A team of MIT researchers led by civil engineer and materials scientist Markus Buehler has finally unraveled the structure of bone—a long-standing mystery—with almost atom-by-atom precision. Doing so took many years of analysis by some of the world’s most powerful computers, results that were confirmed by laboratory experiments.

Buehler says the biggest question was how two different materials—a soft, flexible biomolecule called collagen and a hard, brittle form of the mineral apatite—combine to form something that is simultaneously hard, tough, and slightly flexible.

The constituents are so different that “you can’t take these two materials individually and understand how bone behaves,” Buehler says. Hydroxyapatite is like chalk, he says: “It’s very brittle. If you try to bend it even a little, it breaks into pieces.” Collagen, on the other hand, is what gelatin is made of—the very epitome of a wobbly substance.

The team found that “tiny hydroxyapatite grains [are] embedded in the collagen matrix,” allowing the two materials to “each contribute the best of their properties,” Buehler says. “Hydroxyapatite takes most of the forces in the material, whereas collagen takes most of the stretching.”

The new understanding of bone’s molecular structure and function could help in determining what goes wrong in diseases such as osteoporosis and brittle bone disease.