ONES ARE MoST OFTEN SEEN as pale, dry, flaky objects in museums, or lying in the countryside as the last remnants of a dead animal. They might not seem very interesting, but they do have a life of their own. Our prehistoric ancestors would have had a more intimate knowledge of bones than people today. More than a million years ago, early humans used rock axes and stone hammers to crack open animal bones, presumably to get at the nutritious marrow inside. They would see that a typical bone has a tough skin that we call the periosteum, covering an outer shell of compact bone, which is hard and dense. The compact bone an inner of lighter, honeycombed cancellous or spongy bone, with soft marrow at the centre. When early microscopists peered at thin slices of bone, they were amazed to see a complex and regular micro-architecture, which reflects bone's active life. The structural unit of compact bone is the tube-shaped Haversian system, named after English physician Clopton Havers (1650-1701). He published his observations in 1691, and opened the way for the research of later physiologists, who found that bones had many roles, in addition to holding up the body.