2007-01-11 02:45:31 · 7 answers · asked by Jonathan 2 in Science & Mathematics ➔ Biology
Having the ability to spin our wrist is the result of having two lower arm bones, not the other way around.
The two bones in the distal joint of our limbs (radius and ulna in arm, tibia and fibulla in leg) are a shared derived characteristic that is found in all tetrapod vertebrates, from frogs and salamanders to lizards, dinosaurs, birds, rats, horses, elephants and even whales.
The character is an evolutionary legacy from our Rhipidistian Crossopterygian fish ancestors. This limb bone arrangement (and the condition of having five digits) was inherited from these air-breathing fish that gave rise to the entire tetrapod lineage.
The bones can be traced back to the bony ossicles that formed to support the lobe-fin of those Crossopterygians - one large ossicle close to the body, and two down closer to the fin itself.
It is currently thought that these bones formed by branching - a minor developmental mutation that causes a straight set of segments (as seen in lungfish fins, for example) to split into two sections - which is why the number of splits increases as we go down the limb.
2007-01-11 03:37:24 · answer #1 · answered by Anonymous · 0⤊ 0⤋
I can't say I know for sure, but along the lines of what has been said already, it's because of the joints in our arm.
Our upper and lower arm both have the ability to spin. The upper arm can acomplish this in a controlled way by having a ball on the end which fits into a socket (your shoulder). This takes up space, and that's fine when you're attached to a torso, but to get this same motion in the elbow, we use two parallel bones. This way the attatched muscles can pull on the bones to get a turning or twisting motion.
Again, just a guess.
2007-01-11 03:06:03 · answer #2 · answered by Gerfried 2 · 0⤊ 0⤋
Interesting question....I am guessing here, (obviously) maybe it is because we don't use our upper arm like we do our lower but just think of what we could do if we had more mobility in our upper arms...maybe we could scratch out own backs!
2007-01-11 02:53:49 · answer #3 · answered by Red! 2 · 0⤊ 0⤋
the ones in your lower arm are disigned to spin your wrist
2007-01-11 02:53:33 · answer #4 · answered by Chris the Dude 2 · 0⤊ 0⤋
I think it's so that you can rotate your wrist, even though the elbow is a hinge joint.
Not suer.
I'm putting this one on my watch list!
2007-01-11 02:52:14 · answer #5 · answered by _Jess_ 4 · 0⤊ 0⤋
To articulate the wrist amongst other things.
2007-01-11 03:03:46 · answer #6 · answered by solara 437 6 · 0⤊ 0⤋
Why do we have two in our legs? Probably the same reason
2007-01-11 02:51:11 · answer #7 · answered by cleosmaster123 2 · 0⤊ 0⤋