Picky eaters

Last week a gardener friend and I were talking about the fact that some plants are extreme specialists -- they only thrive in a very narrow range of conditions.

The classic example of this are orchids.  Virtually all orchid species only do well if you can somehow replicate the exact conditions of temperature, soil pH, soil mineral content, sunlight, and so on that they need.  Some also require the presence of symbiotic fungi (such as mycorrhizae) that infiltrate the orchid's roots and aid in nutrient and water uptake.  All of this is why if you ever are lucky enough to see an orchid growing in the wild, resist the temptation of digging it up and bringing it home for your garden.  The chances are nearly one hundred percent that all you'll succeed in doing is killing it in short order.  (Also, if you live in a place with laws against harming endangered species, you might be looking at serious fines if you get caught.)

It's an interesting question to consider why such extreme specialization evolves.  On first glance, it seems like it'd be better for all species to evolve toward becoming generalists -- able to handle a wide range of conditions.  The thing is that while generalists (like dandelions and crabgrass) do thrive just about everywhere, giving them a competitive edge in disturbed habitats (like cities) where not much else grows, they get beaten by the specialists in old, stable ecosystems.  The specialists have evolved to tolerate those specific conditions better than anything else.

It's why in old-growth rain forests, just about everything you see -- plant and animal -- is a specialist.  Along roadside ditches, they're all generalists.

Some recent research suggests that this drive toward specialization in stable habitats is very old.  A study of the distribution of animals in Ediacaran (very late Precambrian) sandstone in Australia found that some of the peculiar animals characteristic of these ecosystems showed a distinct preference for particular parts of the habitat -- a clear hallmark of specialization.

The researchers focused on a handful of species that 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]

Both animals were grazers, feeding on the microbial mat on the seafloor, but their habitat choices differed.  Obamus turned out to have a distinct preference for places where the mat was thickest; Tribrachidium was much more evenly dispersed.  And since both animals were of very low mobility -- similar to modern barnacles -- this didn't just reflect the chance arrangement of where they were when the a layer of sediment, probably stirred up by a storm or landslide, buried them for eternity.

This was a habitat choice -- and the first known example of specialization in the natural world.

"We think about the very oldest animals and maybe you wouldn't expect them to be so picky," said Mary Droser of the University of California - Riverside, who co-authored the study. "But Obamus only occurs where there is a thick mat, and it's a pretty sophisticated way of making a living for something so very old...  There are a limited number of reproductive strategies, especially for animals like these.  There are more strategies today, and they're more elaborate now. But the same ones used today were still being used 550 million years ago."

"It's not like studying dinosaurs, which are related to birds that we can observe today," said Phillip C. Boan, also of UC - R, and lead author of the new study.  "With these animals, because they have no modern descendants, we're still working out basic questions about how they lived, such as how they reproduced and what they ate...  This is really the first example of a habitat-selective Ediacaran creature, the first example of a macroscopic animal doing this.  But how did they get where they wanted to go?  This is a question we don't yet know the answer to."

It's fascinating that we can get some insight into the behavior of a species that lived so long ago, during a time where there was no life at all on land.  Imagine it -- everything alive is in the sea, and the continents were vast, barren expanses of rock, sand, and dust.  The first land-dwelling plants and animals wouldn't exist for another fifty million years (and even then, they were clustered around bodies of water; the central parts of the continents would have been lifeless for a great deal longer).  

But despite how alien this landscape would have seemed, organisms were already evolving through natural selection to have many of the same traits we see today -- including the fact that some of them, like modern orchids, know exactly where they want to be.

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An explosion of understanding

One of the reasons I love science is its capacity for inducing wonder.

Albert Einstein said it best: "Joy in looking and comprehending is nature’s most beautiful gift."  Being able to look around you and think, "Okay, now I understand a little bit more of the universe" is nothing short of a thrill.

I recall having that feeling when I first learned about the Cambrian Explosion, a sudden increase in biodiversity that occurred about 540 million years ago, and which produced virtually all the animal phyla we currently have today.  I think it struck me that way because it was so contrary to the picture I'd had, of evolution slowly plodding along, from something like a jellyfish to something like a worm to something like a fish, through amphibians and reptiles and mammals, finally leading to us as (of course) the Pinnacle of Creation.  That view, it seems, is substantially wrong.  While there has been great change on many branches of the family tree of life, all of the basic branches diverged right about the same time.

