Re: Theoretically, how tall can a human grow?

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