Early Evolution and Development: Ernst Haeckel (2 of 2)

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The Biogenetic Law Is Broken
By the turn of the century, scientists had discovered many cases that defied Haeckel’s so-called law. His followers tried to cast them as exceptions that proved the rule. But Haeckel’s final downfall came with the rise of genetics and the modern synthesis. Haeckel, after all, was promoting a basically Lamarckian notion that evolution had a built-in direction towards “higher” forms. But genes, it was soon discovered, controlled the rate and direction of embryonic development. Individual genes can mutate and cause different changes to the way embryos grow—either adding new stages at any point along their path, or taking them away, speeding up development or slowing it down.

Haeckel imagined humans (“Menschen” in German) to be the “highest” form of life, placing them at the top of his tree of life (left); at right, how the same anthropoid relationships might be shown today.

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.