Symetry

[Jamin4422]

So, what is the symmetry content of DNA? How does it develop a m/2 symmetry of most lives?

"Over 99% of modern animals are members of the evolutionary lineage Bilateria. The evolutionary success of Bilateria is credited partly to the origin of bilateral symmetry. Although animals of the phylum Cnidaria are not within the Bilateria, some representatives, such as the sea anemone Nematostella vectensis, exhibit bilateral symmetry. We show that Nematostella uses homologous genes to achieve bilateral symmetry: Multiple Hox genes are expressed in a staggered fashion along its primary body axis, and the transforming growth factor–β gene decapentaplegic (dpp) is expressed in an asymmetric fashion about its secondary body axis. These data suggest that bilateral symmetry arose before the evolutionary split of Cnidaria and Bilateria."

​

Origins of Bilateral Symmetry: Hox and Dpp Expression in a Sea Anemone

 

[gnx1987]

Short answer: natural selection.

Long answer: random variation in the genes that organise basic body shape will, naturally, alter the body plan. If these alterations are better than the original, then those genes will be more likely to be passed on.

Exactly. So why aren't there more animals alive today with asymmetrical body shapes. Surely it isn't impossible for an animal with an asymmetrical body to survive.

I still also say it's impossible for something based on random mutations to be able to produce anything even close to symmetry. To believe that is equivalent to believing my coin flip analogy is possible. 10 may be possible after a thousand tries, but it would be more accurate to use a minimum 100 coin flips for the example. As in, it could never happen.

 

[The Engineer]

Exactly. So why aren't there more animals alive today with asymmetrical body shapes. Surely it isn't impossible for an animal with an asymmetrical body to survive.

Can't think of a purely asymmetric animal, probably because symmetric animals are simply more effective, which is why this trait is carried on.

If you want an animal without two symmetric sides, that's easy, however. Jellyfish, worms, those animals haven't got two symmetric sides. They aren't purely asymmetric, true, but how should pure asymmetry emerge? If the gene sequences tell a single cell to expand in every direction, a circular life form will emerge; that's nothing special, in fact, it's less special than a life form without any symmetry whatsoever.

I still also say it's impossible for something based on random mutations to be able to produce anything even close to symmetry.

No, it's not. All it has to do is to emerge in a few individuals, and then natural selection kicks in.

To believe that is equivalent to believing my coin flip analogy is possible. 10 may be possible after a thousand tries, but it would be more accurate to use a minimum 100 coin flips for the example. As in, it could never happen.

It would happen if you threw it a million times per month, for several thousand or million years straight.

 

[gnx1987]

Can't think of a purely asymmetric animal, probably because symmetric animals are simply more effective, which is why this trait is carried on.

But asymmetrical could still survive. There should be loads more if evolution was true.

If you want an animal without two symmetric sides, that's easy, however. Jellyfish, worms, those animals haven't got two symmetric sides. They aren't purely asymmetric, true, but how should pure asymmetry emerge? If the gene sequences tell a single cell to expand in every direction, a circular life form will emerge; that's nothing special, in fact, it's less special than a life form without any symmetry whatsoever. Just because they're round doesn't make them asymmetrical. Cut them in half you'll still have to mirrored halves.

No, it's not. All it has to do is to emerge in a few individuals, and then natural selection kicks in. But they have to emerge first. Which is impossible.

It would happen if you threw it a million times per month, for several thousand or million years straight.

No it wouldn't.
.....

 

[The Engineer]

But asymmetrical could still survive. There should be loads more if evolution was true.

How do you know asymmetrical would survive, in the long run?

Just because they're round doesn't make them asymmetrical. Cut them in half you'll still have to mirrored halves.

Do you even read what I write you?! Jellyfish and worms aren't asymmtrical, I never wrote that. What I wrote was that they aren't bilaterally symmetrical, which is true.
I also wrote that there's no reason why asymmetry would emerge in the first place. Point symmetry emerges when identical gene sequences code work outwards from a single point, which is the most probable scenario. It would also yield better results than pure asymmetry.

But they have to emerge first. Which is impossible.

Says who?

No it wouldn't.

Multicellular Life Evolves in Laboratory | Wired Science | Wired.com
According to this, the average rate at which bacteria divide is four time per hour. Lets say we start with two identical bacteria. After an hour you have 16 bacteria. After two hours, you have 256 bacteria. After a day, you have 79228162514264337593543950336 bacteria. A year, and you have 2^35040 bacteria.

If you flip a coin 2^35040 times, you will get the the same side ten times in a row, sooner or later. You will probably get it 100 times in a row. Do it for a million years, and you will get it 1000 times in a row.

EDIT:
Just calculated it. Over the course of a million years, you have 2^17520000000 bacteria. This is such an insanely high number, the calculator just flat-out tells me it's infinity.

 

[Elendur]

EDIT:
Just calculated it. Over the course of a million years, you have 2^17520000000 bacteria. This is such an insanely high number, the calculator just flat-out tells me it's infinity.

Hah, still not as high as my salary!
(For those interested, that was a joke )

Edit: Can also be written as ~10^(10^(9,722))

 

[NailsII]

But random can't create symmetry. Flip a coin just 10 times and the second 5 results aren't going to mirror the first 5. And evolution has loads more room for error.

I agree, randomness could not produce symmetry multiple times.
Evolution is not a purely random process.
In the same way that rules dictate how crystals form, they canform beautiful symmetrical patterns without the need to invoke a designer.

But again, if you look at animals, we are only superficially bilaterally symmetrical - so in reality your question is "how does evolution explain symmetry, except when it is not really symmetrical".

 

[Wiccan_Child]

Exactly. So why aren't there more animals alive today with asymmetrical body shapes. Surely it isn't impossible for an animal with an asymmetrical body to survive.

