Cordain discusses this

  http://www.lifeservices.com/cordain.html  and other places

This seeming paradox (low calcium intake, high protein intake yet strong
dense
bones) may probably be explained by multiple factors. Stone age humans were
more active than modern people and
consequently the everyday work they did resulted in greater bone loading,
which in turn can influence bone density. Further, they
lived outside most of the day so their sunshine exposure and hence vitamin D
status would likely have been superior to most
modern people who work indoors and get little sunshine exposure.

Lastly, and most importantly, the critical dietary factor influencing bone
metabolism and hence osteoporosis is not calcium
intake, nor calcium excretion, but rather calcium balance...


Philip Thrift
http://www.paleofitness.com