Early Evolution and Development: Ernst Haeckel
The evolutionary study of embryos reached a peak in the late 1800s thanks primarily to the efforts of one extraordinarily gifted, though not entirely honest, scientist named Ernst Haeckel (left). Haeckel was a champion of Darwin, but he also embraced the pre-Darwinian notion that life formed a series of successively higher forms, with embryos of higher forms "recapitulating" the lower ones. Haeckel believed that, over the course of time, evolution added new stages to produce new life forms. Thus, embryonic development was actually a record of evolutionary history. The single cell corresponded to amoeba-like ancestors, developing eventually into a sea squirt, a fish, and so on. Haeckel, who was adept at packaging and promoting his ideas, coined both a name for the process "the Biogenetic Law" as well as a catchy motto: "Ontogeny recapitulates phylogeny."
Haeckel was so convinced of his Biogenetic Law that he was willing to bend evidence to support it. The truth is that the development of embryos does not fit into the strict progression that Haeckel claimed. Echidnas, for example, develop their limbs much later than most other mammals. But in his illustrations of echidna embryos, Haeckel deceptively omitted limb buds at early stages, despite the fact that limb buds do exist then. In Haeckel's own day, some biologists recognized his sleights of hand, but nevertheless the Biogenetic Law became very popular, and Haeckel's illustrations even found their way into biology textbooks.
The biogenetic law is broken
Embryos do reflect the course of evolution, but that course is far more intricate and quirky than Haeckel claimed. Different parts of the same embryo can even evolve in different directions. As a result, the Biogenetic Law was abandoned, and its fall freed scientists to appreciate the full range of embryonic changes that evolution can produce an appreciation that has yielded spectacular results in recent years as scientists have discovered some of the specific genes that control development.
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Mouse embryo image courtesy of the Edinburgh Mouse Atlas Project; Tunicate larva image courtesy of Richard Grosberg, UC Davis
Understanding Evolution © 2017 by The University of California Museum of Paleontology, Berkeley, and the Regents of the University of California