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  The concept of homology (Greek, ‘study of sameness’), in the life sciences, refers to the study of evolution where a homologous structure in two or more species indicates commonality of ancestry. The degree of homology is related to the closeness of genetic relationship, though gross structural homologies are difficult to quantify. Leonardo da Vinci first noted the homology between the bone structures of human and horse legs, which superficially look very different but which consist of the same bones adapted for different modes of locomotion. In the 19th century, the paleontologist Richard Owen defined the term along with the concept of analogy, noting that the two may occur in the same structure and may be difficult to separate. However, Owen denied the existence of evolution and the first to postulate that homology indicated a genetic connection between species was Karl Gegenbaur. The identification of homologous features was very important in the post-Darwinian debate on evolution, and in the development of classification systems based on ancestry. Modern techniques have shown that the concept of homology can be extended to the physiological and molecular level: the vertebrate oxygen-carrying protein haemoglobin has conserved homology in that substantial pieces are the same in different species. Gene sequencing has allowed homology to be quantified, as the proportion of the DNA in a gene which is homologous to another can be directly measured. RB

See also adaptive radiation; anatomy; morphology; palaeontology.



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