Re: Will vegetarian's diet choice cause their body to adapt?

Humans, like any other animal, would evolve or adapt to new environmental pressures, given time. The generation-time is much longer for humans, and this would take many generations, assuming that all of the changes are genetic. Some adaptations are physiological, even the bacteria that live in our GI systems may force physiological adaptation. In the case of the lizards, I expect that some of the adaptation (esp the GI tract) is physiological; the scientists studying this do not actually know yet what portion of adaptation is genetic and what is physiologic (Herrel, 2008). Much more work remains to be done to validate this, since the DNA tests run do not exclude the possibility of hybridization of related lizards to produce the "adaptations."

In a similar situation of island isolation, there is now some evidence that the hominin Homo florensis (Brown, 2004), maybe related to Homo Erectus and/or Australopithicus afarensis, may have adapted to its island location over tens of thousands of years. This, I believe, supports that the same could happen to modern humans, but it would ethically unacceptable to run the experiment. One also has to take into account how modern society would affect adaptations: vegetarians supplement their diets to ensure that they get enough protein and vitamins, and our 21st century knowledge allows them to change their diets without ill effect; presumably, this would slow the adaptive response of the body, say to a reduction in a nutrient that might drive adaptation (since humans can replace the nutrient, artificially).

Brown, P., Sutikna, T., Morwood, M. J., Soejono, R. P., Jatmiko, Wayhu Saptomo, E. and Rokus Awe Due (2004), "A new small-bodied hominin from the Late Pleistocene of Flores, Indonesia", Nature 431: 1055.

Herrel, A., Huyghe, K., Vanhooydonck, B., Backeljau, T., Breugelmans, K., Grbac, I., Van Damme, R., and D.J. Irschick (2008), "Rapid large-scale evolutionary divergence in morphology and performance associated with exploitation of a different dietary resource", Proceedings of the National Academy of Sciences, USA, DOI: 10.1073/pnas.0711998105