Why is Ernst Haeckel's theory of recapitulation being given up?

Evolution: biology

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This article discusses frequently cited examples of recapitulations in the course of human ontogeny. Although the methodology that leads to the delimitation and characterization of recapitulated features is increasingly being abandoned in current research (Biogenetic Basic Law - Current), the examples mentioned are still often taken up in popular science as evidence for evolution.

`` Gill arches ''

The Reichert-Gaupp theory

`` Tail ''

`` Swimming skins '', `` paddles ''

Ear muscles

`` Fur ''

`` Yolk sac ''

Reflexes

Conclusions

Summary of all articles on the Biogenetic Basic Law

literature

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`` Gill arches ''

Characteristic of almost all vertebrates is the formation of arched structures in the early embryonic period, which encompass the anterior section of the intestine in the head and neck area (cf. Fig. 294).

Historical review. Heinrich Rathke, who discovered these arches and the invaginations separating them for the first time in a pig embryo in 1825, was completely convinced, due to his natural-philosophical basic position and the expectations associated with it, that he had the temporary system of functioning gills in front of him - corresponding to the development stage of a fish. He therefore concluded: "Even the mammalian embryos are provided with gills at a very early stage of development, and then, with regard to these, resemble the hayfish most" (1825, p. 747). Later he discovered analogous structures in the chicken embryo, among others and finally also in humans, although he no longer called them gills, but spoke of Gill indications, since they already showed clear differences in their embryonic appearance to the embryonic disposition in "bones". Their further research in land creatures showed even before Haeckel that they never correspond to real gills of adult fish in their structure and function. It had also been recognized that structures such as the lower and upper jaw, hyoid bone, larynx, elements of the middle ear ossicles, various glands and lymphatic organs within the neck, etc. emerge from these arches. As early as 1836 Reichert suggested that they should be better visceral arches (visceral = located inside) and not gill arches Nevertheless, the idea of ​​the appearance of anatomically corresponding and functioning gills in the ontogenesis of terrestrial vertebrates remained alive with many scholars (for more on the history see Ullrich 1997.)

Today's rating. The formation of the visceral arches or pharyngeal arches in the course of the 4th week of development is the result of a complex process that is precisely coordinated in terms of time and location, in which the growth processes of the brain and the neural tube, the heart and the blood vessels emanating from it are integrated. The rapid multiplication of cells that migrate from the area of ​​the embryonic brain (neural crest cells) leads to a rapid increase in the volume of the arches and to the creation of pre-cartilage structures (blastemes), muscles, nerves and blood vessels in them. The neural crest cells are genetically determined even before they emigrate (especially by the front (5th) paralogue homeobox genes 1-3) (for the homeobox genes cf. Biogenetic Basic Law - Current and Homeobox genes and evolution). This means that they are set to the formation of a special structure in the head and neck area (upper jaw, lower jaw, ossicles, various chewing muscles, etc.). For this differentiation, however, their interaction with cell associations of the upper part of the intestine (endoderm) and the outer skin layer (ectoderm) is necessary. The four pharyngeal arches, which appear in humans one after the other and with different degrees of strength in the 4th and 5th week of development, are important guide rails for nerves, muscles and connective tissue systems as well as for blood vessels (aortic arches) (see O Rahilly & Müller 1999). Obviously, during the short period of existence of the pharyngeal arches, the structures that later emerge from them are further shaped. Each arch provides specific material for different structures of the facial skull and neck. After reshaping the arches, the determined cells migrate to their final locations in the head and neck area. The muscle cells of the second visceral arch, for example, form the facial muscle (Musculus facialis), which extends from the forehead to the neck area and is only innervated by the corresponding nerve, the facial nerve. A 5th and 6th pharyngeal arch, which would be comparable to the first four, is not created in humans (see below).

