Book - Vertebrate Zoology (1928) 42

From Embryology

Vertebrate Zoology G. R. De Beer (1928)

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Chapter XLII The Evolution Of The Primates And Man

The Primates originated from a stock related to the Insectivora probably in Cretaceous times. Plesiadapis, from the Early Eocene, had characters in common with the Insectivora and the Lemurs, which are the lowest Primates. The characteristic of the higher Primates is that the bony bar separating the orbit from the temporal fossa is complete ; or in other words, the eye-socket is round and protected all round by bone. At the same time, both eyes look more or less forwards so that their fields of vision overlap and may coincide (stereoscopic vision). There are five fingers and toes, and the first digits are opposable to the others, except in the case of the first toe of man. This opposability makes the limbs efficient grasping organs, and is evidence for the fact that the early Primates lived on the branches of trees. It will be seen in the sequel that this arboreal habit had consequences of the highest importance in the evolution of the Primates . Lastly , the most important character of all in the Primates is the great development of the neopallium in the cerebral hemispheres.


In the Eocene the fossil Notharctus is found, representing the earliest member of the group of the Lemuroidea. It was very generalised, for whereas the primitive dental formula in mammals is i| , c \ , p| , m| , that of Notharctus was i| , c \ , pf , mf . From forms of this type the Lemurs must have descended. The Lemurs alive to-day are nearly restricted to Madagascar, though a few occur in Africa, Ceylon, and Malay. They are fairly primitive animals, but show certain specialisations which rule them out from the main line of Primate evolution. Among these may be mentioned the peculiar procumbency of the incisors of the lower jaw, with which they comb their fur.


The tail of a Lemur is long but not prehensile, and its skull may be recognised by the fact that the cavity of the orbit can still communicate with that of the temporal fossa beneath the postorbital bar.


Another Eocene fossil allied to Notharctus is Tetonius, the earliest representative of the group Tarsioidea. Tetonius had an enlarged rounded brain-case and a small face. Its brain must have been relatively larger than in any other known Eocene animal. It also was not on the direct line of descent owing to specialisations such as the loss of the lower incisors, but a close relative of it must have been the ancestor of Tarsius which lives at the present day. In Tarsius the postorbital bar is splayed out and almost but not quite prevents communication between the orbit and the temporal fossa. It shows important advances in the structure of the brain, and of the external ear. In the fact that it has a discoidal placenta with a thickened trophoblast hollowed out into lacunae filled with maternal blood, and in the fact that the mesoderm appears very early in the development of the embryo, Tarsius resembles the higher Primates and Man, and differs from the Lemuroidea.


In the true monkeys, apes and man, or Anthropoidea, the orbit is completely shut off from the temporal fossa. From some Tarsioid ancestor with affinities to Notharctus there diverged a branch which gave rise to the New World monkeys. These forms which are included in the group Platyrrhinas have a broad internasal septum with the nostrils wide apart, and a tail which is usually prehensile. They show a considerable advance in the structure of the brain, and in the fact that their dental formula is reduced to i§, c\, p|, m-?. At the same time they are definitely off the main line of Primate evolution because of the structure of the tympanic bone which forms a ring.


The Old World monkeys, apes and man, form the group Catarrhinae, in which the internasal septum is narrow, the tympanic bone forms a tubular external ear, the tail when present is never prehensile, and the dental formula is reduced to if, cf, p|, inf.



The Catarrhinae must have emerged in the Eocene period from Tarsioid ancestors related to those which gave rise to the Platyrrhinae. In the Oligocene, Parapithecus is found, and from forms related to it the ordinary monkeys or Cercopithecidae must have been derived. These forms are again ruled out from the main line of Primate evolution by specialisations such as the development of two transverse ridges on the molars. At the same time, the Cercopithecidae, which include the baboons and mandrills, show a great development of the brain, which must have undergone an evolution parallel to that which went on in the stock leading to the apes and man.


