Evolution: Biology

Basic Ideas: Biogenetic Law

Content

This article explains what is researched in embryology and why this field of research has particular significance in the questions of origin.  A short historical outline will consider Ernst Haeckel’s Biogenetic Law and discuss critically its value in the research of phylogeny.

Branches of embryology

Stages of ontogenesis

Ernst Haeckel – an influential yet controversial researcher

Causal statements of the biogenetic law according to Haeckel

Descriptive statements of the biogenetic law

Criticism of the causal statements of the biogenetic law

Criticism of the descriptive statements

The present-day value of the biogenetic law

Literature

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Branches of embryology

The process of development from the fertilised ovum to the adult organism (or until death) is known as ontogenesis. The branch of science covering this is embryology.  In contrast to phylogeny, this process can be continually observed. Fig. 263 illustrates the differences between ontogenesis and embryology.

For practical reasons, a distinction is made between descriptive and causal or experimental embryology. Descriptive embryology describes the developmental processes and compares the developmental processes of differing species. Causal embryology tries to explain the causal framework of the processes taking place here.

Embryology also plays a significant role in the question of the history of the organism (evolution), e.g. Can the process from the fertilised ovum to the adult organism be seen as an image of assumed phylogeny? Are temporarily arising structures (e.g. the so-called “gill slit” in humans) vestiges of features of evolutionary forbears (here: fish gills)? (See also: Biogenetic Law- Examples). Do genetic development and control genes  (homeobox genes, see also Homeobox genes and evolution) and embryonic development mechanisms also represent a weak point for the evolutionary mechanisms of species change?

Stages of ontogenesis

The ontogenesis of a human in the uterus is divided into three stages. The first two weeks after fertilisation are known as blastogenesis. During this period the human embryo is transported through the fallopian tube and is implanted in the uterus. After many cell divisions the spheroidal blastocyst  is formed (Fig. 264).

During the second stage, the embryonic period (3^rd to 8^th week of pregnancy; Fig 265), all the organs are formed and the “body shape” develops. In addition to this the placenta is formed, and this provides for all vital functions during development (nutrition, breathing, development of hormones etc.)

The foetal stage (9^th to 40^th week of pregnancy) is characterised by growth and functional maturing of the individual organs and body parts of the foetus, and is completed at birth.

Many different interactions that are exactly related to each other with regard to time and location on a genetic, cellular, tissue and organic level are decisive for the normal ontogenesis process. Despite a great deal of detailed knowledge about embryology, essential correlations leading to the development of an organism with its typical form and functions are still not understood. As with the phenomenon of “life”, it could be that scientific data alone are insufficient to explain the essence and basics of  form development.

Ernst Haeckel – an influential yet controversial researcher

Ernst Haeckel (1834-1919), the zoologist from Jena,  was one of the most successful but also most controversial researchers of the 19^th and 20^th century. His outstanding commitment to Darwin’s theory of evolution created the basis for its unprecedented success in Germany. He became famous predominantly through his attempt to prove the “fact” of  phylogeny by using findings from ontogenesis. In his day there was little detailed knowledge of ontogenesis, and such knowledge was limited to only a  few species. He repeatedly got round this through the presentation of highly speculative sketches, his own arbitrary models and exaggerated generalizations (compare Fig. 267 with modern representations Fig. 268). He repeatedly disregarded the embryological facts delivered by his contemporaries in order to present his view of things in a way that caused less opposition.

Some of his colleagues have rated Haeckel’s oversimplified portrayals as counterfeits or deception (e.g. Rütimeyer 1868, His 1874, Bischoff 1876, Richardson 1997, Gould 2002), although Haeckel himself constantly pointed to the hypothetical character of some of his drawings. The accusation of counterfeit was never directed essentially at methodical errors but at errors of content. For with the criticised diagrams Haeckel was not illustrating the consensus of knowledge of his time, but was distorting known facts and declaring unknown facts as known, with the aim of proving the biogenetic law. His influence on textbooks, morphology and embryology up to the present day probably arises from the fact that his theories and his person were linked with the theory of evolution and in this way received a particular form of authorization.