The four embryonic aortic arch pairs supply the brain and the rest of the body with oxygenated blood. Just like the pharyngeal arches surrounding them, they appear one after the other. The aortic arches are never visible in large numbers at the same time, as in fish (Sharks: 6 aortic arches). They also do not form the typical capillary network of their gills (Fig. 295 and Fig. 296). With increasing, dynamic growth effective kinking of the head in relation to the outflow path of the heart and due to the division of the heart into atria and chambers and other factors, the flow conditions of the blood change, whereupon the strongly developed first two pairs of aortic arches recede. Their appearance and regression is not a recapitulative or unnecessary detour, but a functional requirement within the framework of normal development.

The remaining pairs of aortic arches in the third and fourth pharyngeal arches only partially regress. They are the basis for the large vessels starting from the heart (aorta, clavicle artery, pulmonary artery). If this process of vascular formation is disturbed (genetic defects in trisomy 21, drugs such as thalidomide), severe vascular and heart malformations result.

With reference to comparisons with certain vertebrates (e.g. sharks), some authors count six paired aortic arches or gill arch arteries in humans. They have never been detected in this number in the human embryo. The two main pulmonary arteries, right and left, are initially formed from a fine vascular network between the heart anlage and the future lungs. These are temporarily connected to the aorta via the ductus Botalli, which normally appears only on the left and is considered the 6th aortic arch in comparative embryology. The schemes often presented in textbooks with 6 vascular arches in pairs correspond to phylogenetic specifications, but not the real appearance in human embryonic development (in addition to other findings such as differences in development on the right and left).

During normal development in humans, intact skin remains between the outer pharyngeal furrows and inner pharyngeal pouches, which separate the individual arches from one another. Neck fistulas, these are existing channels in the area of ​​the outer neck skin and the throat, are among other things the result of a pathological destruction of this skin and no gill slits that have remained open (see O Rahilly & Müller 1999). According to Otto (1994), homology between the gill cover of the fish (operculum) and the strongly developing second pharyngeal arch, which makes contact with the heart wall, cannot be spoken of either.

It is conceptually misleading to refer to the pharyngeal arches and the furrows or pockets delimiting them in the embryonic stage of all vertebrates as gill arches or clefts. Only occur in fish and some amphibians from one part the pharyngeal arches also have girdle-bearing structures, but never in the other vertebrates. The comparable occurrence of the pharyngeal arches in the embryos of different vertebrates can be interpreted as homologous in the sense of evolutionary theory (cf. Similarities in morphology and anatomy). This embryonic pattern of features can also be interpreted as a fundamental construction principle of a common creator, which is clearly modified in detail for each basic type - controlled at the genetic level by the integration of analogous developmental genes.

The Reichert-Gaupp theory

The transition from reptiles to mammals, presumed historically, had to result in a significant change in the anatomical conditions in the chewing apparatus and in the middle ear region. With reference to embryonic findings, attempts were made even before Darwin to establish homology relationships between the temporomandibular bones and the ossicles (columella) of amphibians, reptiles, and birds on the one hand and the three ossicles of mammals on the other. The views of Gaupp (1898), who referred to the work of Reichert (1836), settled under the name Reichert-Gaupp theory generally through. After that, the middle ear elements are directed hammer and anvil the mammal from the temporomandibular joint bones Articular and Square of reptiles from (Fig. 297). This means that structures formerly responsible for the chewing process are said to have become sound-conducting elements by relocating them - appropriately redesigned - from the area of ​​the temporomandibular joint region to the middle ear. The findings in ontogenesis in mammals (including humans) are the mainstay of this thesis.

In humans, from the end of the 6th to the 24th week of development, there is actually a continuous connection between the cartilaginous systems of the hammer and anvil with Meckel's cartilage, a cartilage clasp that emerges from the first pharyngeal archFig. 298). The two ossicles mentioned are therefore considered to be ontogenetic descendants of the first pharyngeal arch cartilage or Meckel's cartilage. In reptiles, on the other hand, Meckel's cartilage at its upper end forms the appendages for the temporomandibular joint bones quadratum and articular (Fig. 297). The joint between hammer and anvil that develops in mammals is therefore referred to as the primary temporomandibular joint and is interpreted as the phylogenetic (= phylogenetic) remnant of the former quadrato-articular joint of the reptiles. From a phylogenetic point of view, the Squamoso-Dental joint between the tooth-bearing lower jaw and the temporal bone scale, which is typical for mammals, is the secondary temporomandibular joint, because it is said to have arisen in parallel during the transformation of the primary temporomandibular joint.