The main stem of the Primates leading to the anthropoid apes and man was represented in the Oligocene by the little Propliopithecus. The fact that it was small is important, for so many divergent branches became specialised in the direction of large size, and in the search for the ancestors of the apes and man, choice is limited to forms considerably smaller than those to which they might have given rise. The anthropoid apes have lost the tail, they show a tendency to walk erect, the mechanism of pronation and supination of the hand is perfected, and the brain is greatly enlarged and developed.


A descendant of Propliopithecus was the Pliocene form Pliopithecus, which itself was an ancestor of the gibbon (Hylobates), the smallest of the apes. Other lines of descent from forms like Propliopithecus led to the Orang, and to the chimpanzee and Gorilla, while the main stem culminated in the Hominidae and man. The apes while having highly developed brains and retaining the power to oppose the first toe to the others, have not got brains large enough to enable them to do otherwise than remain brutes, relying on their strength and their long canines instead of on memory, skill, and the neopallium. There are some characters possessed by adult modern man which are present in the young but lost in the adult apes. An example of these is the absence of large brow-ridges in young apes and man.


The fossil record of the Hominidae is not by any means as complete as would be desired, but there is already sufficient evidence to enable an outline to be given of the more important changes and modifications which accompanied the evolution of modern man : Homo sapiens.


The Hominidae include all the members of the human family and it must be noted that they differ from other Primates not so much in matters of kind as in matters of degree. Essentially, the evolution of the Hominidae is a story of enlargement of the brain, reduction of the nose, face, and jaws, perfection of the erect position, and loss of opposability of the large toe, which last feature converted the Quadrumana or apes into the Bimana or men.



Fig. 180. — Professor John I. Hunter's reconstruction of the Piltdown skull, drawn by T. L. Poulton. (From Elliot Smith.)


The nearest approach to the human condition without achieving it on the part of an ape is Australopithecus, the Taungs skull, from South Africa. This fossil betrays kinship to the Gorilla and chimpanzee, but its brain is slightly larger and its face smaller.


The earliest known member of the human family is Pithe- canthropus from Java. This form had a much enlarged brain with a cubic capacity of about 950 c.c, while the maximum volume of an ape's brain is 650 c.c. From the structure of its femur, it walked almost erect. In some respects it preserves primitive features such as the continuity between the occipital and temporal crests on the skull, and many features in the conformation of the brain, but in others it is specialised, as in the development of the large brow-ridges.



Fig. 181. — Skull of Homo rhodesiensis, drawn by T. L. Poulton. (From Elliott Smith.)




The most important of all the human fossils is Eoanthropus, the Piltdown skull from Sussex, The bones of the skull are very thick, and the lower jaw is strikingly similar to that of a chimpanzee, with no chin whatever, and large canine teeth. But the brain-case is dome-shaped and large, with a cubic capacity of about 1170 c.c, and there were no brow-ridges. The latter fact, together with the primitive nature of certain features in the brain, makes it possible to regard Eoanthropus as very close to the line of man's descent, if not his ancestor.


Pithecanthropus and Eoanthropus date from the latest Pliocene or earliest Pleistocene periods. The remaining fossil men come definitely within the period of the great Ice- Age.


The most primitive known member of the genus Homo is Homo rhodesiensis, from Broken Hill in Rhodesia. It had a brain- volume of about 1250 c.c, but still lacked any semblance of a forehead. There was no boundary between the nose and the face, and the palate was very big, but its chief specialisation is the development of enormous brow- ridges. The lower jaw is unknown, but it was probably not very different from that of Homo heidelbergensis from Heidelberg, which unfortunately is known only from the lower jaw. It is large and massive, without any chin. The teeth were definitely human, and primitive in that the last molars preserve five cusps, while those of modern man are often reduced to four.


