According to biologist Ernst Haeckel (1834-1919), the developmental stages that an embryo passes through reveal the evolutionary history of the species. In other words — and brace yourself; it's a mouthful — the ontogeny (the developmental process) recapitulates (summarizes) phylogeny (the evolutionary history).
Haeckel saw in the various stages of mammalian development what he described as stages of development corresponding to specific ancestral species. At one point in their development, for example, human embryos have a tail-like structure, which was thought to signify tailed ancestors. Earlier developmental stages included pharyngeal arches, which were thought to resemble gill slits. This feature was taken to indicate that the developing human embryo passes through a fishlike stage.
Haeckel's investigations were conducted during a period when Darwin's theory of evolution by natural selection and the concept of the common ancestry of all life were stimulating a fair amount of research. The proposition that ontogeny recapitulates phylogeny was seen as being consistent with Darwinian theory, though it was difficult to understand why such a pattern would occur.
Today, scientists understand that the different stages a developing embryo goes through are in no way the equivalents of other species in the tree of life. There is no fish stage in mammalian development, for example. But humans do have structures that are homologous to (share the same ancestor with) structures that a fish has, and some of these structures are most evident at early embryological stages.
Two traits that are homologous have a common ancestor. The wings of birds, for example, are all homologous. All birds are descended from an initial bird ancestor. Go back farther, and you can see that the front limbs of all tetrapods (four-footed animals and the things descended from them — so whales and snakes are called tetrapods, too, as are humans) are homologous. Your arms, the wings of all birds, the front legs of crocodiles, and the front flippers of dolphins all trace back to the first tetrapod. Homologous characters can be similar in appearance, but they don't have to be. Contrast that with analogous. The wings of birds and the wings of butterflies are analogous. They have similar functions, but not as a result of having a common evolutionary ancestor. Insect wings are derived from very different parts than are the front limbs of ducks, for example.
Although Haeckel's idea that ontogeny recapitulates phylogeny turns out not to be true, he was right in thinking that embryology can tell scientists something about evolution. Today, 150 years later, scientists have a much better handle on what that something is.
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