Developmental Processes

People have long been interested in the connection between the development of an organism, its ontogeny, and the evolutionary ancestry of the species, its phylogeny. Anaximander, a philosopher of ancient Greece, noted that human embryos develop inside fluid-filled wombs and proposed that human beings evolved from fish as creatures of the water.

This early idea was a progenitor to recapitulation theory, proposed in the 1800s by Ernst Haeckel, a German scientist. Recapitulation theory is summarized by the idea that the embryological development of an individual is a quick replay of its evolutionary history. As applied to humans, recapitulation theory was accepted by many evolutionary biologists in the 1800s. It also influenced the intellectual development of other disciplines outside of biology, including philosophy, politics, and psychology.

By the early 1900s, developmental biologists had disproven recapitulation theory and had shown that the relationship between ontogeny and phylogeny is more complex than proposed by Haeckel. However, like Haeckel, modern biologists hold that the similarities in the embryos of closely related species and the transient appearance of certain structures of mature organisms early in development of related organisms indicates a connection between ontogeny and phylogeny. One modern view is that new species may evolve when evolution alters the timing of development, so that certain features of ancestral species appear earlier or later in development.