Otto (1984) was able to show that human embryos in the blastema stage (stage of early organ anlage, 5th and 6th week of development), which precedes the cartilage stage, have a completely different anatomical-topographical situation (see also Ullrich 1994). The arrangement of the individual mesenchymal structures (blastemes) of the hammer and anvil on the one hand and the Meckel's cartilage on the other hand in a 39-day-old human embryo show a clear step formation and isolation of both structures. The jointless unit between the Meckel's and hammer cartilage that exists in the following cartilage stage is therefore not ontogenetically original. Only through a growth-functional shift (rostral shifting) does the posterior part of the hammer cartilage blastema reach the area of ​​the first pharyngeal arch, where it then fuses secondarily within the framework of the cartilage with the Meckel's cartilage. That is, the derivation of the hammer and anvil from the first pharyngeal arch, on which the evolutionary recapitulation argument is based, has been empirically questioned. From these facts, alternative, competing conclusions for the homology lists and their phylogenetic interpretation emerge:

1. The ossicles of the middle ear are not homologous to the quadratum and articular, but to the elements of the columella of non-mammals.

2. The hammer-anvil joint cannot therefore be called the primary temporomandibular joint.

3. The diversion of the mammals from the reptiles must have taken place 225 million years ago and, in terms of fossil material, could no longer be equated with the appearance of the modern middle ear.

Current findings seem to support this assumption and thus also underline the methodological uncertainty of the phylogenetic concept of homology for the elucidation of phylogenetic relationships. The configuration of the lower jawbone of an early (dated 115 million years ago) still toothed monotonous shows indications of the presence of an angular, articular, and qudratum. Since mammals living in parallel already have the typical configuration of the temporomandibular joint and the middle ear and today's monotremes are said to have emerged from the fossil described, it must be concluded on the basis of this new find that the middle ear bones arose twice independently of each other in mammals - in the group of Theria (non-egg-laying mammals) about 215-225 million years ago and 100 million years later in the group of monotons. According to Rich (2005), the middle ear bones (hammer, anvil, stirrup) lose their meaning as characteristic features of mammals.

`` Tail ''

When describing the lower (caudal) body section of the human embryo, some authors speak of the appearance of a tail rudiment (cf. Ullrich 2004). This structure is often used in the context of phylogenetic argumentation as evidence for the descent of humans from tail-bearing ancestors (Fig. 299). Most vertebrates have a tail on the other side of the trunk, which is typically made up of skeletal elements and muscles that extend from the spine. There are no intestinal cavities or parts of the CNS in it. As early as 1880, His questioned the legitimacy of the term “tail” in the human embryo based on his examination findings. Current results support these doubts.

Basically, the tail of vertebrates arises from the embryonic system of the caudal eminence (Tail bud), which in turn forms in connection with the attachment of the notochord and secondary neurulation. The ontogenetic formation of the coccyx and the caudal vertebra shows clear peculiarities compared to the development of the other vertebral bodies. This is to be explained briefly using the example of humans.

In the 4th week of development the tail bud of the approx. 2.5-4.5 mm large human embryo becomes visible. A rod-like outgrowth (chorda neuralis) of the neural tube begins to grow from the level of the caudal neuropore in the direction of the caudal eminence, which initially consists solely of mesodermal cells. The chorda neuralis overgrows the lower end of the notochord (cartilaginous rod which, among other things, induces vertebral body formation above the sacrum) and the end of the rectum. The caudal eminence represents the lower end of the embryonic body until the 8th week of development (the embryo is now 27-31 mm in size).

In contrast to the other (cranial (= headward)) sections, the lower parts of the neural tube arise within the notochord through secondary neurulation, which connects upwards (proximally) to the upper part of the neural tube formed by the primary neurulation. The notochord becomes the end of the entire neural tube and is the main component of the tail bud in this phase. There are no morphological or functional similarities with an adult vertebrate tail (see O Rahilly & Müller 1999).

