Stephen J. Gould

Ontogeny and Phylogeny: Bibliographical Excerpts

"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 of 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.
18. Brachycephaly.
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).

These lists from Bolk and Montagu display the extreme variation in type and importance of the basic data presented by leading supporters of human neoteny." (Gould, S.J. (1977) Ontogeny and Phylogeny, Cambridge: Belknap Press. p. 357)

"Humans and chimps are almost identical in structural gens, yet differ markedly in form and behavior. This paradox can be resolved by invoking a small genetic difference with profound effects---alterations in the regulatory system that slow down the general rate of development in humans. Heterochronic changes are regulatory changes; they require only an alteration in the timing of features already present. If the frequency of heterochronic change were known, it would provide a good estimate for the importance of regulation as an evolutionary agent" (Gould, S.J. (1977) Ontegeny and Phylogeny. Cambridge: Belknap Press. p. 9)

"Evolution occurs when ontogeny is altered in one of two ways: when new characters are introduced at any stage of development with varying effects upon subsequent stages, or when characters already present undergo changes in developmental timing. Together, these two processes exhaust the formal content of phyletic change; the second process is heterochrony. If change in developmental timing is important in evolution, then this second process must be very common (if it is predominant in frequency, I will be in even better shape)." (Gould, S.J. (1977) Ontegeny and Phylogeny. Cambridge: Belknap Press.pp. 4)

human evolution