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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