Popular Science Monthly/Volume 27/September 1885/Origin of Man and Other Vertebrates
|ORIGIN OF MAN AND THE OTHER VERTEBRATES.|
THE early part of this century saw the establishment of most of the fundamental principles of the science of physics, especially as applied to astronomy. A few decades later saw the science of chemistry emerge from the empirical and enter the philosophical stage. It has been reserved for the second half of the century to witness the discovery of the facts and principles of the history of life on the earth. The public mind is gradually awakening to the fact that the grandest truths of creation are being placed within their reach by the researches of contemporary science, and that the knowledge of the manner of the origin of the human race is no longer withheld from us.
The study of the fossil remains of animals has revealed an immense number of forms of life which in former ages have peopled the world. The study of geology has shown that the history of our planet is marked by successive deposits in water, which have become beds of rocks. The relation of these beds to each other gives us the relations of the animals and plants whose fossil remains they contain. Thus we have obtained a consecutive history of life from its early appearance to the present day. Before the doctrine of evolution was understood, the successive populations that filled the successive periods were supposed to have been the products of special creations. Now it is believed, with the best of reason, that all forms of life have been produced by changes of structure which arose in the course of descent, the one species coming from the other; and that interruptions in the series of species from older to later periods are simply due to the absence of the means of preserving their remains at certain times during the course of the history of the world. These interruptions indicate periods of dry land, since fossils are not preserved unless they are excluded from the air by a covering of water or of mud.
It is one of the peculiar advantages of the North American Continent to the scientist that the geological structure of its great interior is comparatively simple, so that its history can be easily read. It follows that the history of the succession of life is recorded with the same regularity, and may be read by those who will bestow the necessary labor upon it. Those who have, during the last ten years, devoted themselves to this study have been rewarded by the discovery of the course of development of many lines of animals, so that it is now possible to show the kind of changes in structure which have resulted in the species of animals with which we are familiar as living on the surface of the earth at the present time. Not that this continent has given us the parentage of all forms of animal life, or all forms of animals with skeletons, or vertebrata, but it has given us many of them. To take the vertebrata, we have obtained the long-since extinct ancestor of the very lowest vertebrates. Then we have discovered the ancestor of the true fishes. We have the ancestor of all the reptiles, of the birds, and of the mammals. If we considered the mammals, or milk-givers, separately, we have traced up a great many lines to their points of departure from very primitive things. Thus we have obtained the genealogical trees of the deer, the camels, the musk, the horse, the tapir, and the rhinoceros, of the cats and dogs, of the lemurs and monkeys, and have important evidence as to the origin of man. We have the primitive mammals from which all these kinds that I have mentioned drew their descent, and from which, no doubt, many other lines were derived which we have not yet discovered in North America. Such are the lines of the elephants, the hyenas, the bears, the hogs, and the oxen. The ancestors of the strange, pouch bearing marsupialia, have been found in part. These creatures, now confined (except the opossums) to Australia and the adjacent islands, were, at an early period, widely distributed over the earth. Some of these are found in the fossiliferous deposits of our plains and Rocky Mountains.
So soon as the possibility of learning the manner of creation of animals is admitted, curiosity and speculation are awakened. Many alternatives naturally occur to the mind. Were any of the living kinds of animals descended from any other living species, or have the ancestral animals all disappeared from the earth? Have the giants of ancient periods become reduced in size and strength, or have the giants of to-day grown from weak and insignificant beginnings? Have things grown more and more perfect with the lapse of the ages, or have they degenerated from more perfect ancestors? Have these changes advanced alike in all continents, or have they proceeded differently in different parts of the earth? Such are the questions that confronted the student of North American vertebrate paleontology fifteen years ago, and some of them could only be answered by North American material, not only because its record is the most complete, but because, as the second continent studied, it furnished the first opportunity in the history of the science for a comparison with the record already placed before us by the paleontologists of Europe.
Answering the last question first, it has been conclusively proved that there has been a general correspondence in the progress of vertebrate life in the two continents of the northern hemisphere. The differences, though numerous, are of minor importance. Some families of vertebrata have existed on the one continent, which were absent from the other, but the number of such is not large. Even the same genus occasionally existed on both continents.
