Thy fearful symmetry

Everyone knows that most living things are symmetrical, and the vast majority of them bilaterally symmetrical (i.e. a single line down the midsection divides the organism into two mirror-image pieces).  A few are radial -- where any line through the center point divides it in half -- such as jellyfish and sea anemones.  Even symmetrical organisms like ourselves aren't perfectly so; our hearts and spleens are displaced from the midline toward the left, the appendix to the right, and so forth.  But by and large, we -- and the vast majority of living things -- have some kind of overall symmetry.

True asymmetry is so unusual that when you see it, it really stands out as weird.  Consider the bizarre-looking flounder:

[Image licensed under the Creative Commons Peter van der Sluijs, Large flounder caught in Holland on a white background, CC BY-SA 3.0]

Flounders start out their lives as ordinary little fish, upright with symmetrically-placed eyes, fins, and so on.  But as they mature, their skulls twist and flatten, and they end up with both eyes on the same side of the head -- a great adaptation for a fish that spends its life lying flat on the seabed, and who otherwise would constantly have one eye pointing downward into the mud.

A question I've asked here before has to do with the constraints on evolution; which of the features of life on Earth are so powerfully selected for that we might expect to see them in life on other planets?  (An example of one that I suspect is strongly constrained is the placement of the sensory organs and brain near the front end of the animal, pointing in the direction it's probably moving.)  But what about symmetry?  There's no obvious reason why bilateral symmetry would be constrained, and it seems as if it might just be a holdover from the fact that our earliest ancestors happened to be bilateral, so we (with a few stand-out exceptions) have inherited it down through the eons from them.

What about symmetry in general, however?  If we went to another life-bearing planet, would we find symmetrical organisms, even if they differ in the type of symmetry from ours?

The answer, judging from a paper that appeared this week in Proceedings of the National Academy of Sciences, by a team led by Iain Johnston of the University of Bergen, appears to be yes.

What Johnston and his team did was analyze the concept of symmetry from the perspective of information theory -- not looking at functional advantages of symmetry, but how much information it takes to encode it.  There are certainly some advantages -- one that comes to mind is symmetrically-placed eyes allows for depth perception and binocular vision -- but it's hard to imagine that's a powerful enough evolutionary driver to account for symmetry in general.  The Johnston et al. research, however, takes a different approach; what if the ubiquity of symmetry is caused by the fact that it's much easier to program into the genetics?

The authors write:

Engineers routinely design systems to be modular and symmetric in order to increase robustness to perturbations and to facilitate alterations at a later date.  Biological structures also frequently exhibit modularity and symmetry, but the origin of such trends is much less well understood.  It can be tempting to assume—by analogy to engineering design—that symmetry and modularity arise from natural selection.  However, evolution, unlike engineers, cannot plan ahead, and so these traits must also afford some immediate selective advantage which is hard to reconcile with the breadth of systems where symmetry is observed.  Here we introduce an alternative nonadaptive hypothesis based on an algorithmic picture of evolution.  It suggests that symmetric structures preferentially arise not just due to natural selection but also because they require less specific information to encode and are therefore much more likely to appear as phenotypic variation through random mutations.  Arguments from algorithmic information theory can formalize this intuition, leading to the prediction that many genotype–phenotype maps are exponentially biased toward phenotypes with low descriptional complexity.

Which is a fascinating idea.  It's also one with some analogous features in other realms of physiology.  Why, for example, do men have nipples?  They're completely non-functional other than as chest adornments.  If you buy intelligent design, it's hard to see what an intelligent designer was thinking here.  But it makes perfect sense from the standpoint of coding simplicity.  It's far easier to have a genetic code that takes the same embryonic tissue, regardless of gender, and modifies it in one direction (toward functional breasts and nipples) in females and another (toward non-functional nipples) in males.  It would take a great deal more information-containing code to have a completely separate set of instructions for males and females.  (The same is true for the reproductive organs -- males and females start out with identical tissue, which under the influence of hormones diverges as development proceeds, resulting in pairs of very different organs that came from the same original tissue -- clitoris and penis, ovaries and testicles, labia and scrotum, and so on.)

