I did a search, and while I got some interesting results, nothing that answers my question:
Why do thumbs only have one joint, not two like the other fingers have?
Is this due to the whole “opposable thumb” thing?
I did a search, and while I got some interesting results, nothing that answers my question:
Why do thumbs only have one joint, not two like the other fingers have?
Is this due to the whole “opposable thumb” thing?
There are two. The other one’s right where it connects with the web of skin between it and the first finger. The joint where your thumb connects to your hand is down by the wrist.
With the thumb attaching to the wrist doesn’t that mean it lacks a metacarpal? The details are different but Montfort’s point is the same.
When you talk about evolutionary physiology there may not be a nice neat discreet reason for why thing are the way they are. Perhaps the question should be “why not?”
I was about to paraphrase Montgomery Burns and say that we have opposable thumbs so we can operate firearms but I’ve found that’s not true. Wednesday I learned that unless one has mutant long thumbs it’s difficult to reach the selector on an H&K MP5 when it’s in safe position. I found it far easier to place my right hand thumb on the right side of the pistol grip and control the seletor that way.
First, as Gunslinger indicates, there are two joints in the thumb. Of course, there are three in all the other fingers, which still presents the problem of why the thumb has one less joint than the rest of the fingers (note that the same is true for the ‘big toe’ vs. the rest of the toes - this in itself means there is more at work than mere ‘opposable thumbs’).
Second, the thumb does not attach to the wrist. There is a metacarpal (the first joint, not counting the wrist joint, is formed where the metacarpal joins the first phalanx).
Third, this ‘phalangeal formula’ (23333) is consistant throughout mammalia, except where digits have been lost entirely (e.g., in horses and their ilk).
Fourth, even in other groups, the first digit is always the smallest. Reptiles, for example, have the formula 23453 for the ‘hand’ (manus) and 23454 for the ‘foot’ (pes). Birds retain this foot formula, except the fifth (outermost) digit is lost entirely (23450)
So, we see that pretty much uniformly, the innermost digit is reduced. While no reference I checked actually indicates why this might be the case, after giving it a bit of thought, it seems reasonable that this would be the case. For a four-footed critter, falling over sideways is the biggest balance concern (also, because of the splay, the outermost section of the ‘hand’ will contact the ground first). As such, the outermost digits are going to be most important in preventing this (and would also be bearing the majority of the animal’s weight). The innermost digits, while not unnecessary in this regard, have less evolutionary impact, so to speak. Thus, they become reduced (the sarcopterygia, or ‘fleshy-finned fishes’, from which land-dwellers evolved had numerous, multi-boned ‘rays’ which supported their weight when they made their occassional forays onto land).
Fast forward to humans. We, of course, walk upright. Because we only have two points of contact, the entire foot becomes useful for supporting weight, rather than just the outermost parts. Thus, our big toe is comparatively rather longer than in other primates (who still largely walk on ‘all fours’). However, evolutionarily speaking, it is much more difficult to regain a feature than it is to lose it, so we don’t get extra bones - we are still stuck with two, which our distant ancestors had. When early hominids stopped using their forelimbs for locomotion, they could then turn their hands to other uses, allowing the thumb to become opposable, but again, not gaining any bones in the process.
Of course, the above is largely speculation, but I hope it at least sounds reasonable
:rolleyes: Maybe I should’ve been clearer on that.
You thumb doesn’t attach to your wrist, but near it. Thumbs have the same three joints as the rest of the fingers, they’re just harder to see.
Not so. The thumb does, indeed, attach to your wrist; however, it is offset a bit in humans (this is where much of the ‘opposableness’ comes in). The thumb is composed of 1 metacarpal and two phalanges (thus two joints, plus the wrist joint). All other fingers are composed of 1 metacarpal and three phalanges (thus three joints, plus the wrist joint).