Incomparably the best known of the fossil men is Homo neanderthalensis, specimens of which have been found from a number of localities in Europe, from Gibraltar to Palestine. In addition to the bones themselves, there is considerable evidence concerning these men from the weapons and tools which they fashioned from flint, and which consequently have been preserved. An indication of the degree of mental develop- ment of these people is obtained from the fact that some of the individuals which have been discovered appear to have been intentionally buried. The brain is very big (about 1350 c.c. on an average), but it preserves numerous primitive features. At the same time, the brow- ridges were large as were also the face, palate, and jaws. There was no chin, and the lower jaw preserves the large attachments for the digastric muscles. The vertebrae and the legs show that the neanderthal man did not stand straight up, but stooped considerably. The hip-girdle was still long, and the foot rested mostly on its outer border, as in young children and certain savage races to-day. While having lost the capacity to be opposed to the other toes, the large toe was considerably separate from the remainder. The neanderthal race has gone extinct, doubtless because of its specialisations, and the insufficient development of the brain which handicapped it in the competition with Homo sapiens. Here, the brain has achieved maximum development, so much so that its front wall has been pushed forwards to form a more or less vertical forehead. This vertical wall of bone provides the necessary resistance for the reduced lower jaw to bite against. In the apes, Pithecan- thropus, rhodesian and neanderthal man, where the lower jaw is still large and there is no forehead, the strain of the bite is taken up in the large brow-ridges which are developed in the adult. These brow-ridges are therefore not a primitive feature, but independently acquired as an adaptation in certain groups. Their presence, however, rules their possessors out from modern man's ancestry.


The face in Homo sapiens is relatively smaller than in any mammal, the lower jaw is slender and provided with a promi- nent chin, and the canine teeth are small. The hip-girdle is short and wide, and the vertebral column and legs enable man to stand bolt upright.


In connexion with the expansion of the brain and the assumption of an erect attitude, it is consistently found, on ascending the scale of Primate evolution to man, that the foramen magnum through which the spinal cord joins the brain is movedr elatively farther and farther forward. This fact is obvious when it is considered that the head of an ordinary lower mammal projects forwards horizontally from its neck, whereas man's head is carried vertically above his neck. At the same time, the eyes of lower mammals and of man look hori- zontally from about the middle of the front of the face. There has therefore been a progressive expansion of the hinder and upper part of the skull accompanying the development of the brain, and which moves the face-region farther and farther forwards. The ordinary superposition of median vertical sections through skulls suffers from the fact that it is then difficult to distinguish between differences of actual size and differences of development. This difficulty vanishes when the sections are superposed on a common centre, and then rotated so that certain standard radius-lines coincide. Other lines coincidence up to the top of the skull, and drawing a line at right angles to it through Sollas' centre can then be read-off by angular measurement regardless of the actual size of the skull. The centre of gravity is chosen as the common centre since it is the morphological centre of form. It may be called Sollas' centre. The sections are then rotated so that the radius-lines from the centre to the middle of the foramen magnum coincide. The sections are then " set," and reference-lines are made by continuing the radius of




Fig. 182. — The skeletons of Neanderthal man and of modern man com- pared. (From JBoule.)


One of the most instructive readings is the measurement of the angle made between the line of the foramen magnum, and the line from the centre to the point of junction between the nasal and frontal bones (the nasion). It is essential for this comparison that the sections be taken from specimens of equivalent age, for during development the angle changes. Nevertheless, taking adult material it is possible to make out the following : —

Angle between foramen magnum and nasion in adult :


Gibbon . . 238

Chimpanzee .. 239°

Pithecanthropus 25 1 ° (conjectural)

Homo neanderthalensis • • 253°

Homo rhodesiensis . . 262

Eoanthropus 264 (conjectural)

Homo sapiens circ. 270


These measurements show that the periphery of the brain- case in modern man amounts to three right angles, and it is interesting to note in comparison with lower forms that the accommodation for the increased size of man's brain is obtained by the angular increase in the periphery of the brain-case as well as absolute increase in size.


