I thought that “opposable” was entirely a matter of the setup of the thumb. But it seems like to be opposable there also has to be something against which it’s judged. That is, if we had only one thumb, and no fingers, even if it kept the same configuation it wouldn’t be opposable. Is this right?
It means that you can touch the other fingers with your thumb. Without the other fingers, it couldn’t really be opposable.
It’s a tautological term for two reasons:
“Opposable” is a word that’s only ever really used to describe thumbs.
And if a finger or toe isn’t “opposable”, it’s not a thumb.
In fact, the term “thumb” is so closely associated with an opposing grip that it’s been borrowed for the panda’s “thumb”, which isn’t even a finger, but a protrusion from a wrist bone which acts in a similar manner to a true thumb, ie it allows bamboo to be grasped against the fingers.
Right. It is opposable to something (the other fingers). It’s analogous to opposite, which is meaningless unless there’s an answer to “opposite of what?”.
Then surely plain old fingers are opposable… in that they oppose the thumb?
You’re right that it’s tautologous, and that we should really be saying ‘opposable digits’, because it’s possible for toes to have a similar function.
You know those times when you get the impulse to reach into that really narrow pipe to fish out your car keys which you dropped while trying to look cool by tossing them casually into the air?
When your thumb gets in the way, keeping you from reaching in and getting your entire arm stuck and requiring you to hack it off with a pocket knife, you’ll appreciate that sometimes opposability to dumb ideas is an aid to survival.
Not true; “opposable” in biology can refer to any opposable digits.
Not just primates have opposable digits. They are found in various marsupials. Koalas have two “thumbs” on the front foot or hand, and one on the hind foot. Some opposums also have opposable digits.
Chameleons also have opposable digits, split 2-3 but differently on the front and hind feet. The front feet have three toes on the inside and two on the outside, while the hind have two on the inside and three on the outside. If you regard the inside set as “thumbs”, by analogy with humans, then they have three thumbs on the front foot and two on the rear.
Of course many birds also have opposable digits. Birds such a songbirds and raptors have the toes split 3-1, while woodpeckers, parrots, and others have them split 2-2.
If you had a birth defect, or suffered some accident, leaving you with a thumb but no fingers, I would say you still had an opposable thumb but that there was nothing for it to oppose. Although you could still press your thumb against your palm and grip some things. On the other foot, your big toe is parallel to your other toes and bends in the same direction, so it can’t be said to be opposable.