Thawing the snowball

One of the frightening things about a system in equilibrium is what happens when you perturb it.

Within limits, most systems can recover from perturbation through some combination of negative feedbacks.  An example is your body temperature.  If something makes it goes up -- exercise, for example, or being outside on a hot, humid day -- you sweat, bringing your temperature back down.  If your body temperature goes down too much, you increase your rate of burning calories, and also have responses like shivering -- which brings it back up.  Those combine to keep your temperature in a narrow range (what the biologists call homeostasis).

Push it too much, though, and the whole thing falls apart.  If your temperature rises beyond about 105 F, you can experience seizures, convulsions, brain damage -- or death.  Your feedback mechanisms are simply not able to cope.

This, in a nutshell, is why climate scientists are so concerned about the effects of anthropogenic carbon dioxide.  Within limits -- as with your body temperature -- an increase in carbon dioxide results in an increase in processes that remove the carbon dioxide from the atmosphere, and the whole system stays in equilibrium.  There is a tipping point, however.

The problem is that no one knows where it is -- and whether we may have already passed it.

A piece of research from the Virginia Polytechnic Institute, however, has suggested that this flip from stability to instability may be fast and unpredictable.  A paper authored by a team led by paleobiologist Shuhai Xiao, that was published in the journal Geology, looks at one of the main destabilization events that the Earth has ever experienced -- when the "Snowball Earth" thawed out in the late Precambrian Period, 635 million years ago.

Artist's conception of the Precambrian Snowball Earth [Image is in the Public Domain, courtesy of NASA/JPL]

Xiao and his team studied rocks from Yunnan and Guizhou, China, that are called cap carbonates.  They are made of limestone and dolomite and are deposited quickly in marine environments when the carbon dioxide content of the atmosphere spikes, leading to a dramatic temperature increase and a subsequent increase in absorption of carbonates into seawater (and ultimately deposition of those carbonates on the seafloor).  The cap carbonates Xiao et al. studied were dated to between 634.6 and 635.2 million years old, which means that the entire jump in both temperature and carbon dioxide content took less than 800,000 years.

So in less than a million years, the Earth went from being completely covered in ice to being subtropical.  The jump in global average temperature is estimated at 7 C -- conditions that then persisted for the next hundred million years.

Xiao et al. describe this as "the most severe paleoclimatic [event] in Earth history," and that the resulting deglaciations worldwide were "globally synchronous, rapid, and catastrophic."

Carol Dehler, a geologist at Utah State University, is unequivocal about the implications.  "I think one of the biggest messages that Snowball Earth can send humanity is that it shows the Earth’s capabilities to change in extreme ways on short and longer time scales."

What frustrates me most about today's climate change deniers is that they are entirely unwilling to admit that the changes we are seeing are happening at an unprecedented rate.  "It's all natural," they say.  "There have been climatic ups and downs throughout history."  Which is true -- as far as it goes.  But the speed with which the Earth is currently warming is faster than what the planet experienced when it flipped between an ice-covered frozen wasteland and a subtropical jungle.  It took 800,000 years to see an increase of the Earth's average temperature by 7 degrees C.

The best climate models predict that's what we'll see in two hundred years.

And that is why we're alarmed.

It's unknown what kind of effect that climate change in the Precambrian had on the existing life forms.  The fossil record just isn't that complete.  But whatever effect it had, the living creatures that were around when it happened had 800,000 years to adapt to the changing conditions.  What's certain is that an equivalent change in two centuries will cause massive extinctions.  Evolution simply doesn't happen that quickly.  Organisms that can't tolerate the temperature fluctuation will die.

We can only speculate on the effects this would have on humanity.

This is clearly the biggest threat we face, and yet the politicians still sit on their hands, claim it's not happening, that remediation would be too costly, that we can't prevent it, that short-term profits are more important than the long-term habitability of the Earth.  (Not to mention firing the people and closing the agencies that are currently trying to do something about it.)  Our descendants five hundred years from now will look upon the leaders from this century as having completely abdicated their responsibility of care for the people they represent.

Presuming we still have descendants at that point.

****************************************

The rocks drawn down

Imagine yourself standing on the shoreline, somewhere on the Earth, three billion years ago.

If you're picturing swamps and tree ferns and dinosaurs, you're way off.  The first trees wouldn't appear for another 2.6 billion years or so; the first dinosaurs we know about were 150 million years after that.  Three billion years ago there probably weren't even any eukaryotes -- organisms with complex cells containing organelles and nuclei, such as ours, as well as those of plants, fungi, and protists -- anywhere on Earth.  It's likely there wasn't much oxygen in the atmosphere, either.  This is before the "Great Oxidation Event," when photosynthesizing cyanobacteria reached a population sufficient to dump huge quantities of oxygen into the atmosphere, dooming most of Earth's living things (but simultaneously setting the stage for the rise of aerobic organisms such as ourselves).

