Part of the problem is that with chemicals and other bits of modern research and science, humans can build muscles that exceed the structural capacity of human bone-especially if there is an undiagnosed stress fracture in the bone used to support the heavy lifts. World's Strongest Man competitors break bones like that on a not infrequent basis.
Almost all of us can do the same if adrenaline triggers and we remove our internal safeguards. Most of us who train in martial arts have also broken our own bones at some point by hitting things too hard-that's a different sort of stress but we are still overloading the structural abilities of the bone.
Our ancestors had thicker bones than modern humans but their own heavy musculature forced their bones into a slightly curved shape.
