How does natural selection account for the symmetry of the human body?

In a typical human , the following similarities may be quickly discerned:





The corresponding fingers of each hand (on the OPPOSITE side of the body) are roughly identical in length and shape; the same relationship exists concerning the size and shape of the hands, arms, shoulders, ribs, legs, knees, ankles, feet, toes, eyes, ears, and nostrils.





Additionally, the eyes and ears are located in the same relative positions on both sides of the face. Also, the corresponding teeth in the upper and lower jaw are roughly identical in structure.





What accounts for this symmetry?

How does natural selection account for the symmetry of the human body?
Well, I hope you notice that this symmetry doesn't just apply to the human body ... but practically *all* animals ... all primates, all mammals, all terrestrial vertebrates, even fish.





In other words, it is all explainable by common ancestry tracing back to an early ancestor that had a symmetrical body plan. So natural selection obviously has some role in preserving this symmetry throughout evolution.





The natural selection explanation is that bilateral symmetry provides some benefit ... and so there has never been much pressure to change it. It is useful because it is economical during development of the embryo. The same genetic instructions for building the right hand, will also build a left hand. The instructions for one eye can build the other. Instructions for building the left-side of the rib cage, also builds the right.





A more interesting question is why almost all animals have this left-right (bilateral) symmetry, but don't have dorsal-ventral (belly vs. back) symmetry, or anterior-posterior (mouth vs. rear) symmetry. The answer is that these other kinds of non-symmetry are themselves beneficial.





Dorsal-ventral non-symmetry (the fact that the belly-side of most animals is different from the back-side) traces back to our ancestors that either swam or crawled with belly-side down and back-side up ... so and the dangers from above are different from the dangers from below. Incidentally, there is absolutely no reason this has to be true for we humans who walk upright ... there's no reason our belly is soft and back is hard ... except that this has some benefit for animals that walk on all fours.





Anterior-posterior non-symmetry (the fact that the front of the animal is different from the rear) traces back to our aquatic ancestors, where one end had a mouth, and the other end was for excretion ... so there was obvious benefits to keeping them at opposite ends of the animal.





However, there have never been benefits to left-right non-symmetry ... animals are just as likely to get attacked from the left as from the right ... or to find food on the left more than the right. So the benefits of economy in embryology won out.





Hope that helps.
Reply:Thanks! I was worried that it was too long. But symmetry is a *really* interesting topic.





Star for the *terrific* question. Report It

Reply:Great question, fantastic answer! Report It

Reply:GOD deserves the credit and the glory you atheist.








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Reply:get a life, dave, and go back to your R%26amp;S board Report It

Reply:tHE SYMMETRY of all animal body help us to survive, for example we are more attractive to our mates
Reply:it can't account for that. because natural selection cannot happen without a guiding intelligence.
Reply:I find I must agree with Harry!





Brilliant answer! Concise, pithy, and brilliant.
Reply:the features that allow us to survive are kept and those that dont have those features die off.survival of the fittest.if offspring have a good physical change the offspring survives and passes that gene or feature onward
Reply:The same power that makes sure everything has a perfect mate (otherwise the speces would die out).
Reply:Yep, there is NO way, we were created by chance. We were created by God's brilliant mind.
Reply:Never thought about it before, but now I wonder if it has something to do with we are made in the mirror image of God? Not that that makes sese either, but it is the best I can come up with. That and possibly a balance issue.
Reply:Well, it all goes back to which was first, the chicken or the egg. People just don't get it.... there is only one answer and that is the master engineer, God, made us the way we are, no matter who wants to believe it or not. You simply did not come from nothing, God, put us here, created a perfect creation with a perfect matching mate, and gave purpose to our life. The only thing we have to do as humans, is to find it.
Reply:GOD's design , of course.





( aint it obvious )





Not only that if you look at a woman's body you see pure and perfect intelligent design by GOD.





( every square inch poetry in motion )





( women %26gt;%26gt;%26gt; GOD's last, highest, and BEST creation )








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Reply:Symmetry happened pretty early.


Two of most things was enough and more unnecessary.


As for maintaining it would you choose an asymmetrical mate, (all other things being equal)?
Reply:Intelligent Design, dictated by logic.


I Cr 13;8a





Hi Kidd!



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