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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The road not taken

One of the most intriguing sets of life forms I've ever heard of is the Ediacaran Assemblage.

It dates from the late Precambrian Era -- something on the order of 570 million years ago -- and is named after the Ediacara Hills of Australia, where rocks of that age are exposed at the surface.  They're sometimes conflated with the Cambrian Explosion fauna like the ones in the famous Burgess Shale, but any connection between the two is tenuous at best.  Not only are they separated by almost seventy million years, the Burgess Shale animals are (mostly) from phyla we know about.  A few -- like the bizarre and aptly-named Hallucigenia -- have more obscure relationships to modern life, but most of the fossils we find there are identifiably proto-arthropods or proto-annelids or proto-whatnot.  So while the Cambrian Explosion fauna is fascinating in its own right, by and large it's still fairly familiar ground.

Not so the Ediacaran Assemblage.

These things are downright mysterious.  Take, for example, the group called rangeomorphs.

They may have been animals, although they were sessile (fixed to the seafloor) via stalks, and had weird frond-like structures of uncertain purpose (but which may have been a mechanism either for oxygen extraction or for filter feeding).  So if you were to look at a living one, your initial impression might well be that it was some odd sort of seaweed, and not an animal at all.

A 550-million-year-old fossil of the rangeomorph Charnia masoni, from the Mistaken Point Formation in Newfoundland [Image licensed under the Creative Commons Smith609 at English Wikipedia, Charnia, CC BY 2.5]

Not only are they bizarre-looking, many seem to have no living descendants, including Obamus coronatus (which looks like a French cruller) and the hubcap-like Tribrachidium heraldicum, one of the only known animals to have triradial symmetry.

Artist's reconstruction of Obamus coronatus [Image licensed under the Creative Commons Nobu Tamura (http://spinops.blogspot.com/), Obamus NT, CC BY-SA 4.0]

There's a misconception about evolution -- that it's linear and progressive, that one form supersedes another in some kind of stepwise fashion based upon an identifiable "improvement," such as increase in speed, defensive or offensive capabilities, ability to access food, or intelligence.  While you can find examples where this appears to have happened, there's a large measure of the chaotic involved in the history of life.  Not only do we see sudden and drastic changes in the climate and environmental conditions -- which, after all, are the biggest drivers of selective pressure -- random occurrences like volcanic eruptions and meteorite strikes can create a situation where extinction had way less to do with poor evolutionary fitness than simply being in the wrong place at the wrong time.

The Ediacaran Assemblage seems to have been on the unfortunate end of that particular equation.  As I mentioned, the majority of them apparently left no descendants, not only today but even by the beginning of the next geological era.  None of the bizarre Ediacaran life forms appear in the early Cambrian; the dominant animals five hundred million years ago show almost no resemblance to their predecessors seventy million years earlier.

In fact, the subject comes up because of a paper a few weeks ago in Geology suggesting that the wipeout of the Ediacaran Assemblage represents the Earth's first known mass extinction (not counting the Great Oxidation Event, of which the effect on life was uncertain but probably enormous).  The new study uses recently-uncovered late Precambrian fossil beds that greatly add to the described Ediacaran biota, and the analysis found that we may well have been drastically underestimating the magnitude of the crash.

The researchers' data shows that what is known as the Kotlin Crisis, the biotic collapse that took out pretty much all of the Ediacaran life forms, may have wiped out as much as eighty percent of life on Earth.  This easily places it amongst what paleontologists Jack Sepkoski and David Raup called the "Big Five" extinction events (the Late Ordovician, Late Devonian, Permian-Triassic, End Triassic, and Cretaceous-Tertiary extinctions).  In fact, if the eighty percent number is correct, it would be in second place -- handily beating the sixty-odd percent of life destroyed in the famous Cretaceous-Tertiary extinction, and exceeded only by the cataclysmic Permian-Triassic "Great Dying."

So, what caused the Kotlin Crisis?  At the moment, it's uncertain.  It may have been a series of unfortunate events, including climate shifts, changes in oxygenation of the ocean, volcanic eruptions, and possibly the evolution of carnivory, but honestly, we're not sure.  There are few enough rock outcrops of that age available to study that any determination is likely to be slow in coming.

But what's certain is that these (very) distant cousins of ours represent a road not taken -- a branch of the vast evolutionary tree of life on Earth that led to no descendants.  It always makes me wonder what would have happened had they survived, and perhaps outcompeted, the bilateral, mobile forms that superseded them, and who ultimately became our ancestors.  If -- in evolutionary biologist Stephen Jay Gould's evocative words -- we could re-run the tape, who would now be the dominant life forms on Earth?

