Things do not evolve because they have too. They evolve because what ever parents had a reproductive advantage. A rabbit that can only have one offspring per litter will have a less reproductive advantage over one that can have 20. A great example is that moth in England. There are two types on is dark grey and the other some light color. In the Industrial revolution (Every thing was dirty) the darker one had a reproductive advantage because birds could see it less easily. Where as now or before I cannot remember the other had/has a reproductive advantage becaseu the treebark it sleeps on is/was a lighter color so birds could see it less easily. If human females all want someone taller then them tall men will have a reproductive advantage. Original hominids (according to the Discovery Channel) were about 2 foot five. Do not try to tell me that now we do not eat bugs and have a balenced diet so we are taller. Fine get some animal and give it a balenced diet. See if that makes it taller. Giraffes are getting taller because the the longer thicker necks have a reproductive advantage. They fight for females with their necks.