11) Ernst Haeckel and the Recapitulation Theory: A Scientific Milestone (by Rey J. Rosa Morales)

         Ernst Haeckel (1834-1919) was a German biologist-philosopher, who worked with different scientific areas. He introduced new names for his discoveries in different scientific fields such as the terms ecology, anthropogeny, phylum, the kingdom Protista, phylogeny, stem cell and also proposed the Recapitulation¹ Theory.

Ernst Haeckel (1834-1919) was a German biologist and philosopher.

Images of dog and human embryos, looking almost identical at 4 weeks (most left upper panel) then differing at 6 weeks (left upper panel at right continuously). Also above at right hand side, shown a 6 week turtle embryo and below a 8 day hen embryo, presented by Haeckel in 1868 as convincing proof of evolution. The pictures of the earliest embryonic stages are now considered inaccurate (Image from Richardson et al. 1998).

The Recapitulation Theory was proposed to create a synthesis on basis of Lamarckian and Darwinism theories. The Recapitulation Theory says:  “the ontogeny² recapitulates phylogeny³”,  (look the image above) which it means that the whole development process repeats or resemble the complete successive evolutionary stages, beginning with his common ancestor passing through several organism that he had already evolved until reach his own proper form. It’s said, when an organism is developing, he remakes and pass through every embryonary development stage that compound his whole evolutionary lineage-history. In other words, each of the individual states of a species crosses throughout embryonic development, represents one of the adult forms that appeared in their evolutionary history. For example, Haeckel proposed that the pharyngeal grooves between the pharyngeal arches in the neck of the human embryo resembled gill slits of fish, thus representing an adult "fishlike" developmental stage as well as signifying a fishlike ancestor. Ernst Haeckel made drawings of his observation studies and there were several controversies about Haeckel drawings where he shows the variety of embryology resembling between organisms. There was several misunderstanding about his work at that time.

          During the nineteenth century, there were several scientists agree with Haeckel’s ideas “that all vertebrates looked very similar at an early stage, in what was thought of as a common ideal type”. But others differ proposing different perspectives views like Karl Ernst von Baer. He thought “the early general forms diverged into four major groups of specialized forms without ever resembling the adult of another species, showing affinity to an archetype (is a universally understood symbol, term) but no relation to other types or any Transmutation of species (theory that described the altering of one species into another)”.

            Actually, the Recapitulation Theory does not have any applicability (see figure 1) and is until today rejected by his literacy in modern biology. For instance, if you observe a chick's development, you will see that the chick embryo may resemble the embryos of reptiles and fish at points in its development, but it doesn't recapitulate the forms of its adult ancestors (figure 2). Another example is the axolotl, which evolved from a salamander ancestor that had internal gills in the adult stage. However, the axolotl never develops through a stage with internal gills; its gills remain external (figure 3).

Figure 1. Representation that organisms don't stop in a intermediate state showing successive changes to acquire or resemble the anterior stage of a full adult form see in during the development (Richardson M. et al; 1998). 

Figure 2. The chicken don't recapitulates his adult ancestors such as it was formulated by Ernst Haeckel.

Figure 3.  Salamander possess internal gills as is showing during his development however the axolotl evolved from salamander but never present during the early stage internal gills. If evolution had slowed the rate of shape change of a salamander, but kept everything else the same, we would have ended up with the axolotl  [Salamander image (Pseudotriton ruber ruber) © 2002 John White; Axolotl (Ambystoma mexicanum) image © 2003 Jessica Miller.].

Glossary:

1. recapitulation: to restate briefly : summarize;  to repeat the principal stages or phases of something.
2. ontogeny: ontos means: “to be” and geny means “mode of production”; the history of structural change in a unity, which can be a cell, an organism, or a society of organisms, without the loss of the organization which allows that unity to exist.
3. phylogeny: is the story of the evolutionary development of a group of organisms.

References:

1. Embryo Project Encyclopedia. 2007. Retrieved August 2011. "In 1874 His published his Über die Bildung des Lachsembryos, an interpretation of vertebrate embryonic development. After this publication His arrived at another interpretation of the development of embryos: the concrescence theory, which claimed that at the beginning of development only the simple form of the head lies in the embryonic disk and that the axial portions of the body emerge only later."
2. Ernst Haeckel — Britannica Concise (biography) Encyclopædia Britannica Concise, 2006.
3. Evolution and Development for the 21st Century. Retrieve on Sunday, march 31 2013 from:   Stephen Jay Gould http://evolution.berkeley.edu/evosite/history/modevdev.shtml
4. Learning about evolutionary history.  Retrieve on Sunday, march 31 2013 from: http://evolution.berkeley.edu/evolibrary/article/0_0_0/evodevo_02
5. Michael K. Richardson. 1998. "Haeckel's embryos continued." Science 281:1289, quoted in NaturalScience.com webpage Re: Ontogeny and phylogeny.
6. Richardson MK, Hanken J, Selwood L, Wright GM, Richards RJ, Pieau C, Raynaud A (1998). "Letters". Science 280 (5366): 983, 985–6.