A BOLKIAN VIEW ON HUMAN EVOLUTION
My views on human evolution are inspired by the fetalization theory of Louis Bolk (1866-1930), a Dutch anthropologist who formulated his 'retardation theory' (or 'fetalization theory') in 1917. According to this theory, a directive factor operates in evolution, towards the appearance of Homo sapiens.
Bolks starting point was the phenomenon of human fetalization. See the figure above, that shows the skull of a monkey (Macaca nigra) in three developmental stages (the youngest stage above, adult stage below). The proportions of the skull become less human-like in older animals: the forehead and the cranial sutures disappear, a snout and a sagittal crest have developed and so on. In humans, this transformation is much less pronounced. Humans conserve more of the common traits that are shared by young primates generally. Another such phenomenon, that made a great impression on Bolk, is human nakedness. Monkeys and apes have also a naked skin in the fetal stage, and the development of their haircoat starts from the scalp, and then spreads down to the rump and the limbs. Here too, it seems as if human development is arrested with respect to the development of other primates. As far as ontogenetic development has phylogenetic significance, this phenomenon suggests that apes and monkeys are descendants from a human-like progenitor, rather than the other way round. But Bolk saw this ancestor not as some actual animal, but as a non-material factor, that operates in evolution in the sense of the Aristotelian final cause.About the phenomenon of human nakedness, he wrote:
"The fetal trait considered here, cannot result from some adaptation to external circumstances. There have never been chimpanzees, or ancestors of chimpanzees, with a naked skin and only a hairy scalp. The phenomenon under consideration has to be the expression of a deeper developmental principle. We are forced to conclude that the nakedness of man and the conservation of the haircoat on his scalp result from causes already active in the development of the chimpanzee fetus. These causes thus were of no external nature, and they were active before the appearance of man. An inner developmental factor must have been at work, active already in the great apes, and fully deploying its force in man (...) The full consequence of this view must be that in the first and lowest organism the necessity of ulterior hominization was already present (...) I am fully aware of the risks and of the vulnerability of my position. iI can be argued that by accepting this internal developmental factor controlling from within the changes of the animal forms throughout evolution, I am lead to believe that evolution is something of a determined process (...) This remark is correct, but I do not yield" (Bolk, Hersenen en Cultuur (1918); my translation).
Bolk's original contribution was the linkage between two particularities of Homo sapiens: extreme longivity and very strong fetalization. Although I believe that Bolk was basically right, his argument was imprecise and incomplete. Moreover, Bolk was unable to incorporate phenomena of human hypermorphosis (such as our long legs) into his framework.
Bolk's connection between longevity and fetalization can be formalized in the following way (see scheme below,I). We start from a generalized ontogeny, consisting of a bundle of developmental pathways, that connect the different traits of the adult with the zygote. Each developmental path consists of a number of subsequent 'elementary developmental steps'. In the scheme, the pathways are represented by arrows; the points separate the elementary steps on each pathway.
What happens when an animal, for instance under the pressure of natural selection, wil be induced to alter the generalized ontogeny? Natural selection will tend to interrupt developmental pathways, in order to insert specialized developmental steps into the ontogeny. These specialized steps are indicated by white dots in the scheme (II). Two points are important.
Firstly, developmental steps that occur later in the generalized ontogeny are more likely to become eliminated by specialization. When the interruption of the developmental paths occurs at random, the number of surviving elementary steps will decrease exponentially with developmental time. In the scheme, developmental steps of stage F are much more likely to become eliminated, whereas developmental steps occuring in stage A have a much better chance to survive specialization (from a formal point of view, the process resembles radioactive decay, with the existence of radioactive atoms being interrupted randomly).
Secondly, more generalized steps have to be eliminated to allow the introduction of a few specialized steps. Compare it to a building constructed for maximal height (the Eifel tower, say). If you start changing the plans in the midst of the building process, the final height attained with be lowered, and the building time will be shortened accordingly. In the same way the total number of elementary steps in the ontogeny, and the corresponding life time, will be compressed when specialization disrupts the internal coherence of the original, generalized ontogeny. From this point of view, a longer life time and a less specialized anatomy are logical corollaries. Bolks intuition, that the very long lifetime of humans, and their utter lack of specialization (leading to a 'fetalized' morphology) are connected, becomes understandable when one introduces the concept of 'generalized ontogeny' that is illustrated in the figure above.
A very important prediction follows from this interpretation: later ontogenetic stages will tend to be selectively suppressed in more specialized mammals. Inversely, in more retarded mammals, later ontogenetic stages will encompass comparatively more elementary steps, and will thus transpire more strongly. Organs or body parts that develop later, will tend to be relatively stronger developed in more retarded (and at the same time less specialized) mammals.
The generalized ontogeny is characterized by several developmental gradients. A very striking one is the cephalocaudal gradient. In early developmental stages, the head of a mammal is very big with respect to the rump, the limbs are still very small with respect to the rump, and the legs/hindlimbs are small with respect to the arms/forelimbs. One can observe this phenomenon not only in humans, but in mammals generally.The figure above compares newborns and adults (reduced to the same trunk length) of humans, gibbons and chimps (har coat removed - after Schultz). One sees that the limb become longer with respect to the rump; the head becomes comparatively smaller.
Higher primates are characterized by very long limbs (with repsect to other mammals). In humans, the hindlimbs (legs) are exceptionally long, not only with respect to the rump, but even with respect to the forelimbs or arms. Our very long limbs are an instance of hypermorphosis: these body parts become proportionally longer in retarded animals, because developmental compression selectively suppresses ontogenetic latecomers. In the figure above, you can see that the apes have very long forelimbs. These long arms cannot be explained by the same mechanism (hypermorphosis through retardation) because the arms are ontogenetic forerunners with respect to the hindlimbs (the hindlimbs remain shorter in apes). Moreover, newborn apes have already rather long arms, ready for locomotion (climbing), whereas the legs are still very short in newborn humans. The particularities of limb proportions in apes are to be explained by specialization and adaptation; the limb proportions in humans can be understood as a simple explicitation of the generalized mammalian ontogeny (hypermorphosis through retardation).