Causal statements of the biogenetic law according to Haeckel

Haeckel regarded the findings of individual development as “the most significant torchbearer for the understanding of our evolution”, and he had no doubt about the reality of evolution. Through the biogenetic law he construed “natural law” links between ontogenesis and phylogeny. From the main principles of Haeckel’s understanding  of the interconnection between ontogenesis and phylogeny, we will cite first the causal statements (causes for the process of ontogenesis).

1. Phylogeny is the mechanical cause of ontogenesis. Haeckel held the process of ontogenesis of an organism to be the direct result of its evolution. He considered the same mechanisms (heredity and adaptation) responsible for both processes.

2. The occurrence of recapitulation in ontogenesis is caused by the mechanisms of phylogeny. In the ontogenesis of a more highly developed organism, temporary morphological structures are said to arise which correspond to adult characteristics of phylogenetic forbears. Haeckel called these repetitions recapitulation. Through the addition of new features, older phylogenetic characteristics would be increasingly suppressed into an earlier stage of embryonic development. In the ontogenesis of a higher vertebrate, these characteristics of primitive forbears are thus at the beginning of the individual development and those of immediate forbears at the end (compare Fig 269).

3. The causal mechanism: the inheritance of acquired traits. In a similar way to Lamarck and Darwin, Haeckel assumed that acquired traits are passed on directly to descendants.

Descriptive statements of the biogenetic law

The most important descriptive statements can be summarised as follows:

1. The ontogenesis of an individual represents a shortened form of the most important changes in form of phylogenetic forbears (adult forms) of the same species. According to this, single embryonic stages represent the respective adult forms of corresponding forbears.

2. There is no complete identical representation of  phylogeny through ontogenesis. Haeckel set limits to his law right from the beginning. A differentiation should be made between palingenesis, where the earlier phylogenetic stages are repeated during the individual development (recapitulation), and kenogenesis, the development of new forms that are necessary for ontogenesis but not utilisable for phylogeny. For example Haeckel attributes the so-called branchial arches and vestigial tail of humans to palingenesis. That is: these structures in the human embryo are supposed to indicate evolutionary forbears (fish or later on mammals with tails; but compare – Biogenetic Law - examples).
In contrast to this, the development of the yolk sac, the allantois, the placenta, the embryonic membranes (e.g. amnion) and the umbilical cord are rated as kenogenetic structures. These say nothing about the appearance of forbears. Haeckel also took spatial (heterotopic) and temporal (heterochronic) shifts into consideration in the ontogenetic development of organs when forming comparisons on the basis of phylogeny.

3. Ontogenesis represents first general and later specific characteristics of a species. Following Baer’s typological ideas (1828) in the law of embryonic similarity, Haeckel formulated: “There is thus no difference between a human in the early stages of development and the embryos of birds and reptiles. If even earlier stages of development are considered, one would find no differences between the embryos of these higher vertebrates and those of lower amphibians and fishes” (Anthropogeny  1877, p. 295).

Conclusions. Following the paradigm of “evolution”, Haeckel tried by means of the biogenetic law to present “the close, causal connection” (Welträthsel 1903, p. 36 ) between phylogeny and ontogenesis, and at the same time to set down methodical guidelines for the reconstruction of the genealogical tree. Earlier views from natural philosophy in a phylogenetic interpretation (step ladder, typology) are to be found in the fundamental ideas behind the biogenetic law ( see also the text for experts on “Biogenetic Law”). Because of circularity, incorrect assumptions, exaggerated conclusions (see below) and calculated forms of representation Haeckel faced scientific and personal criticism all his life. Through the biogenetic law, however, he gave an enormous impetus to research on comparative embryology.

Criticism of the causal statements of the biogenetic law

From the moment of its formulation, there was fierce controversy over the biogenetic law’s value for biology in its content and method. The spectrum of reactions ranged from unqualified acceptance to complete rejection. Particularly problematical was Haeckel’s methodical starting point of declaring the theory of evolution to be a proven fact. This inevitably led to circular arguments in the interpretation of ontogenetic data.

