If you stretch out your arms or legs and compare their length, you will, in most cases, find that they are very similar. Indeed, the external features of our body are reliably symmetrical; a noticeably asymmetrical feature is perceived as an abnormality.
Little attention has been given to how limb symmetry is achieved, even though much is known about how bones themselves grow. Limb structures are specified in the embryo and then grow for some 16 years. We do not even know why they finally stop growing or why their growth rate varies at different ages. Arm length matches with extraordinary accuracy, varying by no more than about 2%. Yet much about how this finely tuned growth is controlled remains a mystery. There is no evidence for communication between the right and left limbs: if a child has an accident that causes one limb to have reduced growth, the opposite ...