Wouldn't be us, that's for damn sure.  Maybe something like H. P. Lovecraft's bizarre pentaradial "Great Old Ones:"

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

I can virtually guarantee that whatever it would have been, it'd be something so strange to our eyes that it would give even Darwin pause, despite all his blithe talk about "many forms most beautiful and most wonderful."

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Dawn life

Currently I'm working my way through Mark McMenamin's book The Garden of Ediacara, an analysis of the fossil evidence from the Vendian Period, the last bit of the Precambrian (650-543 million years ago).

The subject of McMenamin's book is undeniably fascinating -- more about that in a moment -- but it's uneven reading.  Part of it is a travelogue of his work in Namibia, Mexico, and Australia, places where there are significant outcrops of late Precambrian sedimentary rocks, but it's obvious from page one that most of what he does is write papers for scholarly journals.  As a result, it's halfway between an introduction to the topic for laypeople and an extended academic paper, and I've been glad as I worked my way through it that I have at least a passing background in paleontology.

Something that struck me right away, however, was that I've been laboring under a serious misunderstanding of the Ediacaran biota; that it overlapped significantly with the Cambrian explosion fauna, the bizarre creatures like Anomalocaris and Opabinia and the aptly-named Hallucigenia.  In reality, there was almost no overlap, and the Ediacaran organisms such as Cloudina and Dickinsonia were almost certainly driven to extinction and replaced by the large predatory forms of the early Cambrian.

A fossil of Dickinsonia costata from Australia [Image licensed under the Creative Commons Verisimilus at English Wikipedia, DickinsoniaCostata, CC BY-SA 3.0]

While the early Cambrians (best known from the Burgess Shale formation of British Columbia) are clearly animals, the bizarre Ediacarans are of completely uncertain affinities.  When McMenamin wrote his book (1998) there was considerable contention about what they were, with various paleontologists arguing vehemently that they were early animals, fungi, algae, or even giant protists (or protist colonies).  Despite the passage of twenty-five years, the issue is still far from settled.  Some make persuasive arguments that the Vendian biota doesn't belong to any of the five modern kingdoms of life (animals, plants, fungi, bacteria, and archaea), but are representatives of a completely different lineage, or more than one, that left no descendants at all.

So I'm grateful to McMenamin and his book for clearing up something I'd misunderstood for years.

I was in the middle of reading The Garden of Ediacara when, coincidentally, a friend and frequent contributor of topics for Skeptophilia sent me a link to an article in Smithsonian magazine about the evolutionary origin of animals.  Another point of contention amongst biologists is determining, out of the entire kingdom Animalia, which group branched off first.  (This is sometimes phrased as which is the "oldest" or "most primitive" -- both terminology I don't like, because every living animal on Earth has an exactly equal length of evolutionary history.  It's just that during that time, some branches have changed a great deal faster than others, and some groups share more recent common ancestry than others do.)

In any case, the argument is about which group of modern animals is the outgroup -- the one that split off first, and therefore is the most distantly related to all other animals.  When I took zoology (many, many years ago) the conventional wisdom was that it was sponges (Phylum Porifera).  And there's certainly a good case to be made there; sponges are weird animals, with no differentiated organs, skeletons made of either protein fibers, bits of calcium carbonate, or slivers of glass, and no nerves, muscles, or digestive tracts.  But genetic analysis has shown unequivocally that there's an even more distantly-related group -- the comb jellies (Phylum Ctenophora).

They look superficially like jellyfish, and that similarity led scientists to put them on the same branch as Phylum Cnidaria (which not only contains jellyfish, but sea anemones and corals).  The genetic studies, though, show that there's only a distant relationship between comb jellies and jellyfish.  The comb jellies, in fact, show more of a genetic similarity to certain species of protists than they do to other animals.

"That was the smoking gun," said Daniel Rokhsar, of the University of California - Berkeley, who co-authored the paper.

So this goes to show that there's a lot we still have to learn about the earliest life on our planet.  And I'm sure that as definitive as this study seems to be, it won't be the last word.  As more evidence surfaces, expect the arrangement to change.  This, after all, is how science works; it has a mechanism for self-correcting.  And far from the reaction I've seen people have -- that the shifting understanding means "it could all be proven wrong tomorrow" -- that capacity for change is science's main strength.

After all, isn't it a good thing to have your model shift to accommodate new information?  Seems like standing firm on what you believe despite strong evidence to the contrary is the cause of a lot of the problems in the world.

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