[Geoff]

Geoff asked the Naked Scientists:
Just wondering why people are asymmetric - eg, their organs, the hemispheres of the brain, etc.

And given that they are, why are most people asymmetric in the same way?

Thanks
What do you think?

[RD]

Our brains are contra lateral, meaning that the left hemisphere controls the movement of the right side of our body and the right hemisphere controls the left side of our body. A left-handed person will therefore be right brain dominant while a right-handed person will be left brain dominant.

Studies have suggested that a 'lefty' may be more talented in areas thought to be controlled by the right side of the brain – spatial awareness, maths and architecture. Right-handers may have better verbal abilities because the left hemisphere of the brain is generally more efficient in processing verbal information. However, this theory is highly controversial and it is by no means set in stone. Other studies have shown that left-handers perform just as well in functions controlled by the left side of the brain.

Better in a fight?

What is known is that about 10–14% of the world population is left-handed and men are twice as likely to be left-handed as women.

Left-handers have long been persecuted but some scientists believe that they have at least one advantage over right-handers. They may be more likely to come out on top in hand-to-hand combat or sports like cricket or baseball.

 
A BBC One television series Secrets of the Sexes explores brain sex differences.
This may be because there are fewer left-handers, so when a right handed person (who is used to fighting mainly right-handers) encounters a left-hander, he is overwhelmed by the unfamiliar experience.

Or it may be that hormones play a part. There is a theory that, on average, left-handed people are exposed to higher levels of the male sex hormone testosterone in the womb. High levels of testosterone are linked with more assertive behaviour.

http://www.bbc.co.uk/science/humanbody/sex/articles/handedness.shtml

Reversal of organs, total: This condition (medically called situs inversus totalis) involves complete transposition (right to left reversal) of the thoracic and abdominal organs. The heart is not in its usual position in the left chest but is on the right. Specifically related to the heart, this is referred to as dextrocardia (literally, right-hearted). And the stomach, which is normally in the left upper abdomen, is on the right. In patients with situs inversus totalis, all of the chest and abdominal organs are reversed and appear in mirror image when examined or visualized by tests such as x-ray filming. Situs inversus totalis has been estimated to occur once in about 6-8,000 births. Situs inversus occurs in a rare abnormal condition that is present at birth (congenital) called Kartagener's syndrome.

http://www.medterms.com/script/main/art.asp?articlekey=5345

[Refractor]

Interesting reading, but how does the asymmetry come about?  When developing in the womb what determines 'leftness' or 'rightness'?  Why is situs inversus totalis such a rare condition?  Is there some evolutionary benefit in having most people have their organs on the same side?

[RD]

In a society where verbal communication is important there will be a evolutionary bias for
 left-brained/right-handed individuals, as left side of the brain is better at processing language.

It is advantageous to have two of something rather than one; one as a back-up if the other fails,
 e.g. two kidneys, lungs, gonads, hemispheres of brain, etc.

Where there are two of something asymmetry is inevitable.

Whether the heart is on the right or left is arbitrary, but some distant ancestor whose heart was on the left side must have had some associated, (but not necessarily consequential), benefit with this chirality which has caused virtually everyone to have this arrangement.

e.g. the gene which caused the heart to be on the left also contained the code for a beneficial feature which right-hearted ancestors did not have.