, but science is silent on the great problems of first causation.
Our fossil record begins with the dawning of the Palaeozoic era, some 500,000,000 years ago, but the date of the origin of life was vastly more remote, for when the fossil record actually begins not only is life fully manifest, but the numerous animal stocks that constitute its invertebrate division are already well established. These earliest known animals are far more diverse and complex than the most primitive imaginable organisms, some of which yet exist—such as the slime molds and the bacteria described as prototrophic (literally "first feeding"). Thus the missing links in the early evolution of animal life constitute for millions of years the entire chain. There is, however, no doubt in the scientific mind that the oldest known fossils imply, with the assurance of certainty, a long antecedent evolution, much of which, in spite of the fact that palaeontology is silent, can be deduced from the sister sciences of comparative anatomy and embryology. The reason the record anterior to this time is blank is because of the nature of the organisms themselves. Composed as they were largely of soft parts with no limy skeletons, or shells, they apparently left little to be preserved in the rocks of ancient time. Consequently, save for certain remarkable impressions, yet to be described, little of the actual nature of these ancestral types is known, except by inference, until they had established what has been called the lime-secreting habit, which was formed by animals in mid-Cambrian time, by plants somewhat earlier. True, there are masses of limestone, iron ore, and graphite in the pre-Palaeozoic rocks, all of which are regarded as largely of organic origin, but, although their presence is indirect evidence of the existence of organisms in this remote time, it reveals nothing of their nature.
One notable exception lies on the flank of Mount Wapiti, near Field, in British Columbia, where, in a small area of slate of Lower Cambrian age, the most marvellous impressions of delicate and fragile organisms have been found—mere films of carbon against the slate—with all their detail of structure preserved with the utmost fidelity. These impressions show many of the relatively high invertebrate types, such as worms, crustaceans, and echinoderms, and here there might well be preserved the ancestral stock out of which the backboned creatures arose, although, so far as actual discovery goes, that record is much later in time.
Except for a persistent type of being variously known as Amphioxus or Branchiostoma, which to this day inhabits the shallow waters bordering the continents, there is again no trace of the important link connecting the vertebrates with their invertebrate forebears. To those of us who are prehistorically interested this is perhaps the most coveted of all the missing links, for its discovery will help to settle arguments that have arisen in favor of this or that method of origin, about which so little is really known.
In Silurian time there came the sharks, known largely from their fossil teeth, some so much like the teeth of modern persistent survivors that the external form and habits of life of some of the early species may be safely surmised by analogy, although others are unlike any that now exist. Of their gradual evolution, especially in the favored lines which were to produce the higher fishes, sufficient, perhaps, is known. Out of one armored group, however, there were to arise the land-living vertebrates. Here again there is more hypothesis than observed fact, and especially desirable would be the discovery of fishes whose fins show potentialities, either in their structure or their implied use, or both, of giving rise to the shore-adapted foot, the point of departure of all terrestrial progression. There is at Yale a single footprint, which speaks volumes to those who read its lesson aright, for it is the most ancient record of a terrestrial vertebrate known to science, although there is still some difference of opinion as to its correct interpretation. But it is baffling in its obscurity, as it tells little of the structure of the impressed foot and nothing at all of any other part of the owner’s frame, except, again, by inference. This footprint (Thinopus) is a most important link in a weak part of our chain; but other links, which thus far are known but vaguely or not at all, are necessary for the chain’s integrity.
The old-time emergents—those that first left the water—were lung-breathers, of course, but they still went back to the water to bring forth their young. Living amphibians, such as frogs and salamanders, yet do this each recurring season, and gill-arches preserved in certain fossils, along with shore-adapted hands and feet, show conclusively that such was their ancient custom.
The passage into the next higher group, that of the reptiles, implies the loss of gill-breathing in the young and the consequent laying of eggs ashore. This passage has given rise to a large, complex egg, which can both nourish the developing embryo and allow it to breathe as well, until at the time of hatching it emerges as a miniature snake or turtle or crocodile, according to its kind, but never as a form reminiscent of the ancestral fish in shape or habit. Here the transitional types are surely known; the uninitiated can not tell whether they are amphibian or reptile, the difference lying in certain technical details of structure that are discernible only to the expert; and here, therefore, our chain has all the requisite strength of continuity.
Links of great interest are those that connect reptile and bird on the one hand and reptile and mammal on the other. Huxley, years ago, spoke of the birds as “glorified reptiles,” and his descriptive term is still very apt, for the birds are merely reptiles that were especially adapted to aerial navigation. Nature essayed this adaptation twice with the reptiles, one product being the pterodactyls or winged reptiles of the Mesozoic era. The pterodactyls and the birds have much in common, even to certain minute details of pneumatic bones with comparable openings of communication one with another: lightness of skull, with a precocious consolidation of cranial bones, and loss of teeth and their replacement by a beak in later forms. The marked distinctions lie in the mechanism of flight—by wing feathers in the birds and by a bat-like wing membrane in the pterodactyls. The two groups are doubtless derived from the same or a related non-flying ancestry, and much of their similarity is probably due to community of habit and its reaction on the mechanical structure.
