Why are our armspans usually almost exactly our height?

[Kerbface]

Is it just coincidence? Obviously this is not true for most other animals. You hear a lot about unusual mathematical relationships between the sizes of certain body parts. Is there any science behind why this occurs?

[Dkmdlb]

There are so many different relationships to choose from it's not surprising that some happen to line up fairly nicely. Even the arm/height thing isn't true for most people.

Most of the alleged unusual relationships between different bodily are based on nothing more than approximations or outright falsity.

[rodion_herrera]

Evolution favors creatures that do cartwheels, and with armspans equal to height, cartwheels are just WHEEEEEEEEEEE!

[Themohawkninja]

It probably has to do with balance. If your arms are the same length as you are tall, and given that it is all about the same density, you get a certain weight distribution, and therefor balance? (Maybe? I'm just guessing to be honest).

[PakledHostage]

In the sport of rock climbing, the ratio of arm span to height is known as Ape Index. There's some debate about whether or not having a higher ape index predisposes someone to being a better climber. If it is true that a higher ape index makes us better climbers then it stands to reason that we'd have long arms. After all, climbing (trees) was certainly important to our ancestors.

Maybe something like running, on the other hand, selects for shorter arms and the balance between the two contradictory pressures is that we have arm spans somewhere close to our height?

Just my WAG...

[Stochasty]

The is mostly just (somewhat informed) speculation on my part, but the one thing that humans do better than any other species on the planet is throwing things. Our shoulders evolved to be able to rotate rapidly through a large range of motion, our hands evolved to be able to dexterously grip and release a thrown object, our musculature evolved to provide maximum torque to our arms, and a portion of our brain evolved to be able to estimate and time trajectories. No other species on the planet comes close to our abilities in this regard; we're the ultimate in poo-flinging monkeys.

I rather imagine that our arm length is directly related to this. Too short, and it's difficult to build up sufficient velocity at the point of release. I don't know what the drawback of having arms that are too long would be, but it probably has something to do with balance during the throwing motion.

Edited June 12, 2013 by Stochasty

[ZingidyZongxxx]

Or it could be a total coincidence that we're all over-analyzing.

[Holo]

Or it could be a total coincidence that we're all over-analyzing.

Could be. But also, it couldn't.

[Dunbaratu]

The is mostly just (somewhat informed) speculation on my part, but the one thing that humans do better than any other species on the planet is throwing things. Our shoulders evolved to be able to rotate rapidly through a large range of motion, our hands evolved to be able to dexterously grip and release a thrown object, our musculature evolved to provide maximum torque to our arms, and a portion of our brain evolved to be able to estimate and time trajectories. No other species on the planet comes close to our abilities in this regard; we're the ultimate in poo-flinging monkeys.

I rather imagine that our arm length is directly related to this. Too short, and it's difficult to build up sufficient velocity at the point of release. I don't know what the drawback of having arms that are too long would be, but it probably has something to do with balance during the throwing motion.

I like this hypothesis. It *is* true that we are amazingly good at throwing compared to other species, but I'm not sure if that may be more attributable to brains than physical body traits. The amount of the brain that is highly active during physical coordinated actions is big and we don't even consciously notice all the lightning fast activity that has to occur beneath our surface thoughts for things like bipedal walking, and rock throwing, to work right. Throwing a rock accurately over a good distance requires quickly resolving objects in your vision into a mental map of the area, using that information to judge the distance and elevation to the target, feeding that into a subconscious guesser that determines how high above the target to aim depending on how fast we are throwing, updating that information in a loop as we feel our muscles carrying out the throw (to correct the aim it if the throw isn't at the anticipated velocity), and the parallax vision correction given that the eyeballs are not directly behind the rock. This is NOT trivial. People have tried programming robots to throw based on vision and it takes a LOT of computing power to pull it off.

[Stochasty]

It *is* true that we are amazingly good at throwing compared to other species, but I'm not sure if that may be more attributable to brains than physical body traits.

It's both. Not only is the software involved in throwing things amazingly complex, the hardware is quite specialized as well. For hunting purposes (which is the reason why these abilities evolved), you have to be able to throw something with both power and finesse. The mechanics behind generating power can be seen in the design of our shoulders, hips, elbows and musculature (just think of how much of the body goes into making a long distance throw), while the finesse comes from having a highly flexible wrist and an opposable thumb. These features are entirely lacking in the other higher apes, and at some point I saw a study suggesting that not even Homo Neanderthalensis could match us at throwing (due to a body built more for brute force, and hence a less articulate shoulder).

That said, again I should note that I'm just speculating here on the basis of things I've read in the past. I'm not a biologist, and this isn't exactly my area of expertise (although one of my professors when I was a grad. student was working on the physics of sports, so I learned some of this stuff from him).

[DisarmingBaton5]

It makes it easier to build rockets!

Sheesh. People over think things...