Fascinating, too, that there were also a variety of branches that left no living descendants, that are so bizarre to our eyes that they look more like something from a science fiction movie.  There's Dickinsonia:

[Image is licensed under the Creative Commons Verisimilus at English Wikipedia, DickinsoniaCostata, CC BY-SA 3.0]

... and Anomalocaris, shown here as a model of what it might have looked like when alive:

[Image is in the Public Domain]

... and the aptly named Hallucigenia, which could be straight out of a fever-dream:

[Image is licensed under the Creative Commons Scorpion451, Hallucigenia Reconstruction Current 2015, CC BY-SA 4.0]

... and my personal favorite, five-eyed, vacuum-cleaner-hose-equipped Opabinia:

[Image is licensed under the Creative Commons Nobu Tamura (http://spinops.blogspot.com), Opabinia BW, CC BY 3.0]

If you'd like to find out more, I encourage you to read Stephen Jay Gould's awesome book Wonderful Life, which will tell you about these four creatures and a great many more besides.

The reason I bring this up is that some research out of Oxford University has elucidated not only the structure of these odd creatures, but the environment in which they lived.  Having fossils from 540 million years ago that were sufficiently intact to determine what they'd looked like while alive is amazing enough; but being able to determine anything about the conditions under which they lived is downright astonishing.  But that's just what Ross Anderson and Nicholas Tosca, of the Department of Sedimentary Geology at Oxford, and their team have done.

Their paper, which appeared in the journal Geology, described microscopic mineralogical analysis of the Burgess Shale of Canada and the Ediacaran Assemblage of Australia, two of the finest deposits of Cambrian Explosion fossils in the world.  And what the geologists found allowed them to make a guess at where the likes of Opabinia and the rest lived: warm, shallow ocean ecosystems that had water rich in iron.

The iron content allowed the formation of the mineral berthierine, which is not only distinctive in its origins, but has an anti-bacterial effect that halted decomposition and prevented decomposition.  This resulted in the phenomenally well-preserved fossils both sites are known for.

"Berthierine is an interesting mineral because it forms in tropical settings when the sediments contain elevated concentrations of iron," Anderson said.  "This means that Burgess Shale-type fossils are likely confined to rocks which were formed at tropical latitudes and which come from locations or time periods that have enhanced iron.  This observation is exciting because it means for the first time we can more accurately interpret the geographic and temporal distribution of these iconic fossils, crucial if we want to understand their biology and ecology."

The whole thing is tremendously exciting.  To not only have an idea of the appearance of these animals, but to be able to picture them in something like their actual habitat, gives us a glimpse of a world five times older than it was during the heyday of the dinosaurs.  It's breathtaking to think about.

I'll end with a quote from another scientist -- Brian Greene, the physicist whose lucid writing about modern physics in his book The Fabric of the Cosmos inspired an equally brilliant NOVA series.  Greene says: "Science is a way of life.  Science is a perspective.  Science is the process that takes us from confusion to understanding in a manner that's precise, predictive and reliable -- a transformation, for those lucky enough to experience it, that is empowering and emotional."

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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.

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

With fronds like these...

One of the most mystifying, and therefore (to me) one of the most fascinating, paleontological finds is the fauna of the Ediacaran Assemblage.

It's intriguing from a number of perspectives.  First, it gives lie to the picture most people have of the evolution of animals, that it was some kind of linear progression.  It's often seen as a climb up the Great Chain of Being, from something like a jellyfish, to something like a worm, to something like a bug, to fish, amphibians, reptiles, mammals, and finally -- at the top, of course -- is the Pinnacle of Evolution: namely, us.

The truth is (predictably) much more interesting.  During the late Precambrian and early Cambrian Periods, in a relatively short amount of time (geologically and paleontologically speaking) all of the ancestors of the major animal groups appeared, as if there was a sudden and drastic push to diversification.  At that point there were proto-arthropods, proto-vertebrates, proto-mollusks, and proto-damn-near-everything-else.

