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All mammals that I can think of have a high degree of bilateral symmetry (In fact, almost every animal I can think of is like this).

So why is the human heart not exactly in the middle of the body? An effect of this is that one lung is slightly smaller. Are there any evolutionary theories on why this came to be?

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    $\begingroup$ You could ask that about all organs of which there are only 1. $\endgroup$ – Richard Tingle Jun 14 '17 at 12:27
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    $\begingroup$ @RichardTingle Not exactly all. The brain, esophagus, nose, and even skin itself all maintain pretty good bilateral symmetry. The stomach, liver, spleen, etc. might make good questions but could get closed as too broad if done all at once. $\endgroup$ – DrZ214 Jun 14 '17 at 12:37
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    $\begingroup$ The reason you think your heart is off-centre is that, when you put your hand on your chest and feel your heart beating, you're actually feeling the blood pulsing through your aorta. Your actual heart is hidden behind your breastbone so you can't feel it. $\endgroup$ – David Richerby Jun 14 '17 at 14:42
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    $\begingroup$ @DrZ214: It could easily be argued that we really have two brains, in much the same way that we have two lungs. But we only have one liver... $\endgroup$ – jamesqf Jun 15 '17 at 6:13
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First of all, let me make it clear that the heart is at the vertical centre of the body -- it is not shifted towards left (or right). However, it is slightly tilted towards the left in most cases.

heart location

In some cases, it is tilted towards the right, and the condition is called Dextrocardia. For why it is so, lets look at what the heart does. Below is a diagram of double circulation (from here).

double circulation

As you see, the highest pressure needs to be generated for pumping oxygenated blood into the body. Thus, the left ventricle needs the thickest muscles for this purpose. And due to these extra muscles, the heart appears extended and seems shifted towards left.

Coming to the evolutionary perspective, it is important to mention that humans are not the only organisms with this feature. Indeed, displacement of the heart towards the left is a conserved feature in all vertebrates (Fishman et al, 1997). See this answer for more information.

Coming to genes, bending of the heart towards one side is actually controlled by the NODAL gene during development. See this diagram (from Jensen et al, 2013):

heart development

Tilting occurs in two phases, one during the first four and a half months of intrauterine life and the other, which is actually a 45° rotation to the median plane, later. During the early development of the heart, a process called cardiac looping happens and the straight heart tube develops a bend (see diagram). The NODAL gene, along with the Lefty1 and Lefty2 genes, regulates the speed and direction of cardiomyocyte movement during the development of the heart, leading to this asymmetry. To confirm it, researchers knocked out the spaw/nodal gene from a zebrafish and found randomized development of heart, even symmetric heart, as the result(!) (see Walmsley, 1958 and Rohr et al, 2008).

Now, talking about why this happened in the first place, and why it is so conserved among vertebrates, we need to ask ourselves a basic question: what good would a symmetrical heart be? External symmetry is preferred (probably) because it helps in locomotion; it would be quite difficult to move with your two legs placed away from your center of gravity. But when we talk about internal symmetry, conditions drastically change. We get a major restrictive factor here: space. And limited space always dominates other factors. Seeing that the structure of the heart is necessarily pointed towards one side, it becomes difficult to make it symmetrical. (The only option IMO is to have another pointed end at the right side.) In this case again, what advantage would a symmetrical heart provide? None. And it might even be harmful since having an even bigger heart would mean making both lungs smaller. Thus, a symmetrical heart would only prove to be a liability rather than an asset. See this question for more information.

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    $\begingroup$ This is a pretty amazing answer, even giving detail of heart development in the womb. Thanks! If you ever come across an evolutionary theory explaining this, please come back and link to it. $\endgroup$ – DrZ214 Jun 14 '17 at 7:04
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    $\begingroup$ Our bipedal stance would also come into play. the torso is in a vertical position with respect to gravity, as opposed to the regular horizontal, so the pumping would be more difficult against other vertebrates of our size. How is the poisitioning in the great apes? $\endgroup$ – user001 Jun 14 '17 at 10:58
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    $\begingroup$ After almost 10 years in academic biology, I stumble across this question and learn that the heart is in the middle of the torso. Thanks, my mind is blown (and +1 of course). $\endgroup$ – AlexDeLarge Jun 14 '17 at 11:18
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    $\begingroup$ "it is not shifted towards left (or right). However, it is slightly tilted towards the left in most cases" - Clearly, the center of mass of the heart is off-center because of that tilt. Sufficiently such that the left lung is smaller than the right, and has a different pattern of vascularization. I do still think it is more centered than most people conceptualize, though, so I agree with the overall point, and your description of the development is excellent, thank you. $\endgroup$ – Bryan Krause Jun 14 '17 at 19:33
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    $\begingroup$ @bryankrause yeah, certainly it is not in exact middle if you see it from average mass, but also certainly it is not off-center if you see it anatomically...there can certainly be confusion about this ;) $\endgroup$ – another 'Homo sapien' Jun 15 '17 at 1:17
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1- Bilateral symmetry means two mirror image halves following sagittal section only along a central axis.In animal kingdom there are examples of no symmetry (amoeba) , radial symmetry (jelly fish) , biradial (comb jelly and bilateral (human - though not exhibiting perfect bilateral symmetry because of heart, gall bladder and liver) Heart is not symmetrical on account of its functions -the left ventricle is thicker compared to the right).The smaller Left lung could be the result of a varying functional load associated with oxygenation of the blood.

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protected by Chris Jan 14 at 12:15

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