Human Symmetry

June 16, 2004 10:38 PM

This is for the biologists. Humans tend to be very symmetrical, either having one thing often in the middle (nose, genitals, mouth), or two things evenly placed (eyes, nostrils, legs). That said, do humans have three of anything? (We have three ear bones, but in each ear.. totalling 6, so that fails!)
posted by wackybrit to science & nature (23 comments total)

There is very possibly an odd number of hairs on my head. Eventually, it might equal exactly 3.
posted by scarabic at 10:50 PM on June 16, 2004

three lobes in the right lung, as opposed to two in the left, i think.
posted by jheiz at 10:51 PM on June 16, 2004

Sometimes people have three nipples.
posted by onlyconnect at 11:51 PM on June 16, 2004

I like to have a three martini lunch.
posted by Kwantsar at 12:10 AM on June 17, 2004

I have three

oh shi...
posted by holloway at 12:16 AM on June 17, 2004

Also 3 lobes in prostate and pituitary and 3 types of granulocyte, not to mention the 3 layers of skin and the
three parts of the small intestine and sternum
posted by shoos at 12:18 AM on June 17, 2004

maybe you should ask this guy. Coincidence?
posted by obloquy at 12:20 AM on June 17, 2004

Actually, the symetrical nature not just of humankind but nature in general has always been a thought I have had as to why the triune Christian diety has been such a mystical object to those who believe.

My thought is that with a traditional bi-polar, parity diety, we can easily see the human example in everyday life simply by looking around but when the concept of The Father, The Son and The Holy Spirit/Ghost is introduced, there is no ready example that we can use as a frame of reference. I am reminded also of the Copenhagen Compromise when quantum physics was starting to make inroad and the old school scientists were bothered that there was no easy way to come up with a laboratory model to test these new laws.

The solution, offered by Niels Bohr if I recall correctly was pithily phrased as, "The menu is not the meal, the map is not the territory." Stating that no matter how well a model a newtonian scale experiment might be, it would never completely mimic its subatomic counterpart.

Not really related to the question at hand but something I thought worth mentioning.
posted by Dagobert at 12:54 AM on June 17, 2004

I think the Holy Spirit is a useful undefinable variable for Christianity. It's the -1^0.5 of the religion. It's a concept that exists to make other concepts easier to explain or define.
posted by wackybrit at 1:15 AM on June 17, 2004

It's worth noting that our internal organs are not symmetrical (although you already knew that).
posted by adrianhon at 3:29 AM on June 17, 2004

I wonder if anyone on the Raman version of Ask Metafilter asked if Ramans had only two of anything.
posted by bondcliff at 6:43 AM on June 17, 2004

Also, we are symmetrical in one plane (and only from the outside, asadrianhon points out), while there are an infinite number of planes on which we are not symmetrical, most notably a horizontal plane (you don't have two sets of legs, one attached at the shoulders).
Also, if you were to look at any of your cells, you'd find that they are completely unsymmetrical. In fact, during development, it is polarity, not symmetry that drives complexity. One of the first changes that the blastula undergoes is determining which group of cells is going to turn into your upper end, and which will turn into your lower end. After that, the part that will turn into your insides is differentiated from the part that will turn into your outsides.
So, really it isn't the symmetry that is interesting, or which drives things, but the polarity (or asymmetry).
posted by nprigoda at 7:10 AM on June 17, 2004

Imagine if Humans were radially symmetrical - like starfish. You'd be asking "Okay, so we've got five of most things - is there anything we have just two of?"
posted by kokogiak at 7:19 AM on June 17, 2004

Why the split between outside-symmetric/inside-assymetric, though? Is this anyway to design an organism? Sounds like the marketing people got into a fight with R&D.
posted by signal at 8:27 AM on June 17, 2004

my uncle has three kidneys. to the best of my knowledge, he is human.
posted by jeb at 10:15 AM on June 17, 2004

as far as i understand things, we don't have enough genes to control everything precisely. so genes tend to specify the big things and the environment within which the rest develops, getting emeergent order out of initial chaos for the detailed structures.

so making things symmetrical reduces the work needed (you don't need to code each side of the body individually in the genes) and is what appears in the basic structure. when it comes to tiny details, like the number of hairs, or the structure of the lungs, or the way the brain wiggles around, there's a lot of variation because it's built by a process that guides and exploits random variations.

but this only a vague impression from reading a couple of chapters of "the tangled wing" last night...
posted by andrew cooke at 10:21 AM on June 17, 2004

oh that was reply to signal. jeb posted while i was editing.
posted by andrew cooke at 10:22 AM on June 17, 2004

Why the split between outside-symmetric/inside-assymetric, though? Is this anyway to design an organism? Sounds like the marketing people got into a fight with R&D.

Seems more like gross-symmetric, versus local symmetric. I'm going to guess the gross bilateral symmetry is useful for ambulation. (faces, are another issue...)
posted by badstone at 11:04 AM on June 17, 2004

when the concept of The Father, The Son and The Holy Spirit/Ghost is introduced, there is no ready example that we can use as a frame of reference.

i always thought that the example was the family. Father, Mother, Child. People certainly can relate to that.

The way i heard it was Christianity inherited the trinity idea from the earlier nature religions it assimilated, which had female and male deities creating everything. But they couldn't put a female deity on the same level as God. hence- Holy Ghost.

i'm sorry, what was your question?
posted by Miles Long at 12:06 PM on June 17, 2004

oh, right, symmetry.

i would guess that symmetry is a simple way to get an organism that can balance itself and ambulate while not having to store a lot of data in the genes about how to build it. You just encode for one side, and then use 'copy mirrored'. or whatever the genetic equivalent command is.

probably not right. a little computer-o-centric.
posted by Miles Long at 12:15 PM on June 17, 2004

Symmetry, it's the way things have to be.
posted by five fresh fish at 12:22 PM on June 17, 2004

A man with three buttocks
posted by apathy0o0 at 2:48 PM on June 17, 2004

If I don't watch my diet I'm going to have three chins.
posted by dglynn at 8:59 PM on June 17, 2004