Introduction to Human General Embryology Developmental Genetics
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What are Vestigial Structures? Human development has been known to be associated with the development of structures which are genetically determined but apparently lost most or all of its ancestral functions. Vestigial status generally relies on the comparison with homologous features in related species. The emergence of vestigiality occurs by normal evolutionary process, typically by loss of function of a feature that is no longer subject to positive selection pressures when it loses its value in a changing environment. More urgently the feature may be selected when its function becomes definitely harmful. Vestigial features may take various forms -patterns of behavior, anatomical structures, or biochemical processes. Like most other physical features, however functional, vestigial features in a given species may successively appear, develop, and persist or disappear at various stages within the life cycle of the organism, ranging from early embryonic development to late adulthood. Vestigial structures are often called vestigial organs, although many of them are not actually organs. Experts say that such vestigial structures are typically degenerate, atrophied, or rudimentary structures and tend to be much more variable than homologous non-vestigial parts. They have also found out that though structures commonly regarded as "vestigial", may have lost some or all of the functional roles they had played in ancestral organisms, such structures may retain lesser functions or may have become adapted to new roles in extant populations. It is on the basis of this that some experts have argued that majority of the so called vestigial organs, especially in man, has a definite use and is not, in any sense, atrophied. Historically, a list of more than 80 organs were considered vestigial amongst which were the thyroid, thymus, and pituitary glands; olfactory bulb, middle ear, tonsils and appendix. Today however, it is common knowledge that all these organs have useful functions, and not infrequently, essential. In the time the list was made, nobody knew their function but as studies were developed by physiologists, the list of human vestigial structures has shrunken. For example: The tonsils previously considered as "worthless" are now know to protect you against infections. The Appendix previously classified as "useless" is now known as an important part of the reticulo-enabthelial system of the body. Like the tonsils, the appendix fights infection. The Coccyx previously declared useless has been found to be useful in the attachment of important muscles (the levator and coccyges). These muscles support the pelvic organs and ensure bowel movements. Without them humans could cannot walk or sit upright. The Thymus once considered “worthless” is now known as the primary central gland of the lymphatic system. Without it, T cells that protect humans from infection could not function properly, for they develop within it. The Pineal Gland is no longer considered as a vestigial structure because it secretes critically needed hormones, including, melatonin which inhibits secretion of luteinizing hormone. The Thyroid Gland also previously considered as a useless organ is now known for sure, to secrete the hormone, thyroxin, which goes directly into the blood. This hormone is essential to normal body growth in infancy and childhood. Without it, an adult becomes sluggish. Either an oversupply or an undersupply of thyroxin will result in over-activity or under-activity of many body organs. Deficiency of this organ at birth causes a hideous deformity known as cretinism. The Pituitary Gland once claimed to be vestigial, is now known to ensure proper growth of the skeleton and proper functioning of the thyroid, adrenal, and reproductive glands. Improper functioning can lead to Cushing's syndrome (gigantism). The Semilunar Fold of the Eye previously claimed to be a vestige of the eye, is now known to be a very necessary part of human eye as it is the portion of the conjunctiva that cleanses and lubricates the eyeball. NB It should be noted that during embryonic development, some embryonic structures persist as modified structures in adulthood. For example, the Urachus, which represents the obliterated Allantois. Download 3.46 Mb. Share with your friends:
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