It is also noticeable that Eoanthropus approaches nearer to sapiens than do neanderthalensis and rhodesiensis, which is an additional reason for including the former but excluding the latter from sapiens' ancestry.


As regards Australopithecus, the following table shows the comparison between it and juvenile specimens of other forms.


Angle between foramen magnum and nasion in young : —

Orang 243 Chimpanzee . . . . . . . . . . . . . . 252 Australopithecus . . . . . . . . . . . . 258 Homo sapiens child . . . . . . . . . . . . 282


It is clear, therefore, that the Taungs skull approaches the human condition in this respect, though still remaining similar to the Chimpanzee.


Having now reviewed the material on which all study of the evolution of man must be based, it remains to consider what causes were probably operative during the history of human descent. It may be said at once that just as the rise of the mammals was due to the development of the brain and formation of a neopallium, so a continuation and perfection of that process led to the rise of the Primates and man, and that this development was largely associated with the sense of sight.


Fig. 183.— Diagrams of longitudinal sections of skulls, superimposed on their centres. (After Sollas.) Showing the difference in angular measurement between the nasion-line (from the centre to the top of the nasal bone) and the line through the foramen magnum (from the centre) : in Gibbon 238 ; in Chimpanzee 239 ; TJ fnP ^° PUS 25I Vo n Nea P derth al man 253°; in Rhodesian man 62 2 , in Piltdown man 264 ; and in modern man 270 .




It has been seen that the history of the Primates can be traced from Insectivore-like ancestors, through Tarsioid, monkey, and ape stages, and that their evolution was accomplished under arboreal conditions of life. Now, the Insectivora, Tarsioidea, monkeys and apes have living representatives at the present day, some of which have changed but little from their Eocene ancestors. Without in the least suggesting that these living forms are on the main line of descent (which indeed it has been shown carefully that they are not), they may be taken and studied for their brains and organs of sight, as showing grades of structure approximately representative of the stages through which it is known that the Primates passed.


Of the Insectivora, Macroscelides (the jumping shrew) may be taken as a primitive mammal, in which the neopallium is developed, but the archipallium related to the sense of smell is still very large. In particular it is important to notice that the region of the neopallium (parietal region) related to the sense of sight is small.


Tupaia (the tree shrew) is related to Macroscelides, and the difference which it shows in its brain is related to the habit of living in trees. Life in trees is conducive to the better development of the sense of sight, for jumping from one branch to another, and inefficient perception of spatial relations would lead to disaster. Accordingly, it is not surprising to find that the visual area of the neopallium of Tupaia is better developed than in Macroscelides, and that in the nature of its retina and other features connected with the eyes, Tupaia approaches the Lemurs.


The stage represented by Tarsius, which is also arboreal, is of great importance, for here for the first time the sense of smell is reduced below the level of the sense of sight, which becomes the dominant sense in the body. The eyes of Tarsius look forwards, and the fact that they have rotated onto the front of the face necessitates the reduction of the nose and snout. At the same time, the senses of hearing and touch are better developed, together with their respective temporal and tactile areas in the neopallium. The development of the tactile area is important because it is associated with that area of the cerebral cortex which is concerned with the performance of delicately adjusted and skilful muscular movements. Such movements are essential for an active arboreal animal, but there is another reason for referring to this part of the neopallium, and that is that a portion of it (the prefrontal region) is concerned with the co-ordination of the movements of the two eyes.



Fig. 184. Diagrams showing the left side of the brains of Macroscelides, Tupaia, Tarsius and the Marmoset. (From Elliot Smith.) Showing the increase in the area of the cortex associated with vision and co-ordinate movements (prefrontal), and the decrease in the olfactory region, in the evolution of Primates.