So symmetry in general seems to have a significant enough advantage that we'd be likely to find it on other worlds.  Now, whether our own bilateral symmetry has some advantage of its own isn't clear; if we landed on the planets orbiting Proxima Centauri, would we find human-ish creatures like the aliens on Star Trek, who all looked like people wearing rubber masks (because they were)?  Or is it possible that we'd find something like H. P. Lovecraft's "Elder Things," which had five-way symmetry?

And note that even though the rest of its body has five-way symmetry, the artist drew it with bilateral wings. We're so used to bilateral symmetry that it's hard to imagine an animal with a different sort. [Image licensed under the Creative Commons Українська: Представник_Старців (фанатський малюнок)]

So that's our fascinating bit of research for today; coding simplicity as an evolutionary driver.  It's a compelling idea, isn't it?  Perhaps life out there in the universe is way more similar to living things down here on Earth than we might have thought.  Think of that next time you're looking up at the stars -- maybe someone not so very different from you is looking back in this direction and thinking, "I wonder who might live on the planets orbiting that little star."

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Thy fearful symmetry

For some of the most fundamental aspects of life, it's uncertain whether or not evolution was constrained.

This has huge implications for the search for extraterrestrial life, and whether or not we'd recognize it if we saw it.  One I've dealt with here before is the fact that terrestrial life is based on carbon -- but is that necessarily true everywhere?  Sure, carbon's pretty cool stuff, with its four snazzy valence electrons and all, but maybe there are other ways to build functional organic molecules.

What about oxygen?  Even here on Earth, we have living things that get by just fine without it; they're the anaerobes, and include such familiar fermenters as yeast and Lactobacillus acidophilus (the bacteria responsible for yogurt), and such bad guys as the causative agents of tetanus, botulism, and gangrene.  Being aerobic certainly seems like a great innovation -- it increases the efficiency of a cell's energy utilization by a factor of 18 -- but it certainly isn't a requirement.  In fact, probably the most common life form on Earth, individual for individual, are methanogens -- deep sea-floor bacteria that metabolize anaerobically and produce methane as a waste product.  By some estimates, methanogens may outnumber all other living things on Earth put together.

So maybe anaerobic respiration isn't as efficient as aerobic respiration, but apparently it works well enough.

There are other features that deserve consideration, too.  How many of the things we take for granted about animal life are ubiquitous not because they were the result of strong natural selection, but simply because one of our ancestors had those features and happened to be the one that survived?  I'm guessing that having the sensory organs, central processing unit (brain), and the mouth clustered together at the anterior end of the animal will turn out to be common; it makes sense to have your perceptive equipment and your feeding apparatus pointing basically in the direction you're most likely to move.  And speaking of movement, that's probably going to turn out to be fairly uniform everywhere, because there aren't that many ways to fashion an appendage for walking, flying, or swimming.

But what about symmetry?  The vast majority of animals are bilaterally symmetric, meaning that there's only one axis of symmetry that divides the animal into mirror-image halves.  (A few have radial symmetry, where any line through the center works -- jellyfish being the most obvious example.)  Even animals like starfish, that seem to have some weird five-way symmetry, are actually bilateral, which is obvious if you look at starfish larva, and in fact is given away by the position of the sieve plate (the opening through which they draw in water), which is off-center.

True multiple-line symmetry doesn't seem to exist in the animal world, and even in science fiction most aliens are depicted as being nicely bilateral.  An exception are the Antarctic Elder Things, an invention of H. P. Lovecraft, which have pentaradial symmetry, if you don't count the wings -- further illustrating that as unpleasant a person as Lovecraft evidently was, he had a hell of an imagination.

[Image licensed under GNU Free Documentation; original available at http://vixis24m.deviantart.com/art/The-Elder-Thing-39576904]

So are most animals bilateral because it's got some kind of selective advantage, or simply because we descend from bilateral creatures who survived well for other reasons?  In other words, is it selected for, or an accidental neutral mutation?

