Biology: Ontogeny and Phylogeny
Biology: Ontogeny and Phylogeny
A famous but scientifically inaccurate phrase, “ontogeny recapitulates phylogeny,” was coined by German biologist Ernst Haeckel (1834–1919) in the 1860s. Ontogeny, from the Greek for “origin of being,” is the growth of any multicellular (many-celled) living thing from fertilized egg to adult form. To recapitulate is to summarize or review. Phylogeny, from the Greek for “origin of a tribe or kind,” is the evolutionary history of a group, often pictured as a family tree. Therefore, the saying “ontogeny recapitulates phylogeny,” also called the biogenetic law, can be restated as “the growth of an individual replays the evolutionary history of its species.”
The biogenetic law asserts, for example, that if all vertebrates (animals with spinal columns) evolved from fish, then early in their development all vertebrate embryos should resemble fish. If the next stage in the evolution of mammals was amphibian, all mammal embryos should next resemble amphibians. This replay of ancestral forms would continue until the adult form of each animal is reached. The process described by the biogenetic law is referred to as recapitulation.
For over a century, biologists have known that the biogenetic law is at best a partial truth. There are many connections between ontogeny and phylogeny, but the biogenetic law is too simple. Ontogeny does not, in general, recapitulate phylogeny.
Historical Background and Scientific Foundations
The biogenetic law was first described by Haeckel in 1866, but its roots go back over two thousand years. Observers have always noticed that human beings do not begin in the womb as tiny adults, but as wormlike creatures that only gradually come to resemble a person. The Greek philosopher and naturalist Aristotle (384–322 BC) taught that the human embryo receives a series of three successively higher “souls” during this process of growth and change—first plant, then animal, finally human.
Aristotle's ideas were widely accepted by Western thinkers until the late 1700s and early 1800s, when a group of German naturalists, inspired by their mystical philosophy of nature (Naturphilosophie), updated Aristotle by linking the idea of staged ontogeny to the idea of recapitulation. In 1821, decades before Haeckel announced the biogenetic law, Johann Friedrich Meckel (1781–1833) said that the “development of the individual organism obeys the same laws as the development of the whole animal series.” Thus, just as various evolutionary theories existed for decades before Darwin and Wallace discovered a workable explanation for evolution (namely natural selection), theories of recapitulation were current long before Haeckel gave them a definitive form.
Haeckel believed that recapitulation was caused by a phenomenon called terminal addition. In terminal addition, a new final (terminal) stage of individual development is added at each new evolutionary stage, squeezing all previous evolutionary stages back earlier into individual development. Thanks to the terminal addition of evolutionary stages, a rapid, highly telescoped history of the species is, Haeckel taught, compressed into embryonic growth: ontogeny recapitulates phylogeny. He also taught, however, that ancestral stages might be dropped or modified from recapitulation, like letters being erased from an alphabet or switched with characters from another alphabet. Ontogeny does not necessarily recapitulate phylogeny perfectly, according to Haeckel.
Controversy arose as soon as Haeckel announced his doctrine. Some biologists took up the idea with enthusiasm, pointing to examples of recapitulation throughout embryology; others argued that resemblances between early-stage embryos reflect not ancestral adult forms but the need of all organisms to achieve their diverse forms starting from a single cell, the fertilized egg. These critics pointed out that some of the illustrations Haeckel used to support the biogenetic law were selective, distorted, or faked. For example, in a book published in 1868, Haeckel used exactly the same woodcut to show embryos supposedly of a dog, chicken, and turtle. His most famous illustration has been reproduced hundreds of times. It shows an array of embryos from eight species of vertebrate (animal with a backbone), namely fish, salamander, turtle, chicken, pig, cow, rabbit, and human. The figure shows each species at three stages of development, arranged in a column. In the first embryonic stage (top row of all eight columns), all the embryos are extremely similar. In the row below it, in the second stage, they are slightly different (except for the fish and salamander, which are already distinct). In the third stage, each embryo is recognizable as a miniature adult of its particular species.
Although Haeckel's famous figure does not pretend to show an evolutionary lineage—nobody has ever argued that humans evolved from rabbits—the figure does show striking evidence for evolution. For example,
the existence of a tail on the human early-stage embryo which disappears later seems to suggest a tail-bearing ancestor. More controversially, the earliest embryo shows a series of slits just below the head. Often called “gill slits,” these features seem at first glance to recapitulate the gills of an ancestral fish.
Haeckel's figure has been used in numerous teaching textbooks, at least into the 1990s. Yet his drawings, which were originally created by re-drawing figures from other published works, not by working from actual specimens, have been shown by many scientific critics to exaggerate similarities between embryos and to systematically omit embryos that would have weakened the impression he sought to create.
Despite flaws in his evidence and rejection of the biogenetic law by almost all embryologists starting in the 1920s, Haeckel's law has been widely believed for generations. The principle of recapitulation was long cited by a number of professionals in related fields: criminal anthropologists (who claimed that criminals are stuck at an apelike stage of ontogeny), racist anthropologists (who claimed that non-white races represent an earlier stage of human evolution), and psychologists (who argued that children recapitulate human cultural history as they mature). Only in recent years has the biogenetic law and Haeckel's defective illustration been more or less purged at last from high-school and college biology textbooks.
