Haeckel's Law of Recapitulation
Haeckel's Law of Recapitulation
German zoologist Ernst Haeckel famously—and inaccurately—uttered, "Ontogeny recapitulates phylogeny ." While investigating the developing embryos of a variety of vertebrates , Haeckel thought they all closely resembled one another. This observation led to his conclusion that embryonic development echoed morphological evolution. Specifically, gill depressions in human embryos led Haeckel to conclude that humans were derived from fishes. He therefore felt that the study of embryonic development, or ontogeny , retold the story of evolution, or phylogeny. As he wrote in 1866, "During its rapid evolution, an individual repeats the most important changes in form evolved by its ancestors during their long and slow paleontological development." (Haeckel)
There are a number of flaws with Haeckel's theory. For example, Haeckel confused a fish embryo with a young human one. Haeckel's drawings strongly suggest that a variety of vertebrates share a common developmental phase, but he does not account for the entire process of development, nor does he compensate for size differences. His drawings were grossly oversimplified and ignored or obfuscated many salient differences. This did not stop Haeckel's law from being widely accepted for the majority of the twentieth century. Many otherwise up-to-date textbooks, such as Molecular Biology of the Cell, written in 1994 by Nobel laureate James Watson and National Academy of Sciences President Bruce Alberts, continue to cite Haeckel.
In 1997 a group headed by Michael Richardson of St. George's Hospital Medical School in London published a serious investigation of Haeckel's claims. Photographs of a variety of vertebrate embryos showed conclusively that, despite some similarities, there is no stage in vertebrate development when all embryos are identical. That said, there are definitely some common features among developing vertebrates. In the nineteenth century, embryologist K. E. von Baer wrote, "The embryo of the mammal, bird, lizard, and snake and probably also the turtle, are in their early stages so uncommonly similar to one another that one can distinguish them only according to their size" (Richards 1992). Common structures do not imply that vertebrate development retells the story of evolution.
Haeckel was a strong supporter of evolution, particularly after reading Charles Darwin's The Origin of Species. However, Darwin argued that natural selection was the mechanism that advanced evolution. Haeckel's view was that developing embryos strove to meet the needs of their environment by adding more and more complex structures. Examining Haeckel's law, one could conclude that it is possible to reach backwards through development and find the alleged evolutionary forerunner of all vertebrates. In normal development, a ball of cells known as a gastrula develops soon after fertilization and eventually becomes the gut. As each embryonic stage was supposed to represent another species, Haeckel postulated the existence of a "gastraea"—an organism that resembled the gastrula and was, by extension, the ancestor of all the vertebrates.
Despite mainstream acceptance of Haeckel's ideas, gastraea do not exist. Nor does evolution advance by adding traits to developing embryos. While there are definite similarities among developing vertebrates, Haeckel's famous utterance can be safely dismissed. Despite the acceptance he found elsewhere, scientists in Haeckel's native Germany considered his findings suspect. He was accused of academic fraud and pled guilty, claiming that many of his drawings were reproduced from memory. When comparing photographs of actual embryos to the drawings, however, one could conclude that Haeckel remembered only one embryo and claimed that all vertebrates looked just like it. As Michael Richardson said, "These are fakes. In the paper, we call them 'misleading and inaccurate,' but that is just polite scientific language" (Times London, August 11, 1997).
see also Ontogeny; Phylogenetics Systematics.
Gould, S. J. Ontogeny and Phylogeny. Cambridge, MA: Harvard University Press, Belknap Press, 1977.
Haeckel, Ernst. The Evolution of Man: A Popular Exposition of the Principal Points of Human Ontogeny and Phylogeny (1866; New York, 1896).
Richards, R. J. The Meaning of Evolution. Chicago: University of Chicago Press, 1992.
Richardson, M. K., J. Hanken, L. Selwood, G. M. Wright, R. J. Richards, C. Pieau, and A. Raynaud. "Haeckel, Embryos, and Evolution." Science 280 (1997):983-984.
Times (London), August 11, 1997.