Recapitulation: Evidence For or Against Evolution?
Recapitulation: Evidence For or Against Evolution?
The idea that embryos of different organisms look similar is not foreign. Anyone in an introductory biology, anatomy, or embryology class knows that chicken embryos are almost identical to the embryos of humans. Embryological development in different organisms diverges from other organisms at different stages in correlation with the complexity of the organisms. For example, human embryos and rabbit embryos diverge at a later time in development than the embryos of humans and fish. This idea was first described by K. E. von Baer (1792-1876) in his biogenetic law that stated that earlier stages of embryonic development of higher organisms resemble those organisms lower on the scale of nature (Pittendrigh 352). It was Ernst Heinrich Haeckel (1834-1919) who coined the phrase “ontogeny recapitulates phylogeny,” adapting von Baer’s ideas of the scale of nature to that of evolution. “Ontogeny recapitulates phylogeny” was a commonly used phrase to explain the evidence supporting evolution. Ontogeny is “the complete developmental history of the individual organism,” and phylogeny is defined as “the complete evolutionary history of a group of organisms,” (Villee 697, 699). Thus, this phase means that the evolutionary history of a group of organisms is repeated during the developmental history of an individual organism of that group. This idea is also known as recapitulation. Recapitulation is a disputed topic used by pro-evolutionists as evidence for evolution and as against by the opposition. The story behind evolution and recapitulation is not as black and white as these suggest. I believe there is a middle ground in which recapitulation neither proves nor refutes evolution.
Haeckel developed his theory of recapitulation in which “ontogeny recapitulates phylogeny” in 1866, only seven years after the publication of On the Origin of Species. At this time, people were jumping at the chance to provide any more supporting evidence for evolution. Haeckel’s, as well as von Baer’s, observations were correct. Embryos of different species closely resemble each other in earlier development. In light of the newly found process of evolution, it was determined that mammals descended from reptiles which descended from fish. It was hard to ignore the observed similarities in the embryos of these groups. All three developed gill slits, reptiles and mammals developed arms and legs, and then during later development, they went their separate ways. It is a natural assumption to explain this by evolution and therefore, use this as evidence for evolution. However, Haeckel took this idea a little bit farther claiming that each embryological stage corresponded to an adult organism stage in the evolutionary line. He proposed, for instance, that there was once an adult animal, a blastea, that was hollow one cell layered sphere corresponding to that of the blastula stage in embryonic development (Weisz 557). If one takes Haeckel’s “law of recapitulation” to mean that there is a strict linear transformation from one organism into another, there is no doubt that this transformation occurred via evolution. Consequently, this “law” was added to fossils and homology as an example of evolution at work and as a result, evidence for evolution. However, almost immediately after the famous words were uttered, “ontogeny recapitulates phylogeny” in the strictest sense was proven false.
“It is now firmly established that ontogeny does not repeat phylogeny. Ontogeny repeats ontogeny, with variations,” (Pittendrigh 352-53). It is now widely accepted that Haeckel’s statement was false. No where in embryological development is an embryo as similar to an adult as it is to other embryos. Those people in opposition to evolution use this evidence as proof that evolution does not exist. They reason that “Haeckel's hypothesis was presented as supporting evidence for evolution, Haeckel's theory is wrong, therefore evolution has less support;” (Recapitulation). It is true that Haeckel presented his theory as supporting evidence for evolution and it is true that Haeckel’s theory, in the strictest sense, is wrong. As a result, one piece of the evidence was logically removed from the basket supporting evolution. Therefore, evolution has less support than it did when Haeckel’s theory was presumed correct. This is not an incorrect conclusion. It comes following a logical process of deduction stemmed from the discredit of the “law of recapitulation.” It could also be argued that if evolution does not explain ontogeny repeating ontogeny in other organisms, then some other phenomenon must be at work; maybe God intended for all organisms to be similar during embryological development or maybe it is just one big coincidence.
As a believer in evolution and a now non-believer in the “law of recapitulation,” I think the concept of recapitulation does not prove nor disprove anything. Haeckel’s story fit his biased observations. If embryos really did resemble their adult evolutionary predecessors, Haeckel would have been entirely correct to say that “ontogeny recapitulates phylogeny.” However, even if this were true, to say it proved evolution would be a stretch. As creatures of one dimensional forward time, humans cannot observe happenings in the past. We cannot watch, as we could in a lab at an embryo, as primitive adult organisms gain characteristics and instantaneously jump to the next level of the phylogeny. One piece of evidence cannot prove a scientific theory or fact. It is with the culmination of large amounts of undeniable evidence that evolution can be “proven.” Even the fossil record alone could not prove that evolution has taken place; many other supporting observations lead Darwin to his story. This idea also works in the opposite direction. Haeckel’s “law of recapitulation” originated after the publication of on the Origin of Species, after the evolution was already proposed. Therefore, the theory of evolution existed before the theory of recapitulation and should consequently, exist afterward. The removal of one piece of evidence from a set should not make the other evidence obsolete. For example, take evidence at a crime scene. Police suspect that someone was murdered because there is toppled furniture and a body. A drop of blood is then found on the counter at the crime scene. This reinforces the assumption that there was a murder. It is later found that the spot was a drop of ketchup, not blood. Does this mean that there was no murder? No, it does not.
“Evolution is a historical process that cannot be proven by the same arguments and methods by which purely physical or functional phenomena can be documented. Evolution as a whole, and the explanation of particular evolutionary events, must be inferred from observations,” (Mayr 13). Mayr understands that one observation such as recapitulation does not prove evolution. There are some aspects of recapitulation that make sense in the light of evolution. Among these aspects are vestigial structures and embryological homology, both of which are used as arguments for evolution. However, complete recapitulation does not occur and therefore, must be taken away from the existing evolutionary evidence. This does not mean that all other pieces of evolutionary evidence should be discarded. In fact, the opposite is true. There should be a heightened focus on these observations so that they can lead to new observations. One cannot prove evolution with one observation nor can one refute evolution based on a wrong observation without first refuting the summation of evidence that already exists. Recapitulation proves nothing nor does it disprove anything.
Mayr, Ernst. What Evolution Is. New York: Basic Books, 2001.
Pittendrigh, Colin S. et.al. Life: An Introduction to Biology. New York: Harcourt, Brace and Company, Inc., 1957.
"Recapitulation theory." Wikipedia, The Free Encyclopedia. 9 Feb 2007, 16:42 UTC. Wikimedia Foundation, Inc. 16 Feb 2007 <http://en.wikipedia.org/w/index.php?title=Recapitulation_theory&oldid=106864370>.
Villee, Claude A.. Biology. Philadelphia and London: W.B. Saunders Company, 1967.
Weisz, Paul B.. The Science of Biology. New York: McGraw-Hill, Inc., 1971.