The long legs of humans are suitable for upright walk; but they are not an adaptation to the erect stance. Rather, a body suited for the erect stance appears, when the generalized developmental scheme of mammals is fully developed, without interruptions and specializations. For instance, the foramen magnum (the hole in the skull, where the spinal chord connects with the brain) has the same position in the skull of humans and simian youngsters. In non-human primates, this position shifts towards the backside of the skull when these animals grow older, and shift more towards the quadrupedal stance (young monkeys show a propensity towards bipedalism, that fades away in older animals). Humans conserve the generalized morphology, that is suited for upright walk.
There are many more instances of hypermorphosis. Some striking examples are:
* intralimb proportions. Adult humans are characterized by hands and feet that are exceptionally short with respect to the more proximal segments of the limbs (see figure above). In the mammalian fetus, one sees that the hands and feet are initially very large with respect to the proximal parts of the limb: the hands and feet are forerunners with respect to the upper arm or the thigh. In later ontogenetic phases, the latter catch up with the distal parts. In humans, this process is extended through retardation, and we end up, not only with very long limbs, but also with limbs that have proprotionally larger proximal segments.
* proportions of digits. Humans have very long thumbs and big toes (see skeletons of feet below). In more retarded primates, one sees the preponderance of digit length gradually shifting towards the side of the thumb and big toe (the 'anterior' side of the digital row). In the human fetus, this process of the anterior digits gradually surpassing the posterior ones has been carefully observed by Schultz. Ontogenetically, the digital row develops from the posterior towards the anterior side; again, the ontogenetical latecomers are strongly developed in Homo. In apes, one sees that the big toe is already surprisingly well developed. Here again, the apes are midway between humans and lower primates.
Again, the strongly developed big toe is suited for the erect stance, and the strongly developed thumb allows for the precision grip, that is so important for human culture. But these traits did not arise through specialization, but through straightforward extension of the generalized developmental scheme of mammals.
* Brain proportions. Humans have very large brains. But more importantly, the ontogenetic latecomers in brain development (especially the neocortex) are proportionally even larger in humans. This proportionally stronger development of the younger parts of the brain (telencephalization) makes our brain to a human one.
Another corrolary of the theory is, that in a row of equivalent organs, the developmental latecomer will not only show quantitative hypermorphosis (it becomes proportionally larger through hypermorphosis), but it will also show qualitative juvenalization (neoteny). For the theoretical background, see Verhulst 1999. Some examples:
* In humans, the anterior digits are more fetalized or neotenic with respect to the posterior digits. For instance, have a look at the hair growth of the middle phalanges of your fingers. Hair growth is sparser on the index finger. The nail of the index finger is flatter (less claw-like) as compared to the nail of the ring finger (our nails are fetalized claws; the claw in a mouse fetus still resembles a nail, but then this nail starts to curve around the finger top en completely circumfuses it, forming a pointed claw). When your hand is relaxed, the ring finger takes on a more curved position with respect to the index finger. This is also a neotenic trait: the fingers are straightish in early growth; bending of the fingers comes later.
All the differences between the anterior and posterior fingers thus spring from a common cause: the anterior fingers are not only hypermorphic, but also neotenic ('or fetalized') with respect to the posterior fingers.
* The human foot is a latecomer with respect to the hand. Also, the foot can be seen as a fetalized hand. There are two major differences. On the one hand, the big toe does not descend, and does not oppose the other toes. This process takes place in the later development of the hand, but does not occur in human foot. On the other hand, the toes remain very short. In monkeys, the foot becomes more or less hand-like in later developmental stages; but in the early fetal stages, the toes of the monkey foot are still very short, and the big toe has not yet descended to a thumb-like position (Verhulst 1993b).
* In the ontogeny of the skull, the bones around the maxillary bones are forerunners with respect to the bones of the skullcap. The maxillary bones fuse early in apes, and extremely early in humans; these bones also develop pronounced pneumatization in humans. The bones of the skullcap remain comparatively neotenic or fetalized: they fuse only in a very late stage, and pneumatization remains very limited. At the same time, the bones in the skullcap are very large (hypermorphic) with respect to the human maxillaries.
More details can be found in the following papers:
* J.Verhulst (1993a) "Louis Bolk revisited: I. Is the human lung a retarded organ?" Medical Hypotheses 40, 311-320
* J.Verhulst (1993b) "Louis Bolk revisited: II. Retardation, hypermorphosis, and body proportions of humans" Medical Hypotheses 41, 100-114
* J.Verhulst (1994) "Speech and the retardation of the human mandible: a Bolkian view" Journal of Social and Evolutionary Systems 17, 307-337
* J.Verhulst (1996) "Atavisms in Homo Sapiens: a Bolkian heterodoxy revisited" Acta Biotheoretica 44, 59-73
* J.Verhulst, and N.Jaspers (1997) "Does human retardation occur at the molecular level?" p.218-228 in: J.Wirz, and E.T.Lammerts van Bueren (editors) "The future of DNA" Dordrecht: Kluwer Academic Publishers
* J.Verhulst (1999) "Bolkian and Bokian retardation in Homo sapiens" Acta Biotheoeretica 47, 7-28
I also wrote a book on the subject: J.Verhulst "Der Erstgeborene.Mensch und hoehere Tiere in der Evolution" Stuttgart (Germany): Verlag Freies Geistesleben (in press).
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