The notochord induces, among other things, the formation of connective tissue in its environment and later the formation of the tailbone ("coccygeal") somites, from which, among other things, the tailbone vertebrae and parts of the pelvic floor muscles arise. At the end of the 7th week, the neural tube is completely surrounded by the coccygeal somite appendages, which in relation to the neural tube now show significantly faster growth in all three spatial levels and which later significantly overgrow (cf. Fig. 300).

In recent years the importance of the Homeobox genes for the differentiation of the structures resulting from caudal eminence has been further elucidated. The paralogue groups 9-13 play a dominant role in the typical formation of the specific vertebral bodies along the body axis (e.g. thoracic vertebrae with ribs, lumbar vertebrae without ribs).

The vertebral bodies usually arise from the upper and lower parts of two neighboring somites. In humans, never more than the remaining 32 to 35 vertebrae (7 cervical vertebrae, 12 thoracic vertebrae, 5 lumbar vertebrae, 5 sacral vertebrae and 3-6 coccyx vertebrae) are created. Compared to the upper body end of the embryo, which dominates the growth, the lower one initially remains relatively behind and therefore appears to taper off in the shape of a cone. In connection with the strong growth of the brain in this phase, the end of the spinal cord, which initially protrudes far beyond the end of the later spinal column and represents the caudal border of the embryo, is shifted upwards. However, your surrounding connective tissue bed (dura tube) remains in its old position and collapses and is more and more enclosed by the expanding spine. In the fetal period, the displacement of the end of the spinal cord is intensified due to the length growth of the spine, which now dominates compared to brain growth. This process continues after the birth. During the birth, the end of the spinal cord is at the level of the 3rdLumbar vertebra, in adults it ends at the level of the 1st-2nd Lumbar vertebra.

From a morphological or functional point of view, one cannot speak of the occurrence of a real tail rudiment in any of the ontogenetic phases listed. Such an evaluation is based only on purely external manifestations, without taking into account the actual ontogenetic background, and must be rejected.

Incorrect developments such as the hypertrophy of individual coccyx vertebrae with above-average expansion in the longitudinal direction of the body and the resulting protrusion of the skin at the tip of the coccyx are extremely rare findings. According to experimental results on the mouse, this malformation is related to the loss-off-function mutation of the genes of the paralog group 13 of the Hox b cluster, as a result of which there is an excessive growth of the structures that are derived from the notochord or are induced by it. A numerical increase in the 3-6 coccygeal vertebral bodies has not yet been described in humans.

Sometimes also referred to as the tail and with a phylogenetic interpretation is the rare occurrence of skin appendages in the lower back area. These formations arise as a result of disturbances in the development of the spinal cord and spinal column, but these are not hereditary. Some of these skin appendages also contain cartilage structures. Similar malformations are very rarely found in the head and neck region of humans with impaired development of the pharyngeal arches. These pathological structures also occur in tailed mammals, which is why it is unfounded to adhere to the above homology. A detailed critical evaluation of atavistic "tail formations" in humans can be found in Ullrich (2004).

`` Swimming skins '', `` paddles ''

The opinion that humans put on “webbed feet”, “fins” or “paddles” in the course of extremity development during embryogenesis is also a common misinterpretation of existing embryonic structures.

The development of the arms and legs in vertebrates proceeds in the proximo-distal direction. This means: First, the structures close to the body (upper arm, forearm, wrist) are visibly created and then those further away from the body (metacarpal, fingers, fingertips). The differentiation of individual cells, e.g. B. in cells of the musculature or the bone, is determined on the genetic level even before the extremities become visible for the first time and controlled by concentration gradients of gene-activating substances and time gradients (progression model, Summerbell 1973). According to the so-called “early specification model” (Dudley 2002, Richardson 2004), the extremity bud of a vertebrate embryo contains strips of cells, each with specific genetic activity patterns, arranged like bands. These Homeobox genes of paralogue groups 9-13, expressed (= read, used) according to the rule of colinearity, form the basis in the cell strips for their subsequent sequential differentiation and structuring into the bony and connective tissue structures of the upper arm, forearm and hand. These results impressively confirm that the early embryonic structures already represent a specific image of their end structures and are not primitive, recapitulated feature structures. (For the Homeobox genes see section “Homeobox genes: key genes of evolution?” In the article Biogenetic Basic Law - Current.)

