Thy fearful symmetry

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

The question has great significance with regards to the possibilities for extraterrestrial life.  I grew up watching Lost in Space and The Invaders and the original Star Trek, and later The X Files and Star Trek: The Next Generation and Doctor Who.  But while those classic shows piqued my budding interest in exobiology, my training in actual biology taught me that whatever the aliens look like, they will almost certainly not be humans with odd facial protuberances and strange accents.  How evolution plays out on other planets is impossible to say, but it's likely to be vastly different from the pathways taken by life on Earth.  I still remember reading Stephen Jay Gould's essay "Replaying the Tape" from his excellent book on the Cambrian-age Burgess Shale fauna, Wonderful Life, and being blown away by the following passage:

You press the rewind button and, making sure you thoroughly erase everything that actually happened, go back to any time and place in the past -– say, to the seas of the Burgess Shale.  Then let the tape run again and see if the repetition looks at all like the original.  If each replay strongly resembles life’s actual pathway, then we must conclude that what really happened pretty much had to occur.  But suppose that the experimental versions all yield sensible results strikingly different from the actual history of life?  What could we then say about the predictability of self-conscious intelligence? or of mammals?

His point was that a great deal of evolution appears to be contingent -- dependent on events and occurrences that would be unlikely to repeat in exactly the same way.  And while there's no way to re-run the tape on the Earth, considering the issue of constraint vs. contingency has profound implications regarding what we're likely to find elsewhere in the universe.  If we did find extraterrestrial life, would we even recognize it if we saw it? 

One good example 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.  The original Star Trek gave a shot at addressing this, with the silicon-based Horta in the episode "The Devil in the Dark."  Silicon, like carbon, has four valence electrons, and thus is capable of bonding into complex rings and chains, and could possibly be the basis of an alternative biochemistry, although its affinity for stabilizing as silica (silicon dioxide), its low solubility in water, and the rigidity of its bonding structure all argue against it being anywhere near as good as carbon.

What about oxygen use?  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 eighteen -- 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, how that's accomplished is 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; it's 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 -- further illustrating that as unpleasant a person as Lovecraft evidently was, he had a hell of an imagination.

[Image licensed under the Creative Commons Tom Ardans - blog - Facebook, Old One by Tom Ardans, CC BY-SA 3.0]

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?

One clue in all this is a discovery in South Australia that was described in a paper a while back in Proceedings of the National Academy of Sciences.  Paleontologists found 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.

The impressions left by Ikaria wariootia [Image credit: Scott D. Evans, UCR]

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 chemical 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 cockier than it already is.

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Life converges

One of the most fascinating features of biological evolution -- particularly as it applies to the possibility of life on other planets -- has to do with the concept of constraint.

Which features of life on Earth are, in some sense, inevitable?  Are there characteristics of terrestrial organisms that we might expect to find on any inhabitable world?  Stephen Jay Gould looked at this question in his essay "Replaying the Tape," from his brilliant book on the Cambrian Explosion, Wonderful Life:

You press the rewind button and, making sure you thoroughly erase everything that actually happened, go back to any time and place in the past -– say, to the seas of the Burgess Shale.  Then let the tape run again and see if the repetition looks at all like the original.  If each replay strongly resembles life’s actual pathway, then we must conclude that what really happened pretty much had to occur.  But suppose that the experimental versions all yield sensible results strikingly different from the actual history of life?  What could we then say about the predictability of self-conscious intelligence?  or of mammals?

Some features that have been suggested as evolutionarily constrained, with arguments of varying levels of persuasiveness, are:

• a genetic code based on some kind of nucleic acid (DNA or RNA, or some chemical analogue)
• internal cell membranes made of phospholipids, to segregate competing chemical reactions from each other 
• multicellularity, with some level of tissue specialization
• in more complex organisms, some form of symmetry, with symmetrically-placed organs
• some kind of rapid-transit system for messages, analogous to our nervous system (but perhaps not structured the same way)
• cephalization -- concentration of the central processing centers and sensory organs near the head end

It's interesting when science fiction tackles this issue -- and sometimes comes up with possible pathways for evolution that don't result in humanoids with strangely-shaped ears and odd facial protuberances.  A few that come to mind are Star Trek's silicon-based Horta from the episode"Devil in the Dark," the blood-drinking fog creature from "Obsession," the giant single-celled neural parasites from "Operation Annihilate," and Doctor Who's Vashta Nerada, Not-Things, Gelth, and Midnight Entity.

So the search for extraterrestrial life requires we consider looking not only for "life as we know it, Jim," but life as we don't know it.  Or, more accurately, to consider to what extent our terrestrial biases might be blinding us to the possibility of what evolution could create.

It's worth considering, however, how often evolution here on Earth ends up landing on the same solutions to the problems of survival and reproduction over and over again, a phenomenon called convergent evolution.  Eyes, or analogous light receptor organs, have evolved multiple times -- some biologists have suggested as many as fifty different independent lineages that evolved some form of eye.  Wings occurred separately in four groups of animals -- birds, pterosaurs, insects, and bats.  (If you include structures for gliding, add flying squirrels, sugar gliders, colugos, flying fish, and flying lizards.)

Even biochemical pathways can reappear, something I find astonishing.  Take, for example, the research that came out this week in Nature Chemical Biology, which found that two only distantly-related plants -- ipecac (Carapichea ipecacuanha), in the gentian family, and sage-leaved alangium (Alangium salviifolium), in the dogwood family, have both come up with complex biochemical pathways to generate the same set of bitter, emetic compounds -- ipecacuanha alkaloids.

The last common ancestor of these two species was over a hundred million years ago, so there's a strong argument that they evolved this capacity independently.  And indeed, when the biochemists looked at the enzymatic pathways, they're different -- they found entirely different chemical synthesis methods for producing the same set of end products.  Weirdest of all, they both evolved an enzyme that cleaves a sugar molecule from the alkaloid precursor, and that's what activates it (i.e., makes it toxic).  In the living plant's tissues, the enzyme and the precursor are segregated from each other.  It's only when they're brought together -- such as when a herbivore chomps on the leaves -- that the sugar is split away from the precursor, the alkaloid is activated, and the herbivore starts puking its guts up.

Clever strategy.  So clever, in fact, that it was stumbled upon by two entirely separate lineages of plants.  The rules organisms play by are the same, so perhaps not surprising there are similar outcomes sometimes.

The whole thing highlights the fact that there is a limited range of solutions for the fundamental difficulties of existence.  It has to make you wonder if, when we do find life elsewhere in the universe, it might look a lot more familiar that we're expecting.  I don't think it's likely we'll bump into Romulans or Ice Warriors or Krillitane, but maybe there are features of life on Earth that will re-evolve in just about any conceivable habitable planet.

But hopefully there won't be any Vashta Nerada.  Those things are terrifying.

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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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