The reason all this comes up is because of a discovery in South Australia described in a paper that came out this week in Proceedings of the National Academy of Sciences.  Paleontologists have discovered a fossil half the size of a grain of rice that is over half a billion years old, and is the oldest truly bilateral animal ever found -- meaning what we're looking at may be a very close cousin to the ancestor of all the current bilateral animals on Earth.

In "Discovery of the Oldest Bilaterian from the Ediacaran of South Australia," by Scott D. Evans and Mary L. Droser (of the University of California-Riverside), Ian V. Hughes (of the University of California-San Diego), and James G. Gehling (of the South Australia Museum Department of Paleontology), we read about Ikaria wariootia, a teardrop-shaped critter whose unprepossessing appearance belies its significance.  This tiny little proto-worm might actually be our great-great-great (etc. etc. etc.) grandparent.

Not only was it bilateral, it had a throughput digestive system (two openings, one-way flow of material), another innovation that has turned out to be pretty important.  "One major difference with a grain of rice is that Ikaria had a large and small end," said study lead author Scott Evans, in an interview with The Guardian.  "This may seem trivial but that means it had a distinct front and back end, which is the kind of organization that leads to the variety of things with heads and tails that are around today."

Of course, this doesn't solve the question of whether bilateral symmetry is constrained or not.  My guess is that if it turns out to be, it will be because mirror-symmetry is easier to produce genetically.  A lot of the homeotic genes (genes that guide the development of overall body plan) work by creating a gradient of some protein or another, so the polarity of structures is established (head here, butt there, and so forth).  It might simply be easier to establish a one-way gradient, with a high on one end and a low on the other, than one with multiple highs and lows arranged symmetrically.

Although we do manage to do a five-point gradient in the development of our fingers and toes, so it's doable, it just may not be common.

In any case, here we have a creature that may be the reason we're arranged bilaterally, whether or not it gives us any sort of advantage.  Kind of humbling that we might come from a millimeter-wide burrowing scavenger.  I guess that's okay, though, if it'll keep humanity from getting any more uppity than it already is.

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Any guesses as to what was the deadliest natural disaster in United States history?

I'd speculate that if a poll was taken on the street, the odds-on favorites would be Hurricane Katrina, Hurricane Camille, and the Great San Francisco Earthquake.  None of these are correct, though -- the answer is the 1900 Galveston hurricane, that killed an estimated nine thousand people and basically wiped the city of Galveston off the map.  (Galveston was on its way to becoming the busiest and fastest-growing city in Texas; the hurricane was instrumental in switching this hub to Houston, a move that was never undone.)

In the wonderful book Isaac's Storm, we read about Galveston Weather Bureau director Isaac Cline, who tried unsuccessfully to warn people about the approaching hurricane -- a failure which led to a massive overhaul of how weather information was distributed around the United States, and also spurred an effort toward more accurate forecasting.  But author Erik Larson doesn't make this simply about meteorology; it's a story about people, and brings into sharp focus how personalities can play a huge role in determining the outcome of natural events.

It's a gripping read, about a catastrophe that remarkably few people know about.  If you have any interest in weather, climate, or history, read Isaac's Storm -- you won't be able to put it down.

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Walk of life

Even considering my background in evolutionary biology, there are a lot of things about the natural world that I take for granted.

For example, bilateral symmetry.  It's so common amongst animals that it's easy to think it's universal, when there's no real reason it should be.  (I recall vividly being startled when I first ran across H. P. Lovecraft's "Great Old Ones" -- who had five-way symmetry.)  What's likely is that a very long time ago, one of our successful ancestors was bilateral, and passed that characteristic down to its descendants -- which include the majority of Kingdom Animalia.

So if we ever find alien life, there's no reason to suspect that it will share some of these probably-arbitrary characteristics with terrestrial life.  Still, there are a few features that are significant enough advantages that it's likely to be found in other living things, wherever and however they evolved.  One of these is cephalization -- having the important organs, including the central nervous system and sensory receptors, near the anterior end.  Having your eyes and nose near your mouth makes a great deal of sense from the standpoint of finding food, and it's pretty likely to be a feature that shows up again and again.  (Consider flight -- such a great adaptation that it's evolved independently at least seven times in Earth's history, in birds, insects, bats, colugos, flying squirrels, sugar gliders, and pterodactyls.)

