Generations of biology students have been convinced--in part because of drawings done 123 years ago by the German biologist Ernst Haeckel--that vertebrate embryos of different animals pass through an identical stage of development. But a report in the August issue of Anatomy and Embryology charges that Haeckel exaggerated the drawings, which persisted in English-language textbooks for almost a century.
Michael Richardson, an embryologist at St. George's Hospital Medical School in London, began to wonder about Haeckel's drawings because they didn't square with his understanding of the rates at which embryonic fish, reptiles, birds, and mammals develop their physical features. So he and his colleagues did their own comparative study, reexamining and photographing embryos of the same age and species--or close approximations--as those Haeckel drew. Lo and behold, the embryos "often looked surprisingly different," Richardson says.
Not only did Haeckel exaggerate the similarities, they report, but he also failed to draw the embryos to scale. Take, for example, the zebrafish. At the stage Haeckel depicted, the fish embryo is about 1 millimeter long, while amphibian embryos at the same stage of development can be as much as 9 millimeters long. "It looks like it's turning out to be one of the most famous fakes in biology," Richardson concludes.
Richardson is not the first to realize that Haeckel--who later admitted having prepared his drawings from memory--took artistic license with these illustrations. His peers at his university in Jena convicted Haeckel of fraud in a university hearing, says Scott Gilbert, a developmental biologist at Swarthmore College in Pennsylvania. However, the drawings were subsequently used in a 1901 book called Darwin and After Darwin and reproduced widely in English-language biology texts.
Recent discoveries that many species share the same developmental genes has renewed interest in comparative developmental biology. And while some researchers like to emphasize the similarity of development--as Haeckel's illustrations did--Richardson stresses that development actually varies quite a bit. Gilbert agrees: "There is more variation [in vertebrate embryos] than had been assumed." And for that reason, he adds, "the Richardson paper does a great service to developmental biology."