The links leading to the pterodactyls are still unknown, for our first fossil record of these creatures are the remains of forms that had already attained sustained flight; of the first steps in that direction we have no direct knowledge. Not all conditions of life, however, are equally susceptible of preservation in the fossil record, and this is particularly true of flying forms, for a prerequisite to fossilization is complete burial either in water-borne or air-borne sediments, such burial as befalls flying forms only in very exceptional circumstances. A single locality, a quarry near Solenhofen, Bavaria, worked commercially for limestone used in the art of lithography, has given us nearly all the flying creatures of Middle Jurassic age that we know. Here are pterodactyls, some of which have the delicate wing membrane preserved in perfect detail; but these are already highly specialized fliers. Here also are the first known birds, feathers and all, and among the whole list of connecting links known to us none is perhaps more satisfying. Three individuals are represented, one by a single feather and the other two by complete skeletons, except that one is headless. This headless form, now in the British Museum of Natural History, is the famed Archaeopteryx; the other, in Berlin, shows sufficient distinction in its preserved parts to warrant a new generic name, Archaeornis. Both are classed as birds largely because of the feathers and implied powers of flight; in other respect they are reptiles. Archaeornis, in fact, has been called a reptile in the disguise of a bird, and the skull might well belong to a reptile, never to a modern bird. Here a none too efficient flight is already attained, but here again the history is missing, and our ideas concerning the origin of bird flight must still remain hypothetical. The scientific visualization of this pro-avian is of a light-built dinosaur-like form, running freely on the hind limbs and occasionally taking to trees for soaring leaps, sustained largely by partly modified reptilian scales. Out of such a scale in turn would evolve that masterpiece of nature, the feather—a prophecy that may in time be realized by fortunate discovery in the older rocks. From Archaeopteryx to the toothed birds of the Cretaceous period is again a considerable unbridged gap, during which the bird became essentially modernized except for the retention of teeth in the jaws, which may well have been due, however, to a habit of eating fish. We can well imagine intervening stages, showing gradual adaptation to greater efficiency in flying. The shortening of the lizard-like tail of Archaeornis, the feathers of which are arranged in a row on either side, to the fan-like tail of the pigeon or crow produced a vastly better device for manoeuvering in flight. The hand has consolidated the old free grasping fingers into a better and stronger wing, and the skull has changed its character in many details. A succession of drawings comparable to a moving picture film might well be made to show these changes with a fair assurance that, one after another, forms showing the comparable actual stages might be found. Here the lack of the connecting links does not seriously disturb the evolutionist, although their discovery would be an event of profound interest.
Our own lineage lies of course in the mammalian line; hence the dawning of mammalian life is of intense personal concern. Here we know much of the truth, for many of the stages have been revealed. The chief distinctions that separate the mammals from their reptilian prototypes are the peculiar methods of nourishing the young both before and after birth, much of the internal mechanism of the mother being directly or indirectly modified as a result of this habit. That the early mammals were egg-layers is attested by the retention of the egg-lying habit in the monotremes, such as the duckbill of Australia, the lowliest of existing mammalian forms, which, with many another evolutionary laggard, is a veritable living fossil, existing in a place remote from the busy competition that impels advance. The palaeontologist cannot trace the development of these diagnostic mammalian characteristics, for they are limited largely to soft parts. The preserved strictly mammalian features that may be compared with features possessed by members of the ancestral reptilian group are few. We find differentiated teeth, which are embedded in the rear of the jaw by more than one root; a single bone in the lower jaw, which is directly articulated with the temporal bone of the skull; and other minor details, largely changes toward greater simplicity.
Back at the beginning of the age of reptiles there existed, mainly in the Southern Hemisphere, a group of reptiles known, from their differentiated dog-like teeth, as cynodonts, from κυνός, a Greek word for dog. That they were not mammals is shown by their complex jaw, which consisted of several bones on either side, and by their retention of a quadrate bone, as it is called, between the jaw and skull. Both of these features had been possessed by reptilian, amphibian, and fish ancestors as far back as we can trace the bony skeleton, and are thus a firmly established heritage of the race. Why the simplifying? And where shall we find the missing bones? These questions have given rise to much argument. At all events, the changes occurred concomitantly with the assumption of other diagnostic features, and in a comparatively short time. The Triassic rocks yield jaws of creatures so near the dividing line between reptile and mammal that only the most intensive modern research has decided their status as reptiles, although for years they were considered primitive mammalian forms. Mammal and bird each have warm blood, which means not only a heat-controlling mechanism, but clothing (hair or feathers) as well. Feathers are seen in Archaeornis, but fossil hair is still unknown in association with ancient mammals, so that in them warm bloodedness cannot be proved, though it may be assumed from analogy. From the mammal-like reptile sprang the reptile-like mammal, out of which true mammals in turn arose. Much of this history is recorded in hundreds of tiny jaws and teeth recovered from rocks of the reptilian age. Certain links in the chain are missing, but the main evolutionary lines are indicated by tangible evidence, not alone by inference.
