When eating a forkful of food, your elbow bends to move your hand toward your mouth. More specifically, your biceps contract and pull on your forearm bones. Because the biceps are anchored to your upper arm bone, your forearm's motion pivots at the joint between these two segments.
Now consider an octopus tentacle. This incredibly flexible and boneless limb can extend, bend, and turn in any direction. Yet despite this freedom, the octopus tentacle surprisingly acts in much the same way as the human arm when transferring tasty morsels from sucker to mouth.
Work published in the journal Current Biology shows that when given a piece of food, an octopus bends its tentacle in three places. This creates three joints just like a human arm with a shoulder, elbow, and wrist. The changing angle of the elbow-like middle bend is chiefly responsible for the food's transfer.
How does the octopus generate this folding motion when there are no obvious structural elements like bone for muscles to pull on? Well, instead, the muscles themselves provide structure. Nerve impulses emanating from two sources, the sucker grabbing the food and the brain, travel along the tentacle. These waves stiffen muscles except where the signals meet mid-tentacle. The relaxed region serves as the elbow-like joint between the rigid segments to either side.
This mechanism solves the problem of how to temporarily locate the joint in the best position no matter where the tentacle ensnares food. In addition, the striking similarity between such distinct features as the octopus tentacle and human arm suggests that jointed limbs represent an optimal means for moving objects from one point to another.