Human evolution theory utilizing concepts of neoteny & female sexual selection
An etiology of neuropsychological disorders such as autism and dyslexia, and the origin of left handedness.
To support the argument that we evolved by retaining juvenile features of our ancestors, Bolk provided lists of similarities between adult humans and juvenile apes: "Our essential somatic properties, i.e. those which distinguish the human body form from that of other Primates, have all one feature in common, viz they are fetal conditions that have become permanent. What is a transitional stage in the ontogensis of other Primates has become a terminal stage in man" (1926a, p. 468). In his most extensive work Bolk (1926c, p. 6) provided an abbreviated list in the following order:
1. Our "flat faced" orthognathy (a phenomenon of complex cause related both to facial reduction and to the retention of juvenile flexure, reflected, for example, in the failure of the sphenoethmoidal angle to open out during ontogeny).
2. Reduction or lack of body hair.
3. Loss of pigmentation in skin, eyes, and hair (Bolk argues that black peoples are born with relatively light skin, while ancestral primates are as dark at birth as ever).
4. The form of the external ear.
5. The epicanthic (or Mongolian) eyefold.
6. The central position of the foramen magnum (it migrates backward during the ontogeny of primates).
7. High relative brain weight.
8. Persistence of the cranial sutures to an advanced age.
9. The labia majora of women.
10. The structure of the hand and foot.
11. The form of the pelvis.
12. The ventrally directed position of the sexual canal in women.
13. Certain variations of the tooth row and cranial sutures.
To this basic list, Bolk added many additional features; other compendia are presented by Montagu (1962), de Beer (1948, 1958), and Keith (1949). The following items follow Montagu's order (pp. 326-327) with some deletions and additions:
14. Absence of brow ridges.
15. Absence of cranial crests.
16. Thinness of skull bones.
17. Position of orbits under cranial cavity.
19. Small teeth.
20. Late eruption of teeth.
21. No rotation of the big toe.
22. Prolonged period of infantile dependency.
23. Prolonged period of growth.
24. Long life span.
25. Large body size (related by Bolk, 1926c, p. 39, to retardation of ossification and retention of fetal growth rates).
"The reduction of body hair and number of sweat glands has gone furthest in that major group of humans in which in every other respect the most advanced morphological developments have occured, to wit, in the Mongoloid major group. Both in the density of hair folicles and in the actual number to hairs per square centimeter there has been an appreciable reduction." (Montagu, Ashley (1989) Growing Young N.Y.: McGraw Hill pp. 57)
"Theorists of human evolution, such as Bolk (1926), Gould (1977), and Montagu (1962,1989), have listed a number of physical and functional neotenous features of humans. ... Bipedality requires a change in the angle at which the spine connects with the skull. The opening in the skull where the spine connects to the skull is referred to as the foramen magnum. In all embryonic mammals, the foramen magnum is located at the bottom of the skull, so that the spine enters the skull at a right angle to the top of the skull and parallel to the plane of the face. During prenatal development, the location of the foramen magnum shifts toward the back of the skull, so that in most species of mammals the spine is essentially parallel to the top of the skull and perpendicular to the plane of the face. However, in humans, the position of the foramen magnum does not change appreciably beyond this embryonic stage. Development is retarded, so that at birth and into adulthood the sharp angle of the spine to the skull is maintained. That is, the foramen magnum maintains its embryonic position, with the result being that the skull sits atop the spine, thus permitting one to look forward while standing upright. Because the foragmen magnum shifts toward the back of the skull in other mammals, forward sight is more easily accomplished when the animal is on all four feet. Thus bipedality results from retention of an embryonic characteristic---development is retarded, setting the stage for major evolutionary change (see Gould, 1977; and Montegu, 1989). (Bjorklund, D.F. (1997) The role of immaturity inhuman development. Psychological Bulletin 122(2): 155)
"Brachycephaly---that is, broadheadedness---in contrast to dolichocephaly (longheadedness) also appears to be a fetal trait. Up to the sixth fetal month the head tends to be long. The "brachycephalic races," Bolk suggested, are the offspring of longheaded ones. In the course of time, he argued, the transforming processes in some groups of humanity were retarded and, growing weaker, "the initial fetal form of the skull was increasingly retained," until, finally, brachycephaly was established as the persisting form of the head. After the sixth month the fetal cephalic (head) index varies from mesocephalic 75.9 to 79.9, to brachycephalic 80.0 to 84.9. (Montagu, Ashley (1989) Growing Young N.Y.: McGraw Hill pp. 51)
"Myelinization---that is, completion of the fatty sheaths of nerve fibers---is not achieved until the fifteenth postnatal month, nor is completion of the tracts associated with the development of many muscular movements---the pyramidal tracts, for example, which pass from brain to spinal cord." (Montagu, Ashley (1989) Growing Young N.Y.: McGraw Hill pp. 83)
"The neotenous traits of the child 1. The need for love 2. Friendship 3. Sensitivity 4. The need to think soundly 5. The need to know. 6. The need to learn 7. The need to work 8. The need to organize 9. Curiosity 10. The sense of wonder 11. Playfulness 12. Imagination 13. Creativity 14. Openmindedness 15. Flexibility 16. Experimental-mindedness 17. Explorativeness 18. Resiliency 19. The sense of humor 20. Joyfulness 21. Laughter and tears 22. Optimism 23. Honesty and trust 24. Compassionate intelligence 25. Dance 26. Song" (Montagu, Ashley (1989) Growing Young N.Y.: McGraw Hill pp. 131)
"Bolk drew attention to the findings of Muller that the Malayans of Java have much smaller thyroid glands than do Causasoids, and that the pituitary gland is heavier is Caucasoids than in Malaysians. A similar difference exists between Chinese and Causasoids. Anatomist J. Shellshear also found that the thymus gland, situated at the front of the neck and passing into the upper part of the chest, persists among the Chinese, sometimes into old age. Shellshear regarded the persistence of the thymus as an expression of retarded development, a more general evidence of which he saw in the neotenous "childlike" appearance of the Chinese---a view which both Keith and Bolk were in full agreement. In recent years investigations of the thymus gland have revealed that it secretes a number of hormones, among them a growth-promoting substance called promine. Dr. Albert Szent-Gyorgi has shown that the thymus reaches its peak when growth of the body is fastest. In his studies on bovines, Szent-Gyorgi found that the thymus is somehow connected with youth, and that extracts injected into old animals make them behave as young ones. In this respect, he stated, promine is similar in its action to the "juvenile hormone," also known as the "Peter Pan hormone," found in insects that undergo metamorphosis into butterflies. In addition to producing a large number of lymphocytes, the thymus also produces a variety of hormones and plays an important role in the development of immunologic competence in fetus and child. There is also good evidence that maturation of fetal liver and splenic cells is dependent on an intact thymus. Removal of the thymus in newborn mice stunts their growth; thymus-derived serum (gamma globulin) injected into such animals causes them to grow faster, to maintain a higher weight average, and survive almost twice as long as thymectomized controls who have been given a neutral serum." (Montagu, Ashley (1989) Growing Young N.Y.: McGraw Hill pp. 47-8)