The following individual points of criticism can be made:

1. Ontogenesis can be understood independent of phylogeny. If Haeckel’s causal approach were correct, an understanding of ontogenetic processes would only be possible through the research of phylogeny. His (1868), who tried to explain  the processes of organ development by taking mechanical laws into consideration, already presented arguments against this. Blechschmidt (1961; 1973) attributed the typical formations during the course of human ontogenesis to differently acting metabolic environments (see appendix to experts’ text – Biogenetic Law – History). During the further course of research, the integration of molecular-genetic and experimental findings in the description of developmental-biological processes over the past decades impressively documents that ontogenesis can be understood independent of phylogenetic concepts. Whilst Haeckel considered ontogenesis explicable only from the point of view of phylogeny, evolutionary researchers today favour the opposite methodical path: from the basis of ontogenesis  they are searching for the causes of phylogenetic change.

2. Ontogenetic and phylogenetic developmental processes are not subject to the same mechanisms. The present knowledge about the genetic basis of hereditary processes is contradictory to the assumption of the inheritance of acquired traits as propagated by the biogenetic law. Experimental embryology has discovered a multitude of connections in the spatiotemporal regulation of  developmental processes. In the epigenetic area (cell nucleus, plasma, cell membrane) there is a complicated interplay between genotype and phenotype. Research on embryonic formation processes is presently experiencing an enormous upswing through increasing knowledge about development regulation genes (e.g. homeobox genes). The interconnections of spatiotemporal regulatory cascades that are recognised here do not however offer any keys to an understanding of the development of new evolutionary forms (see article – Homeobox genes and evolution).

Criticism of the descriptive statements

The descriptive statements of Haeckel’s ideas on the connection between ontogenesis and phylogeny must also be criticised:

It is not possible to place the course of an organism’s ontogenesis on the same level as its phylogeny. Haeckel’s assertion that all the form changes of an organism during its ontogenesis correspond to those of its own phylogeny could not be confirmed by comparative ontogenetic studies. It has been shown that in the ontogenesis of many organs, shifts in their temporal and spatial appearance (heterochrony, heterotopy) are not rare exceptions to their assumed phylogenetic appearance, but are the rule. The comparison of the ontogenesis of organisms that are phylogenetically closely related (e.g. amniota) also shows distinctive shifts, for example in the appearance of structures for heart or eyes, which would lead to absurd results if they were judged as recapitulation. Because of this situation, attempts are now mainly made to prove recapitulation only on an organ level.

In ontogenesis there is no appearance of features from adult phylogenetic forbears. It is not possible to place general homologous adult features of recent species or hypothetical phylogenetic forbears on a level with embryonic or foetal development.  During the embryonic development of the foregut in reptiles and mammals, for example, which is phylogenetically said to be derived from the gill gut of fish and amphibians, at no time can the functional gills of supposed forbears with gill slits or gill lamellas be found.

There is an absence of objective guidelines for distinguishing between palingenesis  and kenogenesis. It is not possible to label ontogenetic structures in themselves as palingenetic or kenogenetic (see above). For the “decision” on whether a certain developmental stage is palingenetic or kenogenetic can only be made precisely if the phylogeny is known” (Siewing 1987, p. 271). However, this prerequisite does not exist (compare Biogenetic Law – Examples).

The path of ontogenesis does not lead from “general to specific”. When comparing the earlier, “more primitive” ontogenetic stages of different organisms, there should be proof of an increasing similarity of germ cells. An individual’s ontogenesis would then proceed through various stages where first simple or general forms of organs and structures would be seen and later more specifically constructed forms, as in phylogeny.

This theory, too, has been refuted. When comparing increasingly earlier ontogenetic developmental stages of vertebrates, an increasing similarity of germ cells simply does not exist. The cleavage types of the fertilized ovum, the blastula or development of the blastocyst, gastrulation and development of the mesoderm as well as neurulation show distinct differences in amphibians, reptiles, birds and mammals (Fig. 270-272).