The other questions must be answered by reference to the genealogies themselves, or phylogenies, as they are called.In tracing back all the lines to which we have yet had access, the same kind of changes is found to have taken place in all of them. Let us take, for instance, the animals with hoofs. These embrace the cloven-footed and odd-toed orders, with their many species and families, which are represented by the ox, deer, camel, hog, and hippopotamus, for the cloven-footed; and the horse, tapir, and rhinoceros, for the odd-toed. Most of these creatures walk on their toes. Many of the first-named group have but two toes, more or less united together, while the horses, of the second group, have but one toe. The bones of the two rows which form both the palm and the sole alternate with each other; and the ankle-joint is a well-constructed tongue-and-groove arrangement. The teeth in many of them are highly complicated by the infolding of the enamel of the crowns of the molars, and this special development of the molars has been accompanied by a corresponding reduction in their number, and in the number of the incisors. In tracing the lines of these animals backward in time we have made the following discoveries: First, the infoldings of the enamel of the molars become shallower, and are finally represented by the valleys between four hills or tubercles, which stand to each other so as to be inclosed by a square figure. The number of the molar teeth increases. If incisor teeth were absent, they appear. The toes increase in number, becoming five on all the feet. The step becomes plantigrade or flat-footed, the heel reaching the ground. The tongue and groove disappear from the ankle-joint, which becomes flat. The bones of the two rows of the carpus and tarsus no longer alternate with each other, but rest, each one of the first on each one of the second row only. In 1874 I foretold that the ancestor of all the mammals above mentioned would prove to be a "pentadactyle, plantigrade bunodont"; that is, a five-toed sole-walker, with tubercular molar teeth. In 1881, seven years later, I obtained evidence that such a type of mammals abounded in North America during the early Eocene Tertiary period, and the prophecy was fulfilled. The best-known genus of this division has been called Phenacodus, and the figures of it will be found in the "American Naturalist" for 1884. In a still earlier formation of the Eocene, nearly all the hoofed mammalia were found to be of this type, showing conclusively that this group, which is known as the Condylarthra, was the ancestor of all hoofed mammals (Fig. 1).
But the Condylarthra were also ancestors of a still more important line of mammals. A remarkable type of quadrupeds known as lemurs at present inhabits Madagascar and some parts of Africa and Malaysia. These creatures, known by the Germans as Halbaffen, or half-apes, present a curious combination of the characters of monkeys and carnivora of the raccoon pattern. They could easily have stood in the position of parents to the monkeys in general; and suspicions to this effect have been abundantly confirmed by the discovery of numerous representatives of the sub-order Lemuroidea in the Eocene beds of both Europe and North America. And these again have been traced as certainly to the Condylarthra as ancestors, so that this group is again proved to be the ancestor of man as well as of the hoofed animals. And here was fulfilled another prophecy made by the writer, along with the one already mentioned, viz., that the ancestor of man also, would be found to be a "pentadactyle plantigrade bunodont."
An especial point of interest in the phylogeny of man has been brought to light in our North American beds. There are some things in the structure of man and his nearest relatives, the chimpanzee, orang, etc., that lead us to suspect that they have not descended directly from true monkeys, but that they have rather come from some extinct type of lemurs. Lemurs, which fulfill this anticipation, have been found in our Eocene beds, and belong to a peculiar genus which bears the name of Anaptomorphus. These creatures have a dentition more like that of the anthropoid apes than any living lemur
Fig. 2.—Skull of the Primitive Lemur (Anaptomorphus homunculus} (Conel. Natural size, except Fig. d, which is four thirds natural size. (From the Eocene of Wyoming.
exhibits. They had the most acute senses of sight and hearing, if we may judge from the bony parts which surrounded those organs. They also had larger brains than those of any other mammal of their period, though they did not differ much in this respect from the existing lemurs. Both of the two species known are small, not probably exceeding a gray squirrel in size. Only the skulls and jaws are known (Fig. 2).
But the Condylarthra prove to be the ancestors of a still greater population of descendants. We have traced all forms of grinding teeth up to a pattern which consists of four tubercles or cones arranged within a square. But it has been possible to show that one of the four tubercles appeared after the other three, as an addition to them, so that the earliest form, of molar or grinding tooth was tritubercular, and that the quadritubercular was an outgrowth from it. Now, one of the three families of the Condylarthra has tritubercular molars, and there is little doubt that it was the ancestor of the two other families. The principal genus of this family is called Periptychus. From this family came an order of hoofed mammals, which never rose to the possession of four tubercular grinders, although the crowns became crested by the modification of the three which they possess. This order, the Amblypoda, had a short life in geological time, and did not grow in the dimensions of the brain, but developed huge skeletons with skulls that sprouted into horns and strange processes.