Ontogeny and Phylogeny Today
Haeckel's argument was not entirely ridiculous. The earliest stages of individual development often differ greatly, but there is a stage, the zootype or phylotypic stage, at which all vertebrate embryos do bear a general resemblance to each other. This is the stage shown in the first row of Haeckel's famous figure (with exaggerated similarity across species). For example, all vertebrate embryos develop “gill slits.” These features are not gills, but the cracks between a series of blobs or fingerlike protrusions of tissue in the neck region of the embryo. All vertebrate embryos, despite their other differences, form these pairs of blobs, usually called the pharyngeal arches, during an early stage of growth. The pharyngeal arches are then used by different species to build their face and neck regions. For example, in all vertebrates the first pharyngeal arch forms the jaw. The third pharyngeal arch forms throat cartilage in humans, but becomes gills in fish. The fact that all vertebrates pass through a stage of rough similarity and then build their parts from similar structures does point toward their descent from a common ancestor which developed in this way.
Moreover, instances of actual recapitulation can be found. For example, baleen whales have no teeth, but filter food from the ocean using large brushy organs (baleen) in the mouth. Yet the embryos of some species of baleen whales develop teeth and re-absorb them before birth, strong evidence that baleen whales evolved from ancestors who did have teeth as adults. There is other evidence that recapitulation of specific characters can occur, though not recapitulation of entire stages of growth.
Thus, the relationship between ontogeny and phylogeny is understood by modern biologists to be complex and to vary from species to species. Genetic changes can act to revise any stage of embryological growth. Timing of the development of features can be shifted in several different ways. Not only can adult characters be relegated to embryonic development, as Haeckel believed (for example, the baleen whales' embryonic teeth), but youthful characters can be pushed into adulthood, a phenomenon called neoteny. Experts in human evolution believe that neotenous changes to our apelike ancestors helped evolve human beings: the cranial proportions of human adults closely resemble those of immature chimpanzees. Chimpanzees are our cousins, not our ancestors, but are thought to more closely resemble our common ancestor than we do.
The modern field of evolutionary developmental biology, sometimes called “evo-devo,” which matured in the late 1990s, has found that the genes controlling ontogeny are deeply conserved (preserved with relatively little change) through phylogeny. In particular, all multicellular animals make use during ontogeny of a unique set of 180 DNA base pairs called the homeobox. (Plants use a different homeobox.) Several genes, called Hox genes, contain copies of the homeobox. The Hox genes are always arranged on the DNA (deoxyribonucleic acid) molecule in a row, like beads on a string in a certain order. During development, they are expressed—that is, the substances they code for are manufactured by cells—in body sections of the embryo that are lined up in the same order as the Hox genes on the DNA strand. The substances coded for by the Hox genes control the expression of other genes to shape the basic plan or pattern of the developing body.
Scientists discovered in the 1980s and 1990s, to their amazement, that species as far apart on the evolutionary tree as insects, mammals, and jellyfish all use the same Hox genes in the same order to regulate body-plan development during ontogeny. The core genes controlling ontogeny have thus been conserved through about 800 million years of branching evolution. In 2000, one specialist in evolutionary development, Sean Carroll, wrote in the journal Cell) that “One of the most surprising biological discoveries of the past two decades is that most animals, no matter how different in appearance, share several families of genes that regulate major aspects of body pattern.” Ontogeny does not recapitulate phylogeny, but ontogeny and phylogeny are nevertheless deeply intertwined.
Modern Cultural Connections
Some opponents of evolution, such as Jonathan Wells and Phillip Johnson, have recently cited Haeckel's faked drawings and the failure of the biogenetic law as typical of the poverty or nonexistence of the evidence for evolution. For example, Johnson has said that “Only by ignoring the early stages of [individual] development can one fit Darwin's theory to the facts of embryology … The later stages of development are no more inclined to cooperate with Darwinian expectations than the earliest stages” (Darwin on Trial, 1991). Wells has said that the Haeckelian theory of recapitulation is part of “our best evidence for Darwin's theory” and that its failure undermines the theory of evolution (Icons of Evolution, 2000). These and a few other writers have proposed that miraculous intervention, not evolution, has created the complexity of life. However, Haeckel's law has in fact never been a basic element of evolutionary biology, and its failure does not undermine evolution. It was not announced until seven years after Charles Darwin (1809–1882) published his classic work On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life in 1859. Today, over 99% of biologists affirm that Darwinian evolution is the organizing principle of biology and nothing in modern life science makes sense except in its light. Numerous, independent, convergent, ever-more-detailed lines of evidence prove that all species have descended from shared ancestors. Understanding the complex footprints that the branching pattern of evolution (phylogeny) has left on individual development (ontogeny) is the subject of intense ongoing research.
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Gould, Stephen Jay. The Structure of Evolutionary Theory. Cambridge, MA: Harvard University Press, 2002.
Carroll, Sean B. “Endless Forms: The Evolution of Gene Regulation and Morphological Diversity.” Cell 101 (2000): 577–580.
Goodman, Corey S., and Bridget C. Coughlin. “The Evolution of Evo-Devo Biology.” Proceedings of the National Academy of Sciences 97 (2000): 4424–4425.
Richardson, Michael, and Gerhard Keuck. “Haeckel's ABC of Evolution and Development.” Biological Reviews 77 (2002): 495–528.