In the development of the human hand, in the 6th embryonic week, the connective tissue systems of the wrist and the cartilaginous rays of the metacarpal become visible, which - and this is quite normal - are connected by tissue bridges (no webbed fins). This is the prerequisite for the following normal differentiation and the exact growth of the fingers. The finger appendages then lengthen through preferential growth at their free ends and through the suppression of cell proliferation (physiological cell degeneration in the area between the fingers). Similar to the regulation of growth in the proximo-distal direction, the differentiation in the anterio-posterior plane of the limbs also takes place in parallel. That means: The place where the thumb (anterior) and the little finger (posterior) develop is determined, among other things, by the concentration gradient of certain protein substances, which among other things influence the expression of the Homeobox paralogue groups 10-13. Another important factor in normal hand development is growth grasping (Fig. 301).

Adhesions between fingers (syndactyly) or fingers that are too much applied (polydactyly) can be traced back to pathological disorders of these processes, which are precisely coordinated in terms of time and location.

Ear muscles

Many mammals, including some primates, have the ability to move their auricles to optimize sound pickup. The presence of functionless ear muscles in humans seems to be evidence of a rudimentary structure that refers to postulated ancestral ancestors. In contrast, Blechschmidt was able to trace the existence of these muscles back to an important embryonic development principle. All muscles are created in so-called expansion fields (see appendix in the article Biogenetic Basic Law - Current). The growth of the auricle in a 15 cm large fetus is embedded in such a stretching field, since in the course of the development of the skull the occiput pushes backwards and upwards relative to the auricle. The formation of the ear muscles is understandable in terms of growth constructive, although today it is obviously no longer of any importance in postnatal care. It cannot therefore be ruled out that in extinct human forms the mobility and functionality of the auricles were better developed and functionally significant.

`` Fur ''

Some biologists see that Lanugo hair of the human fetus, which occurs from the end of the third month to the 8th month of development and is then rejected, a recapitulation of the fur of ancestral ancestors. This interpretation is incorrect from a developmental and comparative anatomical point of view. In the course of human life, different types of hair are created, first the lanugo hair, which is replaced by the replacement hair after birth, and finally the terminal hairs appear after puberty.

During the early fetal period (9th-12th week), the hair systems develop in the superficial ectodermal layer as epithelial cones that sink into the underlying mesenchymal tissue. The ectodermal layer facing the amniotic fluid grows relatively slowly compared to the lower one, which is adjacent to the nourishing stroma. The uneven growth of the two skin layers has the consequence that the hair roots that emerged from the epithelial cones in the deeper layers are placed at an angle, analogous to the course of the mesenchymal connective tissue fibers. This corresponds to the resulting pattern of the very dense first hair, lanugo (lat. lana = fine wool). In addition, lanugo hair is of functional importance for the fetus. According to Blechschmidt, the fetus partially absorbs hair shed in the amniotic fluid through the mouth. The dietary fiber (keratin) in them serves to stimulate the peristalsis of the child's intestine. The lanugo hair also ensures that the cheese smear that appears later (Vernix caseosa) is anchored on the fetal skin. It consists of detached skin cells, secretions and lanugo and is a biologically high-quality substance that protects the delicate skin, among other things, from aggressive substances in the amniotic fluid.

That the lanugo hairs of the human fetus cannot be a recapitulation of the fur of suspected ancestors also follows from the fact that z. B. a Lanugo is also formed in the monkeys, which is later replaced by the fur (terminal hair) (see O Rahilly & Müller 1999).

