I am curious, we are fairly symmetrical. With: Two eyes; ears; arms; legs; lungs; hemispheres of our brain. But when it comes to our internals: Heart: stomach, liver, etc. we loose the symmetry. Is there an answer as to why?
the simple answer is that there is no actual answer. Evolution by natural selection is a random process in where the more fit are more likely to survive and propogate. We are interally asymmetrical assumingly because we were more fit that way, and/or no random mutation for a more fit symmetrical version of humans survived/existed.
Well, if you require stuff like the heart, stomach, pancreas etc. to be internally symmetric, theres only one place you can put them which restricts how much stuff you can pack into the body. OTOH, if they aren’t you can pack them tighter.
The simple answer is embryological:
During development, the internal organs are pretty bilaterally symmetric – the intestines are a single midline structure, the liver is relatively midline, and the paired organs (kidneys) are symmetric. However, as the gut (digestive tract) expands, it literally outgrows the abdoment – it extends out through the region of the umbilicus.
Later, it is drawn back into the abdomen, it rotates, so the intestines are “loosely” coiled in loops (usually clockwise, as you face the patient). You can see this most clearly at the beginning (the initial leftward curve of the stomach) and the end (the 3/4 circle clockwise sweep of the large intestine)
The development of the heart and aorta follows a similar coiling process that turns a muscular tube into a coil, which fuses, and open and closes foramina (openings) in several stages to form the four chambers.
If you examine the lungs, you will see three lobes on the right, and only two lobes on the left, but both lungs originally had three lobes. The middle lobe of the left lung remains as “the lingula” - a vestigial lobe that never fully expands, because the heart, moved to the left by the curling if its development takes up the space the Left middle lobe would ordinarily occupy.
There are many other directional processes in embryology. The causes and roots are complex, but the preferred direction of ciliary movement in cells (cellular microtubles rotate clockwise) may be the most important to your question. There is a condition called Ciliary Immotile Syndrome where cellular cilia don’t operate correctly (leading to lung problems, because in adults, cilia sweep dirt and mucus out of the lungs). Half these patients have their hearts on the right [dextrocardia] and many have their abdominal organs reversed as well (or instead). It’s as if the coiling process, deprived of cell cilia motion, had to pick a random direction.
Like most of my posts, this was probably more than you wanted to know. I keep expecting a Mod to tell me to shut up.
I have been informed, thanks. [Slight highjack] So, these people with their hearts on their right, do they survive? Are there any dopers with their hearts on the other side? I didn’t want to appear to ungooglie, so I found a site: (using google)
Thanks for your insight.
Theres a very rare disese where your internal organs are flipped the other way around and people with it live perfectly normally.
There are many accidentally discovered cases of dextrocardia (heart only) and situs inversus (reversal of the abdminal organs with or without the heart, depending on whose definition you use). Often, they didn’t have any symptoms prior to the discovery.
However, many cases of dextrocardia are caused by conditions like Ciliary Immotile Syndrome which cause problems unrelated to the reversal of organs. Since the organ orientation seems randomly chosen in CIS, we have plenty of right- vs. left- sided CIS cases to compare. There’s little difference in health.
Some texts use the term “Kartagener’s Syndrome” for CIS with situs inversus, but I was taught (apparently wrongly) to use CIS and Kartagener’s interchangeably. It’s a good search term if you want to learn more. So is situs inversus.
External symmetry is a sign of health. Healthy creatures are more likely to appeal to potential mates, and therefore more likely to procreate.
Internal asymmetry is generally not apparent to a potential mate, and is therefore not relevant in determining reproductive success.
I wish you had taught my biology class. Great post!
Evolutionarily speaking, asymmetry is likely born out of necessity. Things stayed symmetric until there was an advantage to being asymmetric. Abandoning the simplicity of symmetric for the flexibility asymmetry provides. Other animals are more symmetric than us. And the ways in which they are more symmetric they are also less specialized.
I’ve heard that identical twins are usually mirror images, externally and internally. (Actually, I’m not sure whether that should be always, usually or sometimes, or whether it’s just a legend.) One of the **Lord Peter Whimsy ** mystery stories was based on a man with his heart on the wrong side, and an episode of Quincey too.
Conservation of "e"s.
The evolutionary pupose of this is so you can tell with a simple physical exam which is the evil one.
If this were true, what happens to identical triplets? Is the third one internally symmetrical?
Ian Fleming’s Dr. No had this condition, and it was a minor plot point in an episode of CSI, which prompted Grissom to make a ref to Dr. No.
About 25% of identical twins exhibit some mirror characteristics. Handedness is a common manifestation. To have everything mirror inside and out is rare.
It’s not really a disease then is it, if it doesn’t affect your health?
I’m curious now. Are people with this condition (reversed internal structure) unsuitable for receiving organ donations? And likewise, unsuitable to donate their organs?