So an accurate picture of what you'd experience: land that is nothing but an enormous expanse of bare rock and sand, devoid of a single living thing; murky water containing a soup of organic compounds, generated by the reducing atmosphere and frequent lightning storms; and unbreathable air mostly made of nitrogen, methane, carbon dioxide, carbon monoxide, sulfur dioxide, and ammonia.

I can just hear Mr. Spock saying, "The planet appears to be entirely inhospitable to life, Captain."

It's hard to imagine that our lush, verdant, temperate world evolved from that, but it did.  Consider, too, that the continents weren't even remotely in the same positions as they are now.  Where I sit writing this, in upstate New York, I'd have been about at the same latitude as I am now -- maybe a little bit farther south, about thirty degrees north.  But that's by far the exception.  See where you'd be on this map between about 2.5 and 1.5 billion years ago, when all of Earth's land masses were fused into a supercontinent named Columbia.  [Nota bene: this is not Pangaea.  This is two supercontinents before Pangaea.]

[Image licensed under the Creative Commons Alexandre DeZotti, Paleoglobe NO 1590 mya-vector-colors, CC BY-SA 3.0]

This was, in fact, not long after the continents formed.  We have continents because there are two basic kinds of rocks in the Earth's crust: felsic rocks, which are rich in silica, low in iron, and relatively lightweight; and mafic rocks, which are the opposite.  Most of the continental land masses are made up of felsic rocks (like granite and rhyolite), so they float in the denser rock of the mostly-mafic upper mantle.  (It's hard to imagine something as gigantic and heavy as a continental rock mass floating, but that's what it does.)  About three billion years ago was when there was sufficient separation of felsic and mafic chunks of crust that we started to see continental cratons form, and these blocks have been so stable thereafter that they're basically the same land masses we have today (albeit much cut apart and rearranged).

The reason this comes up is the discovery of evidence of what might have been one of the Earth's earliest megaquakes.  It occurred in what is now South Africa, part of the Kalahari Craton (as you can see from the map above, it'd have been in the northeast corner of the Columbia Supercontinent, at about the current latitude of Oslo, Norway).

The Barberton Greenstone Belt is one of the oldest relatively undisturbed chunks of rock in the world, and the current study, which was published two weeks ago in the journal Geology, suggests that it shows evidence of an overturned layer of chert that formed from a humongous underwater landslide of the type we see with megathrust earthquakes.  This, the researchers say, is the smoking gun that plate tectonics was already up and running three billion years ago -- that the reshuffling of continental blocks still going on today started not long after the blocks themselves formed.

The Barberton Greenstone Belt, about 350 kilometers east of Pretoria, South Africa [Image credit: Simon Lamb, Victoria University]

We think of the Earth as unchanging, don't we?  "Solid as a rock" is close to a cliché.  And yet, as we've seen, everything shifts, melts, moves; life comes and goes, evolves and falls to extinction; even the continents beneath our feet break up and recombine.  It's been going on for billions of years, and will continue for billions more.  The whole thing puts me in mind of Percy Shelley's evocative poem "Mont Blanc," which seems a fitting place to end:

Yet not a city, but a flood of ruin
Is there, that from the boundaries of the sky
Rolls its perpetual stream; vast pines are strewing
Its destin’d path, or in the mangled soil
Branchless and shatter’d stand; the rocks, drawn down
From yon remotest waste, have overthrown
The limits of the dead and living world,
Never to be reclaim’d.  The dwelling-place
Of insects, beasts, and birds, becomes its spoil;
Their food and their retreat for ever gone,
So much of life and joy is lost.  The race
Of man flies far in dread; his work and dwelling
Vanish, like smoke before the tempest’s stream,
And their place is not known.  Below, vast caves
Shine in the rushing torrents’ restless gleam,
Which from those secret chasms in tumult welling
Meet in the vale, and one majestic River,
The breath and blood of distant lands, for ever
Rolls its loud waters to the ocean-waves,
Breathes its swift vapours to the circling air.

****************************************

The story of the bands

Something that strikes me about many scientific discoveries is how they so often come from someone noticing something the rest of us had overlooked or thought insignificant -- and afterward, most importantly, the person asking, "Why?"

A now-familiar example of this is the discovery by the father-and-son team of Luis and Walter Alvarez of the thin iridium-bearing clay layer at the boundary between Cretaceous rocks and Paleocene rocks -- analysis of which led to the discovery of the dinosaur-killing Chicxulub Meteorite Impact.  Without their questioning why there was a narrow layer of heavy-metal-enriched clay at the boundary, everywhere on Earth where there are rocks of that age, we might never have found out about one of the major events in the history of life on Earth.

