Ontogeny refers to the development of an organism. With humans, ontogeny begins with the fertilized egg and continues through embryonic and fetal development, birth, maturation, and ultimately senescence. Clearly, many developmental phenomena in humans are found in other animals and seem to be related to events in the evolutionary history of the group. Phylogeny relates to the development of a group. Phylogeny is a history of a group of organisms from the beginning of life to the present time.
The terms ontogeny and phylogeny are grouped together frequently because of the fundamental biogenetic law espoused by the German scientist Ernst Heinrich Philip August Haeckel (1834-1919). Haeckel's law, also known as the theory of recapitulation, states that ontogeny is the short and rapid recapitulation of phylogeny. Haeckel, as had others before him, noted the similarity of embryonic forms within a group. Certainly a mouse, elephant, and human embryo appear remarkably similar in contrast to the vast differences in the adults of those forms. The similarity is thought to be due to common descent from a more primitive form. Some embryological similarities are ancient in origin. Consider the aortic arches of vertebrates. Aortic arches connect a ventral aorta to the dorsal aorta in the pharyngeal region of an early embryo. Blood is pumped from the beating heart via the ventral aorta to six arteries which arch around the pharynx. Blood collects in the dorsal aorta and is distributed throughout the body and head of the embryo. During early embryonic development there are six aortic arches in all vertebrates. In fish and amphibian larvae, the posterior arches sprout a capillary bed and ultimately, the arches and their capillaries become the respiratory gills of the organism. Reptilian, avian, and mammalian embryos, being mostly terrestrial, have absolutely no need for functional gills. Nevertheless, as stated previously, they develop six aortic arches. That includes humans. The first and second arches of humans are mostly lost, portions of the third pair form the common carotid arteries, the left fourth contributes to the arch of the aorta, the right fourth forms part of the right subclavian artery, the fifth pair are lost, and the sixth contribute to the pulmonary arteries. One cannot but be impressed that humans, standing at the peak of organic evolution, have a vascular system with an early development greatly similar with the early development of lowly fishes and amphibians. Most would argue that this suggests descent from some common ancestor. Other human organ systems can be shown to have similarities in their development with corresponding organs in lower vertebrate forms.
Is this similarity in developmental pattern recapitulation? The answer to that rhetorical question is a firm negative. At no time does a human recapitulate the ancestral forms to which the human may be related. Humans never have the morphology of a mature fish nor do they have the anatomy of a mature amphibian. And, of course, they never recapitulate a mature mouse or mature elephant form. Thus, ontogeny does NOT recapitulate phylogeny. Clearly, the history of human evolution is not repeated in detail and thus, Haeckel was wrong. However, the very existence of the six embryonic aortic arches in human embryos witnesses the close relationship of humans with all other vertebrate forms.
Phylogeny is a history. Not much historical information can be deduced from the study of embryos. Indeed, it may be stated that vertebrates are related and have similar early embryos. But, how are they related? Comparative anatomy which involves seeking homologous structures in contemporary related forms provides much better information on relationships and so too does the paleontological record.
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