The Condylarthra with three tubercles are probably also the ancestors of the carnivorous orders. The lions, tigers, wolves, and bears of to-day can be shown to be descendants of animals absolutely intermediate between themselves and the animals just mentioned. These half-carnivores, or Creodonta, have, like the ancient hoofed mammals, more numerous teeth than their modern representatives, and differ from the true carnivora in just the ways, in limbs and feet, that we have seen that the ancient hoofed mammals differ from their modern descendants. Creodonta were not such dangerous animals as the carnivora, with some possible exceptions, because, although they were as large, they generally had shorter legs, less acute claws, and smaller and more simple brains.
This genealogy, it will be seen, does not show us the ancestors of the Condylarthra. This remains for future discovery. It is, however, probable that they will be found in the earlier geological periods, among some of the marsupial mammals only known thus far from the jaws and teeth. It must also be noted that a number of these ancestral groups are represented in the existing fauna by a few genera. Of the Condylarthra, a near relative exists in the Hyrax, or cony, which now inhabits Africa and Western Asia. Of the Creodonta, several genera exist in Madagascar, West Africa, and the northern hemisphere. The mastodons are late representatives of an ancient type, and their phylogeny has not yet been fully made out. But they certainly also came from the Condylarthra.
There is a remarkable likeness between the history of the development of the reptiles and that of the higher mammals, in one respect, and that is, that they have apparently all been derived from a single order, which occupied the earth at one (an early geological) period. As reptiles are inferior to mammals in the scale, so they are of earlier origin. The primitive reptilian order first appeared in force on the earth during the Permian epoch—that populous time which immediately followed the age of the true coal. If it existed during the coal measures proper, it has not yet been found in them in North America. This order has been named the Theromorpha. Its representatives have been found in Russia, Germany, South Africa, Illinois, Texas, and France. I give the names in the historic order of discovery. It embraced both carnivorous and herbivorous forms, and species of sizes from that of the Malayan tapir downward. Those with piercing teeth occur everywhere, and those with grinding teeth in North America only. South Africa furnishes us with genera with leaf-shaped teeth, and others with no teeth at all. This order represents the first air breathing land-population of vertebrates, and they evidently fulfilled most of the functions of the mammalia of to-day, though none of them were fliers, so far as known. Many of them had strange physiognomies, with blunt noses and large nostrils, and long teeth mingled with other smaller ones. Besides having given origin to most of the reptilia, this order presents many points of resemblance to the mammalia. Some of the bones resemble very closely those of the duck-bill or Platypus of Australia, and some of the bones of the skull are more mammalian than the corresponding parts of any other reptiles. It is probable that the lowest order of mammalia, which is to-day represented by the duck-bill (the Monotremata), were derived from the Theromorpha. (See "Proceedings" of the American Association for the Advancement of Science for 1884.)
The different lines of reptiles have been traced less completely than those of the mammalia, partly because their history is more ancient, and the formations where their remains are preserved have suffered greater disasters. The changes that have appeared with advancing time have been in the bones of the shoulder-girdle and pelvis, in the limbs, vertebra?, and skull. Certain changes in these parts resulted in the appearance, in the period immediately following the Permian (the Triassic), of the orders of the sea-saurians, the flying saurians, and the land-saurians or Dinosauria. In the next period, the Jurassic, we have the first certain knowledge of the tortoises and lizards; while, in the following ages of the Cretaceous, we get the pythonomorphs and the snakes. All of the existing orders were in the world by the beginning of Tertiary time, but the great monsters that characterized the middle period of the earth's history were only represented by the crocodile branch of the Dinosauria.
The changes of structure which these several lines underwent in the course of the ages were quite different from those which the history of the mammalia exhibits. Instead of becoming more perfect organs of locomotion, the limbs, if we except those of the flying reptiles, lost some of their special characters, becoming more remote from the mammalian type. The pelvis became weaker and more open. The shoulder-girdle lost parts in some orders, and gained some expansion in others. The vertebræ became more perfectly articulated by the bodies, and in the case of the snakes by the arches also. Finally, the ribs lost one of their points of articulation with the vertebræ, and the jaws became looser and more open, and especially adapted to swallowing large bodies whole. The history consists of a successive departure from the mammalian type, and a running into a specialization, which, in some cases, means degeneration.