Unusually thick hair (Hypertrichiosis) during childbirth, which affects the whole body or only individual areas, is the result of a genetic and hormonal pathological proliferation of hair follicles. The complete lack of hair (atrichia) is also genetic. Both forms can be traced back to pathological processes and not the representation of recapitulated feature patterns (cf. Leroi 2004)

`` Yolk sac ''

The term “yolk sac” usually causes incorrect associations with structures of the same name, which are characteristic of fish, amphibians, reptiles or birds. The eggs of these various vertebrate classes have yolks as a necessary reservoir for feeding their germs during ontogenesis. With regard to the size of the eggs, the yolk content and the yolk distribution, there are considerable differences in the groups mentioned (cf. Fig. 271). This also essentially determines their clearly deviating course of early embryonic development. In contrast, the mammalian egg has no yolk or a yolk sac surrounding it. The partly so-called secondary yolk sac in humans emerges from the blastocyst cavity (which is sometimes referred to as the primary yolk sac) and is lined by cells of the lower germ layer (endoderm). This remainder of the blastocyst cavity is integrated into the umbilical cord during embryonic development and is therefore technically correct as Umbilical vesicles (Vesicula umbilicalis) (O Rahilly 1999).

Functionally, the umbilical vesicle plays an important role in the structure of the placenta (uterus) and as the first place of blood formation in human ontogeny. The conceptual equation with the structures of other vertebrate groups cannot apply here as a genealogical justifiable homology, even if the morphogenetic, anatomical and functional peculiarities are taken into account.

Reflexes

In order to test the functional maturity of the nervous and motor systems of a newborn, one tries to trigger characteristic reflex movements (with the Brazelton test, among other things, 20 reflexes are specifically recorded). Reflexes arise involuntarily, i.e. contractions of muscle groups occur independently of consciousness when the nerves supplying them are activated by a certain external or internal stimulus. For example, the stretching movement of the slightly bent lower leg is known when the patellar tendon is hit with a reflex hammer (patellar tendon reflex). Some of the temporary reflexes in the newborn ( Fig. 303) are usually referred to as primitive reflexes. Behind this is the assumption that they would recapitulate old movement patterns of ancestral ancestors.

Reflex patterns depend on the functional and morphological maturation of the nerves, the spinal cord and the brain. This correlates, among other things, with the gradual myelination of nerve fibers (specific sheathing of the nerves with myelin sheaths) in the peripheral and central parts of the nervous system. At the end of the third month of development, the first portions of myelin can be detected in the spinal cord. Finally, myelination takes place in the brain and is only completely completed in the young adult. In the newborn, myelin sheaths are found only in the brain stem, in the white matter of the cerebellum and in some central parts of the conduction pathways of the brain (rear leg of the internal capsule). The reflexes that can be provoked in the newborn are an expression of this developmental maturation of the brain and the sensorimotor system and at the same time are functionally meaningful. For example, the searching, crying and clinging or grasping reflexes are a continuation of fetal movement patterns, while the sucking and swallowing reflexes protect the newborn. Their existence is the basis of later movement and functional patterns. From the middle of the 6th month of development, these reflexes are replaced by so-called early childhood reflex patterns, which mark the beginning of conscious and / or reactive movements on the influence of perceived spatial stimuli (balance sensations) (e.g. righting reflex, landing reaction).

To clarify the above-mentioned relationships, a few detailed comments on the palmar grasping reflex ("Bracket reflex") appended. The child grasps the examiner's finger when it touches the inside of the hand. According to some authors, this reflex is intended to recapitulate the typical grasping movement through tree branches of monkeys or the instinctive clinging of the young to the mother's fur. From the developmental physiological context, there are justified doubts about the explanatory value of this representation. In the course of early differentiation, the arm systems carry out a developmental movement which, according to Blechschmidt, is called "growth gripping" (Fig. 302). At the unconscious embryonic level in the course of arm development, a preparation for the later consciously controlled grasping occurs. From the 5th week onwards, the arms of the embryo grow to the side and then to the front. Then there is a slight curvature in the elbow and thus a slight approach of the hand to the front chest wall. The hand is then brought to the mouth. A maturation-related and growth-stimulating grasping movement can then be observed there in 20 mm embryos (7th week of development). This can still be triggered in the newborn as a hand grip reflex and is lost in the postnatal development as an expression of the further maturation of the brain and the nervous and motor systems. Similar correlations also explain the foot reflex (plantar grasping reflex), which can be demonstrated up to the 10th month after birth.

