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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