The analogy of hand and
grapple assumes you use your hand palm up to scoop a load and simply hold the load in place with your thumb. That is how the most common type of
grapple works. Gravity does the work for L shape or "open bottom" grapples like those sold by
EA. The load sits on the bottom tines so there really isn't a need to have more than a single thumb to secure the load.
Rotate your hand so palm is facing down when you grasp an object is like using a clamshell
grapple. A clamshell doesn't take advantage of gravity but rather fights it as the load is held only by clamping pressure. Open a clamshell
grapple and the load falls out immediately. Open an L shape
grapple and the load just sits on the bottom tines....unless you move or have a load that's center of gravity is outside the
grapple.
Throwing a split fingered fastball is an example use of the remarkably flexible human hand that allows us to use the same implement to swing an axe or hold a baby or pick up a pin without needing SSQA.
Bottom line is that grapples and humans do well with one thumb.