Sunday, January 16, 2011

The Pinnacle (and Nadir) of Evolution - Asymmetry

This blog was born out of interesting argument had by biologist Lynn Rothschild and James Hanken (seen at the very end of the talk here: about if bilateral symmetry was inevitable in multicellular life.  If so, and if other planets have multicellularity, it would have arisen elsewhere in the universe.  Let's do a quick review to make sure we understand what an awesome question that is and then talk about the how evolution even takes that complexity one step forward.

Arguably evolution follows a progression of body plans ascending in complexity, specificity, mobility, dexterity and difficulty of development.  




James Hanken argued that this progression naturally goes toward bilaterality, because predation is inevitable.  It's always easier to steal energy than it is to make it.  Once that threshold of predation is crossed an arms race between predator and prey will follow and the most logical defense is physical separation by movement.  At that point speed becomes essential and once speed is essential the simplest form of body plan is bilaterality since it deals with both gravity and directionality (up/down, back/front).  It's just plain good for speed since it's easier to be really fast in one direction than many.

An interesting, almost compromise, between radiality and bilaterality is echinoderms.  As plankton they're bilateral, both showing a little family history coming from bilateral organisms, and quite possibly a desire to be more mobile in the water column.  Later in life they'll settle down on the ocean floor and live a much more sessile life adopting a near radial make up usually consisting of 5 radial quadrants, which has other advantages like having spare parts, etc.

I'll just mention that speed isn't the only advantage to bilateral make ups.  Having a one way gut and your anus away from your mouth is helpful, too.  Flowers seem to like the idea of bilaterality, too.  It's been shown that the simplicity of radial flower symmetry (in may cases) is being exchanged for the specificity of bilaterality.

There's another step beyond even bilaterality that I think is often not appreciated - asymmetry.  I'll show how wide spread it is before commenting on the function and developmental difficulties.

Trilobites or should I say, trilobites.  The fossil record for trilobites show something odd.  There's a two to one preference for bite marks to appear on the right side of trilobite.  Numerous explanations abound - left could be more lethal, could have to do with the way they curl up, could be sampling bias, etc.  But, more likely than not, this is a very interesting example of ancient asymmetry, handedness.

Univalves- you might find articles like this that show how one sided asymmetry has major effects on predator/prey (snake/snail) interactions.  Snails that were spiraled the opposite way as most of their relatives got away since one particular snail eating snake species seem to always tilt their head one way when feeding.

Crustaceans - it's exceedingly common for crustaceans to have asymmetrical claws that serve different functions.

Giant Land crab

Flounders/Soles - Interesting picture showing how handedness and flounder/sole species go...hand in hand!!

Penises - animals with more than one penis (way more common than you'd think) have favorites.  See ear whigs.

Us!  Here's a short list of ways that we are intentionally asymmetrical (laying aside developmental abnormalities)

Heart - it's on the left side.  Why?  You tell me!

Lungs - the right lung has three lobes and the left lung has two.

Stomach/Spleen - left side.

Liver/Gallbladder - right side.

Colon/Intestines - the actors exit stage right.

Face - you've seen these, but they're cool and they do prove a point - our faces are asymmetrical and maybe for a good reason! (Or maybe not!)  People are also dominant eyed which has major ramifications for archers and sharp-shooters.

Testicles - many males typically have the left testicle hang lower.  Apparently, this has to do with the vascular attachment.

Brain - the asymmetry of the brain is so well known it even enters our vernacular with being 'right brained' or 'left brained'.  There is actually good science for this and the differences go much deeper than just functionality like many colloquial generalizations about the left brain's  logic and right's creativity and into actual structural differences like the planum temporale.  

It's also this brain sidedness that causes our hand handedness, which has considerable effects in our vocabulary (handedness, sinister, etc.), writing, instruments, desks, driving, guns, keyboards, video game controllers, scissors, catcher gloves, computer mouses, golf clubs, power tools, kitchen utensils, cameras...

It all comes down to specificity. It's better to have one really dominant hand that's extremely adept at tasks than two mediocre hands (since learning complex motions can take considerable time). Likewise, it's better two have two different kinds of claws as opposed to one. The body plan must fit the environment and tasks needed.
There's a reason asymmetry has taken so long evolutionarily to show up, though - it's difficult. You not only have to distinguish up and down like in radial symmetry, and front/back like bilateral symmetry, but also distinguish right and left - it's a whole other level of complexity.

Only as recent as 1995 have that we've started to get a grasp on the embryological steps that make it possible. The development of up/down in embryos could be something like gravity, front/back could be caused by a sperm entry point, but it takes something far more complex like chirality (handedness) of molecules to create intentional developmental asymmetry, which is much more difficult, hence why it's both the zenith of evolutionary complexity in body lay outs.

Pictures from here, here, here, here, here, here, here, here, here, here, here, and here.


  1. Typically the right hangs lower?? Call me atypical...

  2. haha! I just fact checked and it looks like you're within the majority! Normality restored! I corrected my mistake above! You were /right/!


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