Locomotion itself is one of those abilities that is so useful that it's likely to show up wherever life occurs, but it's one of those things that's so universal we tend not to think about it, or even be aware there are exceptions.  (In fact, when I got students in my introductory biology classes to brainstorm for characteristics they thought were true for all living things, "able to move" was the most common wrong answer.)

This comes up because of a paper that was published in Nature last week, my awareness of which I once again owe to my sharp-eyed friend Andrew Butters of the brilliant blog Potato Chip Math.  In it we learn about a fossil that seems to be the earliest direct evidence we have of locomotion in an animal.  The fossil, which has been dated to around 540 million years ago, is the trail of a critter named Yilingia spiciformis ("spiky creature from Yiling"), about which the authors, Zhe Chen, Chuanming Zhou, and Xunlai Yuan (of the Chinese Academy of Sciences), and Shuhai Xiao (of Virginia Technological College of Sciences) have the following to say:

The origin of motility in bilaterian animals represents an evolutionary innovation that transformed the Earth system.  This innovation probably occurred in the late Ediacaran period—as evidenced by an abundance of trace fossils (ichnofossils) dating to this time, which include trails, trackways and burrows.  However, with few exceptions, the producers of most of the late Ediacaran ichnofossils are unknown, which has resulted in a disconnection between the body- and trace-fossil records.  Here we describe the fossil of a bilaterian of the terminal Ediacaran period (dating to 551–539 million years ago), which we name Yilingia spiciformis (gen. et sp. nov).  This body fossil is preserved along with the trail that the animal produced during a death march. Yilingia is an elongate and segmented bilaterian with repetitive and trilobate body units, each of which consists of a central lobe and two posteriorly pointing lateral lobes, indicating body and segment polarity.  Yilingia is possibly related to panarthropods or annelids, and sheds light on the origin of segmentation in bilaterians.  As one of the few Ediacaran animals demonstrated to have produced long and continuous trails, Yilingia provides insights into the identity of the animals that were responsible for Ediacaran trace fossils.

So what this represents is not just the dawn of motility, but the dawn of bilateral symmetry, and Yilingia may have been one of the earliest animals that had both.  It might not be our direct ancestor, but certainly was a close cousin to whatever was, and many of the features we now see in virtually all animals were locked in around that time.

It's awe-inspiring to look at this simple little fossil, the tracks of a critter that marched its way on the seafloor half a billion years ago, at a time when there was not a single thing living on the land, when the continents were bare rock, sand, dust, and dirt as far as the eye could see.  And even more amazing to realize that this innovation -- the ability to move -- was passed down through all that time, refined in a thousand different ways, and is the direct ancestor to our ability to walk, run, crawl, and jump.

So far from feeling demeaned by our connections to our primitive ancestry, as the creationists would frame it, I feel exalted by it -- we are linked in an unbroken chain of relationships to every living thing on Earth, and everything we can do, every structure in our bodies down to the molecular level, is directly due to inheritance that stretches back to the very first life in the primordial seas.

And if that's not a mind-blowing thought, I don't know what is.

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This week's Skeptophilia book recommendation is pure fun: science historian James Burke's Circles: Fifty Round Trips Through History, Technology, Science, and Culture.  Burke made a name for himself with his brilliant show Connections, where he showed how one thing leads to another in discoveries, and sometimes two seemingly unconnected events can have a causal link (my favorite one is his episode about how the invention of the loom led to the invention of the computer).

In Circles, he takes us through fifty examples of connections that run in a loop -- jumping from one person or event to the next in his signature whimsical fashion, and somehow ending up in the end right back where he started.  His writing (and his films) always have an air of magic to me.  They're like watching a master conjuror create an illusion, and seeing what he's done with only the vaguest sense of how he pulled it off.

So if you're an aficionado of curiosities of the history of science, get Circles.  You won't be disappointed.

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]