Aristotle was the earliest person in recorded history to study embryos. Observing embryos of different species, he described how animals born in eggs (oviparously) and by live birth (viviparously) developed differently. He discovered there were two main ways the egg cell divided: holoblasticly, where the whole egg divided and became the creature; and meroblasticly, where only part of the egg became the creature. Further advances in comparative embryology did not come until the invention of the microscope. Since then, many people, from Ernst Haeckel to Charles Darwin, have contributed to the field.
Many erroneous theories were formed in the early years of comparative embryology. For example, German biologist and philosopher Ernst Haeckel proposed that all organisms went through a "re-run" of evolution he said that 'ontogeny repeats phylogeny' while in development. Haeckel believed that to become a mammal, an embryo had to begin as a single-celled organism, then evolve into a fish, then an amphibian, a reptile, and finally a mammal. The theory was widely accepted, then disproved many years later.
The field of comparative embryology aims to understand how embryos develop, and to research the inter-relatedness of animals. It has bolstered evolutionary theory by demonstrating that all vertebrates develop similarly and have a putative common ancestor.
Hall, Brian K. (1992). "5: Embryological archetypes and homology: establishing evolutionary embryology". Evolutionary Developmental Biology (2 ed.). Dordrecht: Springer Science & Business Media (published 1999). p. 84. ISBN 9780412785900. Retrieved 20 Mar 2019.
5.5.5 Evolutionary embryology and the search for vertebrate ancestors
The 1870s and early 1880s were an active and exciting time for those involved in the classification and ordering of animals, in comparative embryology and in forging an evolutionary embryology. The grand schemes of animal classification that emerged then are, by and large, with us today. Much energy went into searching for ancestors.
Rieppel, Olivier (2010). "8: Linking the Facts: Tracing the Traces". Evolutionary Theory and the Creation Controversy. Heidelberg: Springer Science & Business Media. p. 169. ISBN 9783642148965. Retrieved 20 Mar 2019.
Comparative embryology offered not only powerful arguments in support of a branching order of nature, but also powerful insights into the deep relationships between natural groups of organisms such as reptiles, birds, and mammals. In fact, Darwin found comparative embryology to offer a key to the recognition of common ancestry.