7
$\begingroup$

Although the human body is mostly left-right symmetrical, it consistently has asymetries. The heart is consistently slightly to the left side of the body. The liver is consistently on the right side.

This is quite perplexing when we consider that the human body has developed from a single cell. It seems like a single cell is mostly left-right symmetrical. A single cell could be oriented any which way in the womb, and it is unclear how the developing body would "know" which way is left or right.

So, how is it that the body develops the clear assymetry. Can anyone explain this interesting phenomenon in layman's terms?

$\endgroup$
  • 2
    $\begingroup$ You are hitting nerve of many biophysics researchers here :) This phenomenon is called Symmetry Breaking is part of active research. $\endgroup$ – Dexter Jul 26 at 14:21
4
$\begingroup$

Developing symmetry is is just a matter of Homeotic genes creating axial patterning, genes that produce a chemical gradient across a cell or group of cells. This is how developing organisms orient individual cells, up, down, left, right, anterior, posterior, medial, lateral, are all just gradients of different chemicals, the cells can detect the difference in concentration one one side or the other to orient itself. Most of these start getting produced when the embryo is only around a few cells so there is not much room for ambiguity. Later more homeotic genes are added to create gradients for things like segments and limbs. Note polarity is often exhibited in a single cell, even in single celled organisms, this is a chemical gradient that orients the inside of the cell, this can easily be turned into orientation of dividing cells.

As for why things move away form this symmetry that's easy, evolution can mostly only work with what it has, evolution is about good enough not perfection. Often good enough creates a kluge, a rube goldberg type solution then descendants then get stuck with. The earliest vertebrate ancestor pikaia is completely symmetrical even down to the gut, A fish is nearly perfectly symmetrical, with the only deviation being to increase the length of the gut without increasing the length of the fish, which would waste a lot of material The solution is to coil and pack the intestines instead of having a straight line. This does not just happen in vertebrates you can see these solutions again and again in mollusk, insects, and even echinoderms, keeping the body compact while making the gut longer means asymmetry. As Some organisms progressed further and further away form the "fish" body plan alteration would result in asymmetry. Lungs for instance force a big asymmetry because you are taking what was a simple one way path into two divided systems, blood now has to return to the heart twice before making a complete circuit. More energetic animals specialize even further abandoning or partitioning part of the circulatory system to close off these two parts entirely so they can have high pressure blood flow without exploding their lungs. Now asymmetry is not a bad thing limbs and sense organs stay symmetrical because there is a stronger evolutionary pressure to keep them symmetric, moving with asymmetric limbs wastes a lot of energy (try running with only one shoe on).

enter image description here

Image sources.

$\endgroup$
  • 1
    $\begingroup$ Sorry but I don't think this is right answer. It is not matter of 'just' Homeotic genes. There are lot of factors which plays important role in symmetry breaking even before these genes are expressed. One of the earliest symmetry breaking even is asymmetrical cell division (long before even tissues are formed). There are lots of hypothesis currently which explains this including maternal mRNA deposition, cilia rotation, external chemical gradient, uneven protein distribution etc. $\endgroup$ – Dexter Jul 26 at 19:37
  • 1
    $\begingroup$ This question could easily be construed as too broad if addressing every form of symmetry and asymmetry in biology. I was trying to restrict myself to the specific examples the OP gave. vertebrate symmetry and organ asymmetry. If you want to address other occurrences feel free to create an answer. $\endgroup$ – John Jul 26 at 20:08

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.