|MadSci Network: Anatomy|
Jon: The physiological mechanisms controlling human height involve the interplay between genetics and endocrinology. Overshadowing these controls is the level of nutrition and overall health of individuals. As described in the 10th edition of The Textbook of Medical Physiology (Guyton & Hall, 2000; W.B. Saunders Co. PP 851-854) the anterior pituitary produces a peptide called growth hormone (somatotropin). Growth hormone (GH) stimulates the liver to produce several small peptides called somatomedins or Insulin-like growth factors (IGF). The coordinated activities of GH and IGF result in increased cartilage formation, increased bone growth and increased muscle growth. The lack of a specific IGF (Somatomedin C) has been shown to be the probable cause of African pygmies' and Levi-Lorain dwarfs' short stature. Also important in the overall height of human individuals is the timing of puberty as gonadal steroids (testosterone and estrogen) function to speed the ossification of the epiphyseal plates at the ends of the long bones, therefore limiting long bone length and overall height. It seems that nutritional deficiencies and general health are closely linked to growth rates and final height. As detailed at the following web site, http://www.plimoth.org/Li brary/l-short.htm these non-genetic variables have as much as a 10% influence on final height. Furthermore, the Maasai, while a nomadic people, have a diet high in protein, including meat, milk and blood. Likely providing the nutritional building blocks for their tall stature. Michael Dougherty at the BSCS in Colorado Springs writes http ://www.sciam.com/askexpert/biology/biology30/biology30.html that the upper limit of human height probably is around 7'1" (average) as the increases in human heights have leveled off over the last few decades. There are certainly examples of very tall people in history, see the next two web sites for examples: http://members.home.net /harbord/heights.html http://www.altonweb.com/hist ory/wadlow/ As a general rule these individuals were "suffering" from some endocrine disorder, ie exessive GH synthesis and release. Physiologically, it may be possible for humans to maintain heights in the 15' range as your question suggests. We have a modern day mammal which averages 18' in total height, that being the male giraffe http://www.n aturalia.org/ZOO/AN_TERRA/e_giraffa_m.html. Obviously, the mammalian skeleton and cardiovascular system can be modified to support such heights. However, the "evolutionary pressure" to attain such genetic results is likely lacking. Finally, some scientists indicate that if man ever lives on planets with significantly less gravity than earth, it may be possible to evolve longer bodies. ..David Mallory, PhD; Department of Biological Sciences, Marshall University, Huntington, WV
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