In this study, crack initiation and growth in four different groups of human cortical bones, i.e., young, aged, diseased (osteoporosis) and treated are investigated numerically with a zero-thickness Cohesive Element Method, employing statistical realisations of randomly distributed microstructural constituents. The obtained simulation results demonstrated distinct crack paths in bones with varying microstructures, based on analysis of initiation, propagation and branching of multiple cracks, with supporting fracture toughening mechanisms. It is shown that superior mechanical properties and fracture resistance in the young and treated groups originated from both the qualitative and quantitative features of microstructural constituents.