Octopus Sheds Light on Arm Evolution
The arm of an octopus is rubbery, but the octopus uses it in much the same way that animals with three joints in their arms do.
When grabbing something, the octopus flexes its arm to form three "joints," located in the same spots as the shoulder, elbow, and wrist in humans. The middle "joint" is like an elbow, dividing the octopus' arm into two segments of equal length, like the upper arm and forearm among humans. This similarity is not an accident.
An Agence France Presse article explains:
Millions of years of evolutionary pressure has determined that the triple-jointed arm is the simplest and most efficient way of achieving this, the study suggests.