Mammalian divergence has followed several lines, all traceable to a few parent stocks. These lines led to the hoofed cohort, to the clawed carnivores, and to those groups which went down to the sea and became adapted marvellously to life in the great waters—whales and sea cows of diverse lineage. Other feebler folk became the inhabitants of trees, and these belonged to a very ancient group, retaining certain primitive traits of limbs and teeth which left them far behind the others in evolutionary status. But out of these, largely through mental supremacy, were to come our forebears and, later, actual humanity itself.
Certain recorded links couple lines which to-day are so divergent that students of recent animals would hardly imagine their relationship. Compare the manatee, of spindle-like body, broad swimming tail, flipper-like fore limbs, and no visible hind limbs at all, with the majestic elephant, conspicuous for his peculiar features of limbs, head, and trunk. What community of structure do they show? And yet there has come out of the Libyan desert of Egypt a fossil form that is neither one nor the other but partakes somewhat of the nature of both—a synthetic swamp-dwelling type, whose descendants came to the parting of the ways and went severally to the sea and to the land—to grotesque aquatic efficiency on the one hand and to noble terrestrial majesty on the other. Here the link has been found. Were it lost, who could tell the tale of this strange kinship?
One of the most completely known of all evolutionary chains is that of the horse-like forms, though here again the hypothetical five-toed ancestor is still undiscovered. We could, however, restore him in detail, so complete is our evidence of the course of equine evolution, and, if the chain includes other missing links we can formulate their characteristics with mathematical certainty, as well as their distribution in time and place. Our material for tracing the horses is so abundant that it resolves itself not into a single chain, but into many, all of which have arisen from the original stock to follow lines that may be closely parallel, or, again, widely divergent, but invariably in response to climatic and vegetative environment. Most of these chains have come to a natural end at one time or another, so that we now have but two, or at most three, terminal lines, represented by the true horses, the asses, and the zebras. The lineage of each of these forms is apparently traceable back through millions of years, until the lines ultimately merge.
But it is with the lineage of Man that we are chiefly concerned. Out of some unknown line of primitive mammals arose those small beasts of prey whose lack of prowess necessitated their feeding on such feeble folk as they could overcome—worms, slugs, insects, and small lizards. These beasts are represented in the living fauna by the Insectivora, such as the shrew, the mole, and the hedgehog, which represent ancient lines of descent and display rather conservative forms. As primitive offshoots of this group came on the one hand the stronger feral animals, known as the Carnivora, and a group of tree folk, the Primates. Where the original home of the Primates was we do not know, but we strongly suspect that it was some circumpolar region of salubrious climate where extensive persistently green forests afforded them asylum and an abundance of easily obtainable food. Fossil plants of Eocene age comparable with those living in Cuba today have been found in Greenland and Spitzbergen. It is safe to assume, therefore, that the climate of those lands was then also comparable. From early Eocene time we have a definite fossil record of Primates in England and in Wyoming, some of the American forms being well-nigh identical with the living lemurs or half-apes of Madagascar, an identity implying that these again are persistent types, or "living fossils." In North America these early Primates lingered throughout the Eocene but disappeared there when a change of climate destroyed the tropical forests that had been their home. They migrated through Central America to South America, where their somewhat altered descendants yet persist. In Europe they had a similar history except that they reappear from some Asiatic or African fastness, flourish for a while, and die out again until the coming of Man. The monkeys of Gibraltar are really African, despite their foothold in this outlying rock.
Living monkeys, both tailed and man-like, are found to-day in both Africa and Asia, and although all are derived from the same root as mankind, the great or man-like apes, especially the gorilla and chimpanzee, are our next of kin, being separable from man, aside from spiritual values, by what in other creatures would be considered inconsequential details. It is only our lack of perspective that makes us imagine a considerable gulf between us.