Even with this knowledge, the idea was still maintained of defining stages that are particularly conservative and thus have a high phylogenetic significance (compare “sand-glass model”, fig. 273). Extensive comparative studies have led to the result, however, that the degree of similarities in the individual developmental stages of different species of vertebrates is not greater in the assumed “conservative” stages than it is in any other stage, and is thus in contradiction to the “sand-glass model”. There is no stage in the early embryonic development of vertebrates that can be called phylogenetic conservation (Richardson et al. 1997).

The present-day value of the biogenetic law

Ernst Haeckel proclaimed his biogenetic law as a scientifically founded representation of the connection between phylogeny and ontogenesis. In doing this he used a direct methodological approach to explain this connection, which he considered to be close. He regarded phylogeny as directly causal for ontogenesis. However, embryology was able to confirm neither the causal nor the descriptive statements connected to it. This means: ontogenesis allows no direct inferences to be made from phylogeny and phylogeny does not explain the processes of ontogenesis. Haeckel’s law has nonetheless continued to exist, albeit in a very limited and modified form (e.g. as “biogenetic basic rule”) and relates to individual organs.

The method of direct application of stages of ontogenesis of an organism or stages of individual organs to their phylogenetic development is now only occasionally practised (e.g. the derivation of chordates from tunicate larvae; fig. 274).

On the other hand, the indirect way of researching phylogeny by means of ontogenesis is seen as an important and by many also as the only possible way. Here ontogenetically arising features are compared with embryonic and adult structures of other species in order to identify homologies (see article “Similarities in morphology and anatomy”). Homologies determined in this way can be interpreted as evolutionary or possibly as recapitulation and in this way understood as indications of phylogenetic correlations. Examples of recapitulation determined in this way are the embryonic dental germs in baleen whales, the primordial primary mandibular joint in mammals, the chorda dorsalis in mammals, the development of claws in the hoatzin nestling or the alteration of the suture lines in ammonites during the course of their ontogenesis. The developments that are interpreted as recapitulation are frequently viewed as “detour developments” as they run a seemingly “unnecessarily complicated” course. The embryonic primordial pharyngeal or branchial arches (“gill arches”), the chorda dorsalis (flexible rod of cartilaginous tissue) or the ductus botalli (blood vessel between lungs and aorta) in mammals seem accordingly uneconomical and historical. It is said that only certain residual functions or a take-over of other, secondary functions within the structure of the organism have caused them to be retained  during the course of evolution. However, these explanations are losing increasingly more ground. Thus the chorda in mammals, for example, serves as a place-holder and inductor for the later development of the spinal column and neural tube. The dental germs in toothless mammals are essential for the proper development of the jaw. The usefulness of organs that appear only temporarily during ontogenesis is obvious. However, to assess their functional significance correctly, organs must never be considered in isolation.

Literature

Bischoff TWL (1868) in: Sitzungsbericht der mathematisch-physicalischen Classe der königlich bayrischen Akademie der Wissenschaften. 6. Band, S.1-2

Blechschmidt E (1961) Die vorgeburtlichen Entwicklungsstadien des Menschen. Freiburg.

Blechschmidt E (1973) Die pränatalen Organsysteme des Menschen. Stuttgart.

Gould SJ (2002) I have landed. New York

His W (1874) Unsere Körperform und das physiologische Problem ihrer Entstehung. Lepizig.

Richardson MK, Hanken J, Gooneratne ML, Pieau C, Raynaud A, Selwood L & Wright GM (1997) There is no highly conserved embryonic stage in the vertebrates. Anat. Embryol. 196, 91-106.

Rütimeyer L (1868) Rezension zu: E. Haeckel, Über die Entstehung und den Stammbaum des Menschengeschlechts. Berlin 1968 und E. Haeckel, Natürliche Schöpfungsgeschichte. Berlin 1868. Arch. Anthropol. 3, 301-303.

Siewing R (Hg, ³1987) Evolution. Stuttgart.

Translator: Reinhard Junker, 16.06.2010

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Author: Henrik Ullrich

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