Evolution is the change in organisms over time that gives rise to new species. Development is the process by which a fertilized egg, or embryo, generates the cells, tissues, and organs of a new individual and assembles them into their proper form. Evolution produces the body shapes of the animal kingdom; development produces the body plan of individuals.
Biologists have been making connections between these two processes since the 19th century. But in the last decade, these studies have intensified and even spawned a new field of study: evolutionary developmental biology, or, as it's often known, "evo-devo.'' Using new techniques of biology and genetics, researchers are now investigating development at the molecular level, the genes that regulate and orchestrate the unfolding of a new life. Moreover, the genes not only serve as a construction and operating manual, they also contain a record of the evolutionary history of the organism, because many of the same genes were used by direct ancestors. "Evo-devo'' researchers investigate the ways that evolution has modified embryological processes, and, conversely, how developmental mechanisms have influenced evolution.
Even before Darwin, biologists recognized that species that looked quite different as adults often had close similarities as developing embryos. Many four-legged animals go through embryonic stages that have similar features -- gill arches, a notochord, segmentation, and paddle-like limb buds -- as they develop into different adults. To Darwin, the embryonic resemblances were strong support for the theory of evolution.
One of Darwin's contemporaries, German biologist Ernst Haeckel, summed up the argument in a famous, pithy statement: "Ontogeny recapitulates phylogeny.'' That is to say, in the process of development, an individual passes through the adult forms of all its ancestors. So, Haeckel proposed, by examining the development of an embryo you could read its entire evolutionary history in the transition from one stage to another. In fact, this isn't strictly true, and the drawings Haeckel made exaggerated the embryonic similarities between species.
But biology now has new tools, from microphotography to molecular biology, with which to examine the process of development in embryos. These new tools reveal that different descendants of a common ancestor do indeed usually go through embryonic stages that resemble each other and their common ancestor The processes that guide embryonic development are conserved by evolution and reused again and again.