Indeed it's not. However, bilateral symmetry was a trait evolved by an early ancestor of ours, and we've simply inherited it all the way down. Since it evolved so very early on in the history of Animalia, it forms one of the biggest subdivisions of animals, along with Cnedaria (jellyfish and the like) and Ctenophora (tiny animals a few cells thick).

So, why are so many animals bilaterally symmetric? Because it was a trait evolved by one of the first sub-divisions of animals, and has been inherited by its descendants. These descendants include everything from dragonflies to komodo dragons.

Non-bilaterally symmetric animals do exist (starfish, jellyfish, sponges, etc), but bilateral symmetry is something that evolved so early on in our history as animals, that the majority of animals have it.

I still also say it's impossible for something based on random mutations to be able to produce anything even close to symmetry. To believe that is equivalent to believing my coin flip analogy is possible. 10 may be possible after a thousand tries, but it would be more accurate to use a minimum 100 coin flips for the example. As in, it could never happen.

Your analogy leaves out two crucial components of evolution: reproduction, and natural selection. Evolution can do what it does because a) species reproduce, b) reproduction causes variation, and c) natural selection favours some variations over others. Your analogy only factors in randomness, and falsely assumes that genes are composed wholesale in one grand swoop by a random coming-together of chemicals. They're not.

If you were able to flip 100 coins, then flip 100 sets of 100 coins, and pick which of those 100 sets most closely matches the original 100, hold their similarities constant and flip whatever coins are left (in other words, there's a selection pressure that preferentially selects some coins over others to reproduce into the next generation), then that'd be more accurate.

And, of course, the left and right hand side of the body don't match up by sheer chance, there's essentially a genetic mechanism which says "and now do exactly the same on the right".

My snowflake analogy holds here, too: the water molecules on one arm of a snowflake have no idea what the molecules on the other arm are doing, yet they form beautiful hexagonal symmetry, all by the sheer random motion of energetic atoms. How? Because they all work in the same conditions under the same rules.

 

[DaneaFL]

Considering that we've observed the natural, spontaneous formation of DNA-like double helix structures, it's not amazingly improbable that we are naturally bilaterally symmetrical...

http://www.lci.kent.edu/ipp/pdf/20chien.pdf

Oh, and snowflakes... Does God mold each snowflake in His hands or is it just natural chemistry?

Why should our bodies be any different?

 

[juvenissun]

As large portions of our body are not symmetrical, I don't see how your question has any meaning.

Embryology explains the overall symmetry, maybe you should add that to your 'must read' set of subjects.

That is a good point. We have m/2 class external form. But our guts have no symmetry. If the nature of symmetry is controlled by genes, why is there such a difference? Why are our organs not all symmetrical like the lungs? If not, then why are the lungs symmetrical?

 

[juvenissun]

The genes just produce proteins when they're turned on. What turns them on is a cascade of signalling molecules in the developing embryo, starting with the fertilized egg. That's where the symmetries of the body plan are generated, not in the DNA for the genes. Roughly speaking, there is a gradient of a signalling molecule across the embryo. At the center of the embryo, the level of a signal is high enough that genes responsible for starting limb formation are turned off. Toward the edge of the embryo, they're low enough that those genes can be turned on -- provided other switches controlled by other signalling molecules say they should be. Those other signals tell how close the cells are to the head of the embryo, and whether they are near the front or the back, and probably lots of other things that I don't know about.

(Note: this is a cartoon version from someone who does not know developmental biology at all well. But that's the basic idea.)

Thanks. That IS the level of explanation I need.

It is amazing that you were a physicist. How did you learn all these?

 

[Mr Strawberry]

Thanks. That IS the level of explanation I need.

It is amazing that you were a physicist. How did you learn all these?

There are these things called books...

 

[Wiccan_Child]

There are these things called books...

Or Google.

Still, at least he's willing to learn; that's more than most Creationists on here will do.

 

[juvenissun]

There are these things called books...

Why don't you understand geology much? I can give you a copy of geology 101 textbook.

 

[CabVet]

Why don't you understand geology much? I can give you a copy of geology 101 textbook.

The paragraph above has no relation to the exchange below. How can you conclude that he doesn't understand geology from this?

How did you learn all these?

There are these things called books...

 

[Mr Strawberry]

Why don't you understand geology much? I can give you a copy of geology 101 textbook.

Your need is clearly greater than mine.

 

[juvenissun]

The paragraph above has no relation to the exchange below. How can you conclude that he doesn't understand geology from this?

Not only him, but you too.

 

[Elendur]

Not only him, but you too.

That didn't answer the question.

 

[NailsII]

That is a good point. We have m/2 class external form. But our guts have no symmetry. If the nature of symmetry is controlled by genes, why is there such a difference? Why are our organs not all symmetrical like the lungs? If not, then why are the lungs symmetrical?

In evolutionary terms, the intestinal tract is one of the oldest structures of animals - and I'll stick my neck out here, waiting to be shot down by someone who knows more, but I believe that it evolved before bilaterism and so may not be under it effect.
In embryology, the GI tract grows from the anus to the stomach and is also on of the first structures to develop.

This may explain the difference - it is made by different genes, which function earlier.

I don't think the lungs are symmetrical, they most certainly are not in snakes.

One major point which is missing from this thread, is that being roughly the same on both sides externally is a massive advantage for locomotion.
Could you imagine how a fish like Nemo would really swim?

 

[Jamin4422]

That IS the level of explanation I need.

Development is controled by the Hox Gene. I tried to figure out once what it would take to get people to grow a new set of teeth. It would be easy to grow a tooth. The difficult part would be a set of teeth. Even sometimes something goes wrong somewhere and they come out crooked. Still when people are 12 years old, their temps fall out and they get a new set of teeth.