Human postcranial skeleton gracilized relative to primates and Neanderthals. LRP5 gene regulates bone mineral density via Wnt pathway. Positive selection on LRP5 in sub-Saharan Africans and Southeast Asians. Non-Africans enriched for low BMD alleles versus sub-Saharan Africans. Archaic introgression minimal in LRP5 except three East Asian haplotypes. LRP5 region depleted of Neanderthal introgression in Europeans. Modern human mutations occur in more conserved genomic regions than archaic ones. Archaics faced stronger purifying selection or moderns relaxed selection on LRP5. Five archaic missense mutations identified in LRP5 first β-propeller. Four archaic mutations enhance Wnt pathway activation versus wild-type. Two archaic mutations resist DKK1 inhibition like human HBM mutations. Archaic p.A67V boosts Wnt 1.78-fold. Archaic p.R186Q boosts Wnt 1.55-fold. Protein stability changes in archaic variants match human HBM mutations. Genetic factors contribute to weaker modern versus robust archaic skeletons.