A curious specialization which supervened on thetype is that of the birds. Various saurians exhibit unmistakable approximations to the birds. The land-saurians include types that walked on the hind legs and had many bird-like characters of the hinder feet and of the pelvis. The flying saurians present affinities in the same direction. The class of birds presents many perfections both general and special. Their brains are larger than those of reptiles, and they acquired warm blood. In their own specialty of flight they display wonderful power, while the highest orders add that vocal skill which makes them so pleasing to man. Here is a good example of advance in evolution. Cases of degenerate evolution are to be found in birds, but they are few.
The next lowest class, that of the Batrachians, prevailed during the coal-measure period. They expanded enormously during the Permian, and were worthy contemporaries in size and numbers of the theromorph reptiles. Their numbers diminished subsequently, as the record now stands, though some of the species maintained their bulk during the Triassic period. In modern times they are comparatively insignificant; frogs, toads, salamanders, sirens, and cœcilias not playing an important part in the existing fauna. In tracing the successive changes of structure of these creatures, one is forced to believe that degeneracy has played an important part. The bones of the skull have so diminished in number as to leave it in some cases in a condition comparable to that of the primitive fishes. In not a few modern types the metamorphosis is never completed, the animals remaining permanently breathers in the water. Whatever we may call such changes, they are plainly a specialization which has carried them further and further away from their starting-point; and, as in the case of the reptiles, this starting-point has been near to orders higher than itself. The Permian Batrachia are nearer in structure to the Permian reptiles (Theromorpha) than any subsequent form of Batrachia has been (Fig. 3).
Professor Agassiz pointed out that the early fishes presented relations to other vertebrata, as I have since shown to be true of the Batrachia and Reptilia. Some of the primitive fishes he called "sauroid" or "reptilian" fishes. Batrachian fishes would have been a more accurate designation, for it is highly probable that it was from one of the early orders of fishes that the Batrachia took their rise. Omitting from consideration the lowest vertebrata, the sand-lances and lampreys, which are not fishes, we have remaining a body of animals which present
great varieties of structure. Of the four great sub-classes into which they naturally fall, but one can be called true fishes. The others embrace the sharks, the chimaeras, and the lepidosirens. It is interesting to note that these four divisions are more closely approximated during the Permian period than at any later time. An order technically referred to the sharks, and known as the Ichthyotomi, combines many of the characters found separately in three of the sub-classes. The creatures which especially deserve the name of batrachian fishes, the ceratodonts, etc., also abounded during the Permian period. From this time the true fishes began to run their course. They have peopled all waters, and have branched into a greater diversity of form than any of the other divisions of vertebrata. And paleontology and zoology show that they have pursued a course which is in its essential principle the same as that displayed by the Batrachia, and in a less degree by the Reptilia. They have diverged further and further away from the batrachians, which they once resembled, and in so doing have left behind them the general ascending line that led some of the land animals to become Mammalia. They have become specialized into types which have special modes of life adapted to special localities. Some of the lines of descent are clearly degenerate, as indicated by a loss of parts. Some of these degenerate lines inhabit the deep sea; others have become movably sessile, attaching themselves to fixed bodies. Others have found protection in an external armor of bony plates rather than in activity and sensibility. But many fishes are in their especial way wonderful exemplars of animal energy, though none of them rise high in the scale of intelligence.
In review of the results obtained from the recent study of vertebrate paleontology, certain principles may be clearly discerned. These are as follows: 1. The earlier types were more generalized, the later ones more specialized. 2. The specialization is sometimes upward or progressive, and sometimes downward or retrogressive. 3. The retrogressive development has been more general in early geological periods, the progressive more general in the later geological periods. For a more detailed exposition of these principles, see "American Naturalist" for February, March, and April, 1885.
It is not my intention in this article to do more than to display the facts of the case. The exposition of the hypotheses of evolution which explain these facts must be reserved for another article. Suffice it to say here, that the study of the changes of structure displayed by the lines of evolution, has brought to light some very definite exhibitions of the application of energy. The illustration of the modus operandi of this creative energy is a very important chapter of evolution, and one that interests mankind practically, even more than as food for his intellectual activity.