In the prefrontal area of Marsupials there are centres which control the eye-muscles and therefore the movements of the eyeball of the opposite side. The movements of the two eyes are linked together in higher forms, and this is especially significant in the Primates, where the visual axes of the eyes become parallel. Further, whereas in lower vertebrates the fibres from each eye all go to the other side of the brain (the crossing at the optic chiasma is complete), in the mammals a certain number of fibres remain uncrossed, and go to the same side of the brain. Now, in the Platyrrhine stage of evolution, represented by the Marmoset, the co-ordination between the movements of the two eyes is perfected, and both eyes are able to follow one and the same object. A consequence of this is that " corresponding points " are developed in the retinae of each eye, on which the images of one object are formed, and the most important of these points is the macula lutea or spot of optimum sensitiveness.


Consequent on the power of making conjugate eye-movements, the Anthropoidea have evolved a macula lutea, and this still further increases the importance of the parietal (visual) and prefrontal (skilled movement) areas, which features already distinguish the brain of the Platyrrhine from that of Tarsius. A continuation of the process of enlargement and perfection of the parietal, prefrontal, and temporal areas can be gradually


EVOLUTION OF THE PRIMATES AND MAN 473 and serially traced through the Catarrhine, the ape (Gorilla), Australopithecus, Pithecanthropus, Eoanthropus, Homo rhodesiensis, Homo neanderthalensis to Homo sapiens. There is further the very interesting fact that in human development, the regions of the neopallium which are the last to be formed are precisely these parietal, prefrontal and temporal areas.


There is therefore good reason to believe that the perfection of these areas of the cerebral cortex and of the functions with which they are associated played the major part in the evolution of man. The brain developed first, and other features such as the reduction of the face and assumption of the erect attitude followed. It is to be noted that the perfection of the parietal and prefrontal areas is directly or indirectly concerned with the function of vision, so that it may be said that sight was of capital importance in the evolution of man. In this connexion, mention may be made of some other aspects of the bearing of sight on evolution.


In the first place, it will be remembered that the eyes are " distance-receptors, " and that the responses which they evoke on the part of the animal are anticipatory rather than consummatory movements. Next, there is the fact that in man, the number of nerve-fibres entering the brain from one eye vastly exceeds the number of all the other afferent nerve- fibres of one side put together. From the physiological side, it is found that in the higher Primates including the monkeys, apes, and man, the eyes assume great importance in regulating the posture of the organism, a regulation which in lower forms is principally dependent on the semicircular canals of the ear. Lastly, from the psychological point of view, experiments on the behaviour of chimpanzees when confronted with problems shows that the eyes play a very important part in solving the problem. Cases of great interest are those in which there lies close at hand some instrument, such as a stick, and by using which the ape would be able to solve its problem easily. Unless the instrument to be used is seen by the ape in the same field of vision as the object or goal for which it is to be used, it pays no attention to it. Without this optic co-presence, the ape does not " see " the solution to the problem.


Perhaps the most important of all the consequences of the perfection of the sense of sight in the Primates is the fact that it is the neopallium which undergoes commensurate development in the brain, and the neopallium is the physical companion of memory, of the ability to profit by experience, and of the arbitrator of possible responses, known as the will. There is also to be noticed here the importance of remaining unspecialised. For if the great development of the sense of sight had taken place earlier in evolution, in an ancestor of the mammals, it would have been not the neopallium, but the optic lobes which would have undergone specialisation, and for a number of reasons these are unsuited for the development of the higher mental faculties. The success of man is therefore also due to the fact that his ancestors did not shoot their bolt of specialisation prematurely.


A consequence of binocular vision and conjugate movement is the power to converge the eyes on an object. In the first place, this enables an estimate of distance to be made, which is important in leaping from branch to branch. Feeling of the degree of convergence is conveyed by stimuli from proprioceptive sense-organs in the eye-muscles by afferent fibres in the eye-muscle nerves. When, however, the eyes are converged on an object, that object occupies the attention of the animal, and the stereoscopic vision which it now enjoys enables it to become aware of the true geometrical and spatial relations of the objects in the world around it.