Human and simian relationships are not yet clearly traceable; they are inferred from close structural and functional similarities, the distinctions being referable to differences of habit and habitat. That man and the great apes are cousinly descendants from a common stock all scientists believe; they do not believe, as some imagine, that man is an exalted ape or that the ape is a degenerate man, though what one should call the common progenitor, if not an ape-like form, I do not know. This precursor of ours has been long sought, and he has been called the "missing link," as though one link could constitute a chain covering many thousands of years of descent. Pithecanthropus, the form found by Dr. Dubois in Trinil, Java, in 1891-92, stands not far from the ancestral form and more than any known today deserves this title. This type, which existed half a million years ago, was then already erect, though not perhaps of god-like carriage, showing how remote was the attainment of this human characteristic; but in form and capacity of skull he lies about midway between the gorilla and the Neanderthal type of man that lived in Europe some 25,000 to 40,000 years ago. In that sense he is a connection, but he is not a common ancestor. Heidelberg man and his lineal successor, the man of Neanderthal, constitute the first and last links of another chain whose slow change is unrecorded for three hundred and fifty thousand years. But these forms are evidently not in the line that led to modern man, although the Neanderthal type was yet alive when Man appeared on the European stage, 25,000 years ago. What lies back of our own species is still unrevealed, but we are probably out of an unknown though extremely ancient Asiatic stock. That Asia is the birthplace of humanity most authorities now agree, except the few who, because of the primitive character of the African natives and the antiquity that was Egypt, infer an African origin for the higher races. This inference seems unwarranted, for, although the negroes are primitive in most respects, in others they show a higher specialization than either the yellow or the white races. Fossil forms are now coming to light, especially in the Siwalik region of India, which could be ancestral to the great apes, and possibly to man, but one looks for the final solution of this problem farther to the north, in the comparatively unknown Asiatic plateau.
That there are gaps in our revealed record of the continuity of life—gaps due in part to incomplete exploration, in part to natural causes—is manifest; but that the record is sufficient to uphold the principle of continuity is equally manifest. Sedimentary rocks form the repository of this record, and sedimentation is always locally discontinuous because of the wearing down of the earth's surface until the force and carrying power of the streams have well nigh ceased. This wearing down of the land is followed periodically by its re-elevation through crustal movement, with consequent rejuvenation of the streams, which begin once more the endless round of scouring, carrying, and depositing. When the crustal movement is so profound as to be dignified by the name revolution there may be marked organic change, stimulated usually by stress of climate, which, either directly or indirectly, has a vast influence upon life. These times of change are therefore critical, both in the elimination of old types and in the acceleration of the evolution of persistent types, so that the whole aspect of nature as revealed by the fossils is profoundly altered. It is not at all surprising, therefore, that the older naturalists, whose orthodoxy caused them to adhere to the doctrine of special creation, imagined a succession of great catastrophes, by which the older faunas were completely destroyed and life was recreated twenty-seven times, the number chosen corresponding to the greater divisions of geologic time. This same explanation was applicable locally to the lesser breaks in the record. But geologists are now convinced of the uniformity of physical conditions throughout geologic time and of the all-sufficiency of the observable phenomena of the present world—of changes in temperature, of rain, snow, and ice, of erosion and geochemical action, of earthquakes and volcanic activity—to account for any and all of the changes, however great their apparent magnitude, in the geologic past. And as a necessary corollary of this doctrine of uniformitarianism in the physical world comes that of continuity with evolutionary change in the organic world. That the evidence for this organic continuity seems meagre is due in part to our lack of perspective, in part to our prepossession with false conceptions or pseudo-conceptions, and in part to our proneness to magnify imperfections that merely mar but do not destroy a most magnificent, clearly unified, and deeply impressive moving spectacle of creation, which at length makes Man the heir of all the ages.
REFERENCES
- Darwin, Charles. The Origin of Species. 1859.
- Lull, R. S. Organic Evolution. Macmillan, 1920. The Ways of Life. Harper, 1925.
- McCurdy G. G. Human Origins. Appleton, 1924.
- Osborn, H. F. The Origin and Evolution of Life. Scribner’s, 1917. Men of the Old Stone Age. Scribner’s, 1915.
- Scott, W. B. A History of Land Mammals in the Western Hemisphere. Macmillan, 1913.
- Shimer, H. W. An Introduction to Earth History. Ginn & Co., 1925.
- Schuchert, Charles. Historical Geology. Wiley & Sons.
- Schuchert, Charles, and Le Vene, C. M. Earth Rhythms. Appleton, 1927.
“The testimony yielded in the superficial layers of the earth and In the caves, embracing hundreds of specimens of the fossilized remains of man, more or less ancient, more or less complete, invariably, without a single exception, testify to the gradual ascent of man from a lower to a higher state, gradually dropping one primitive bit of anatomy after another until the high, intelligent, fully human aspect is attained.”—H. F. Osborn.
“Determinate evolution is victorious all along the line. It only remains for us either to accept the inevitable, or to take up a position as belated outsiders, living in dreamland away from the practical realities of science, rethinking the childish thoughts of primitive folk.”—C. Lloyd Morgan.
“The time has come when scientific truth must cease to be the property of the few—when it must be woven into the common life of the world.”—Agassiz.