An understanding of the developmental processes of embryos is crucial to understanding evolution and the relationship between organisms. More closely related species have more similar developmental stages and developmental processes. Young human embryos resemble fish and actually have structures that look like gills. In fish these structures develop into gills, but in humans they develop into the ear, jaw, and neck.

Ernst Haeckel developed a hypothesis about vertebrate embryo development that was widely accepted by the eminent scientists of the early 20th century. It was called "ontogeny recapitulates phylogeny," which means that growth and development repeats evolutionary history. His hypothesis suggested that during development an organism experiences all the stages of its evolutionary history until it reaches full development. In this scenario humans first develop into fishes, then amphibians, then reptiles, then rodent-like mammals, then babies.

Haeckel's hypothesis is no longer accepted, but his general observations still ring true: The earliest stages of development in uterro (i.e., in the womb) resemble primitive ancestry, and the later stages resemble more recent shared ancestry.

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