The prenatal form development, as shown in the example of arm development, happens in the sense of the development of future performance. "The creation of an organ is already the beginning of its function" (Blechschmidt). This functional explanatory approach as well as the facts of the ontogenetic maturation processes make the retrospective interpretation of reflexes in the sense of phylogenesis superfluous.

Conclusions

Attempts to better analyze the human ontogeny or individual embryonic and fetal organ structures or postnatal reactions with recourse to the theory of descent do not show any success. The human embryological findings document the unsuitability of the principles of the Biogenetic Basic Law and its modifications. Regardless of this, the textbooks repeatedly contain references to apparent phylogenetic relics in human development (gill arches, tail, fur). However, these and other so-called recapitulations can be explained without recourse to hypothetical phylogenesis with regard to their appearance and function.

Man does not become man, but is Human from fertilization. He does not develop into a human being but as a human being. Being human is not a phenomenon that results from ontogenesis, but a reality that is the prerequisite for its ontogenesis. Erich Blechschmidt has this fact in Law of the Preservation of Individuality contrasted with the phylogenetic explanatory approach. It says that with fertilization the individual specificity of an organism and its metabolism is given and is maintained until death. What changes during development is the appearance.

Summary of all articles on the Biogenetic Basic Law

1. A natural law link between the observable, constantly repeating ontogenesis and the hypothetical, unique phylogenesis has not yet been proven. In terms of evolutionary theory, gradual modifications of ontogenetic processes are favored today in order to explain numerous micro- and macroevolutive changes.

2. The same data from embryological research have been and are interpreted differently in the course of the history of science with recourse to the respectively favored development models of life (step-ladder thinking and typology, evolutionary theory, creation theory).

3. The Biogenetic Basic Law as amended by Ernst Haeckel (1866; 1872) turned out to be wrong with regard to its causal and descriptive statements. However, some of his basic ideas were still used in phylogenetics in a modified form, e.g. as a basic biogenetic rule (derivation of individual organs, family tree research). However, the associated methods have proven to be unsuitable and unreliable.

4. Comparative studies of vertebrate embryos have shown that during early ontogenesis there is no “phylogenetically conserved stage” with a higher degree of similarity to other developmental phases.

5. Current approaches and results in comparative and experimental embryology have put embryology back into a central position in evolutionary research under the heading "Evo-Devo". Despite now possible quantifiable Findings of differences and comparisons in the occurrence of events in development sequences do not give these methods any answers to the question about the causal Mechanisms of postulated phylogenetic change.

6. The causal penetration of the dimension of the phylogenetic change of ontogenes is much more difficult due to the discovery of the Homeobox genes and the Hox complexes and can no longer be represented with the previously proposed mechanisms of the synthetic theory of evolution alone. The presumed macroevolutionary change is opposed by numerous developmental constraints (constraints), which are even more important due to the recognized operating principles of the Homeobox genes.In contrast, microevolutive changes at the level of the basic type with relatively rapid diversification become more transparent.

7. The similarities of the germs of different organisms observable during the embryonic development can be interpreted on the assumption of macroevolution, but such an interpretation does not necessarily result from the findings of their individual ontogeneses. An alternative interpretation of this data, e.g. in the sense of the doctrine of creation, is possible without contradiction and with equal rights.

8. The example of human ontogeny shows that every embryonic system and every organ in form and function is necessary and must be understood without the prerequisite for evolution. Despite fundamentally detectable similarities to the development processes of other vertebrates, humans are undoubtedly human from the beginning with unmistakable individuality in every phase.

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Author: Henrik Ullrich, 06/14/2006

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