The repair of long bones has not been optimized by Mother Nature. One would think that such a complicated process, leading to replacement of damaged tissue and restoration of function, would be the final result of millions of years of evolution. However, if our hairy and four-legged ancestors were to survive a fracture, they would have needed restoration of long bone function within days. Otherwise, without being able to run, jump or climb perfectly, they would have starved to death or been eaten up by carnivores. Because long bone fracture led to almost certain death, even a mutation that could shorten healing time by half would still not save the animal. Thus, there can hardly have been any evolutionary pressure towards optimized long bone repair. Still, long bones do repair remarkably well: if the animal is helped to survive, the fracture gap will be replaced by newly developed bone with a complex architecture. This is a kind of regeneration rather than repair—quite different from scar formation, which is seen at most other locations. It seems paradoxical that we have a truly regenerative process in the absence of any evolutionary pressure. The explanation is probably that long bone repair uses the same genes as repair of trabecular microfractures. Such fractures often occur with minimal symptoms, and require a repair system for the animal to survive.