To return to the face, it is obvious that when the nose and snout are reduced as a result of the eyes coming on to the front of the face, the mouth itself can no longer so easily be used as a food-obtaining organ, as it is in lower forms. Here, the hands come to the rescue, and being five-fingered and with opposable thumbs, capable of pronation and supination, they undertake the function of carrying food to the mouth. At the same time, the development of the prefrontal area of the neopallium enables delicate movements to be made, in the course of which the animal acquires skill. It is an interesting fact that in the higher Primates, the focal length of the eyes for most acute vision should be just within the reach of the hands. The assumption of the erect posture which is made possible by the increased power of co-ordination of the brain, relieves the hands from the service of locomotion, which is performed solely by the feet. The latter therefore lose the opposability of the big toe.


Lastly, in connexion with the greater development of the temporal region of the neopallium, the power of hearing became more acute, and with it came the development of speech. There is clinical evidence that in man, one of the lobes of the temporal region is concerned with the faculty of stringing words together into sentences with a logical meaning, and it has been shown above that this is one of the regions of the neopallium which has undergone progressive development in the evolution of man. It is not claimed that man is nothing more than a mammal which sees, hears, and co-ordinates his movements better than other mammals. All that is intended is to show that the development and perfection of these functions of sight, auditory discrimination with which must be coupled speech and language, and muscular skill, bring about changes which are prerequisite for the development of that peculiarly human attribute — the higher mental faculties.


Literature

Boule, M. Les Hommes fossiles. Masson, Paris, 1921. (English translation, Oliver & Boyd, Edinburgh, 1923.)


Elliot Smith, G. Essays on the Evolution of Man. Oxford University Press, 1927. Gregory, W. K. The Origin and Evolution of the Human Dentition.


Williams and Wilkins, Baltimore, 1922.


Sollas, W. J. Ancient Hunters. Macmillan, London, 1924.


Sonntag, C. F. The Morphology and Evolution of the Apes and Man. John Bale, Sons and Danielsson, London, 1924.


Thomson, A. A Consideration of the more Important Factors concerned in the Production of Man's Cranial Form. Journal of the Anthropological Institute. Vol. 33. 1903.





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Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)
Vertebrate Zoology 1928: PART I 1. The Vertebrate Type as contrasted with the Invertebrate | 2. Amphioxus, a primitive Chordate | 3. Petromyzon, a Chordate with a skull, heart, and kidney | 4. Scyllium, a Chordate with jaws, stomach, and fins | 5. Gadus, a Chordate with bone | 6. Ceratodus, a Chordate with a lung | 7. Triton, a Chordate with 5-toed limbs | 8. Lacerta, a Chordate living entirely on land | 9. Columba, a Chordate with wings | 10. Lepus, a warm-blooded, viviparous Chordate PART II 11. The development of Amphioxus | 12. The development of Rana (the Frog) | 13. The development of Gallus (the Chick) | 14. The development of Lepus (the Rabbit) PART III 15. The Blastopore | 16. The Embryonic Membranes | 17. The Skin and its derivatives | 18. The Teeth | 19. The Coelom and Mesoderm | 20. The Skull | 21. The Vertebral Column, Ribs, and Sternum | 22. Fins and Limbs | 23. The Tail | 24. The Vascular System | 25. The Respiratory system | 26. The Alimentary system | 27. The Excretory and Reproductive systems | 28. The Head and Neck | 29. The functional divisions of the Nervous system | 30. The Brain and comparative Behaviour | 31. The Autonomic Nervous system | 32. The Sense-organs | 33. The Ductless glands | 34. Regulatory mechanisms | 35. Blood-relationships among the Chordates PART IV 36. The bearing of Physical and Climatic factors on Chordates | 37. The origin of Chordates, and their radiation as aquatic animals | 38. The evolution of the Amphibia : the first land-Chordates | 39. The evolution of the Reptiles | 40. The evolution of the Birds | 41. The evolution of the Mammalia | 42. The evolution of the Primates and Man | 43. Conclusions | Figures | Historic Embryology



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