Even more fascinating is that there were a number of animal groups around during that time that are of uncertain affinity to the others, and who apparently left no descendants.  There's the bizarre Anomalocaris, probably related most closely to early arthropods (its name is Greek for "abnormal shrimp"), with two jointed, spike-lined tentacles and a mouth shaped like a pineapple ring.  Opabinia was equipped with no less than five compound eyes and a proboscis like a vacuum-cleaner hose.  Most famous is the aptly-named Hallucigenia ("it creates hallucinations"), a worm-like critter with giant eyes, tube-like legs, and a double row of formidable spines down the back.

All three of these are probably branches of the huge group Protostomia, which are still today the most numerous animals on Earth.  But there are other fossils from the Ediacaran Assemblage that are even more mysterious, and one of the weirdest ones is the group called rangeomorphs.

They were almost certainly 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]

If Anomalocaris, Opabinia, and Hallucinogenia are problematic in terms of their evolutionary affinities, the rangeomorphs are complete ciphers.  They have no obvious connections to any living animal group, and in some ways more resemble fungi, although that too is speculation.  They were apparently quite common during the late Precambrian, so the sea bottom would have been covered with their frilly fronds gently waving in the currents -- but at the moment, exactly what they were is a mystery.

And the mystery just deepened considerably with a discovery that was the subject of a paper last week in Current Biology.  The rangeomorphs had another perplexing and unusual feature -- they were connected by thread-like filaments, some of them up to four meters long, that seem to have hooked populations up into a huge network of interlinked individuals.

The purpose of these filaments is unknown, but it could be that the individuals in a network were all clones, and were functioning as a colonial organism a little like modern corals.  What it immediately put me in mind of was groves of aspens, which look like bunches of individual trees but are all linked underground by a network of rhizomes -- some of the colonies cover many acres, and one in Colorado is said to be over eighty thousand years old.  (This calls into question what we mean by the word "organism;" is each of these trees a separate organism?  Is the whole grove a single organism?  If so, and you dug a trench down the middle and cut the rhizomes, have you just created two organisms?  Like many terms in biology, this word only seems simple until you push on it a little.)

In any case, the rangeomorphs apparently had the world's first social network, but what exactly it was used for we can only speculate at.  They were strange animals to say the very least.  These sorts of discoveries always make me wonder what the Earth looked like back then -- given how infrequent fossilization is, and how unlikely it is for a rock to remain undamaged through all those millions of years, the chances are that for every one species we have a reasonably good picture of, there are hundreds that we know nothing at all about.  The Precambrian water-world of the Ediacaran fauna would have looked a very alien place to our eyes, even though the seeds of all of our modern life-forms -- including ourselves -- were there in those oceans.

Some of those seeds, though, failed to leave behind any progeny, and it seems likely that the rangeomorphs were one of those.  Whatever they were, they certainly show no obvious connections to any modern group, animal or otherwise.  To me this only increases their fascination -- and with it, the hope that further discoveries may shed some light on this and other groups whose origins are lost in the depths of time.

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This week's Skeptophilia book-of-the-week is brand new -- science journalist Lydia Denworth's brilliant and insightful book Friendship: The Evolution, Biology, and Extraordinary Power of Life's Fundamental Bond.

Denworth looks at the evolutionary basis of our ability to form bonds of friendship -- comparing our capacity to that of other social primates, such as a group of monkeys in a sanctuary in Puerto Rico and a tribe of baboons in Kenya.  Our need for social bonds other than those of mating and pair-bonding is deep in our brains and in our genes, and the evidence is compelling that the strongest correlate to depression is social isolation.

Friendship examines social bonding not only from the standpoint of observational psychology, but from the perspective of neuroscience.  We have neurochemical systems in place -- mediated predominantly by oxytocin, dopamine, and endorphin -- that are specifically devoted to strengthening those bonds.

Denworth's book is both scientifically fascinating and also reassuringly optimistic -- stressing to the reader that we're built to be cooperative.  Something that we could all do with a reminder of during these fractious times.

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

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!]