Another example, less well known, has to do with the banded iron formations found in locations all over the world, including Australia, Brazil, Canada, India, Russia, South Africa, Ukraine, and the United States.  They're striking in appearance, sometimes hundreds of meters thick, with alternating layers of light-colored iron-poor and dark, reddish-brown iron-rich chert or limestone.  Here's an example from near Fortescue Falls in Western Australia:

[Image licensed under the Creative Commons Graeme Churchard from Bristol, UK, Banded iron formation Dales Gorge, CC BY 2.0]

Most of us, I think, would say "pretty rock formation" and leave it at that; a smaller number would recognize the fact that they were sedimentary, and wonder why the colors alternate.  Geologist Preston Cloud, though, took it several large steps farther -- and what he came up with is a little mind-blowing.

What first struck Cloud as curious about banded iron formations is that they're all about the same age.  Regardless of whether they're in Australia or Ontario, just about every banded iron formation studied was deposited around 2.4 billion years ago.  But what could create this pattern not just in one location, but in widely-scattered spots all over the planet?  Whatever the process was must have happened everywhere simultaneously -- and rapidly.

Cloud's hypothesis, which is now well-accepted, is that banded iron formations represent the fingerprint of something called the Great Oxidation Event.  Here's basically what we think happened.

Early living things were largely scavengers, living from abiotically-produced organic compounds dissolved in seawater (and the decomposing bits of dead cells).  These compounds were abundant -- an anoxic atmosphere, rich in reducing compounds like ammonia, methane, and carbon monoxide, together with an energy source like ultraviolet light, generates organic compounds of all sorts.  (As the Miller-Urey experiment conclusively demonstrated.)

But there's always competition between species, and sometimes mutations can create proteins or structures that allow organisms to able to access resources faster or more efficiently than their neighbors.  And that's what happened when a single-celled bacteria evolved a gene to produce chlorophyll, which can quickly capture energy from visible light and store it as chemical energy.

In other words: photosynthesis.

This had only one downside, but it was a huge one.  Photosynthesis generates molecular oxygen.  Oxygen is highly reactive, a strong oxidizer (thus the name), and tears apart organic compounds as quickly as they form.  The presence of oxygen, first dissolved in seawater and then liberated into the atmosphere, did three things.

First, it shut off the abiotic production of excess organic compounds, eliminating the food source for most of life on Earth.

Second, it was directly toxic to most cells, except for the (very) few which had detoxifying enzymes like superoxide dismutase to cope with living in an oxygenated environment -- or which were capable of metabolizing it, using a pathway we now call aerobic respiration and which we have become completely dependent upon.  (It's amazing to think about, but our energy-production system originally evolved as a way to mitigate the poisonous effects of molecular oxygen.)

Third, the oxygen reacted with dissolved ferrous (II) ions in seawater, and altered them to mostly-insoluble ferric (III) ions, which settled out on the ocean floor.  This process, however, bound up the available oxygen, so the reaction dropped oxygen levels, and for a while any iron eroded into the oceans was dissolved as ferrous ions again.  But eventually the photosynthesizing bacteria pumped out enough oxygen that the iron precipitated once more.  The result: alternating layers of iron-poor chert when the oxygen levels were low, and iron-rich chert when the oxygen levels rose.

Eventually, of course, the oxygen rose and stayed high.  By this time, damn near all life on Earth had died; the only ones left were anaerobes that could hide (like the bacteria we still have in deep-sea mud and other anaerobic habitats), and aerobes like our own ancestors that had metabolic pathways to cope with the presence of oxygen.

And the alternating pattern of light and dark layers in banded iron formations chronicle the rising and falling of oxygen during one of the pivotal moments of Earth's prehistory.

Certainly a large part of being a successful scientist is intensive training in a specific field, but I think sometimes there's not enough attention given to another facet of it -- the role of creativity.  The scientists who make important discoveries are usually the ones who notice things the rest of us might just walk past, wonder about them, and most importantly, draw connections between disparate realms to find answers (in this case, geology, chemistry, and biology).  Without this combination of technical knowledge, curiosity, and insight, we would know far less about the universe we live in -- and what an impoverished understanding we would be left with.

****************************************

Life inside the snowball

Right now, here in the wilds of upstate New York, it's cold, gray, and snowy.  They say our area has a "four-season climate" -- but usually neglect to add that the four seasons are Almost Winter, Winter, Still Fucking Winter, and Road Construction.

On the other hand, if you know something about prehistory, it could be a whole lot worse, and in fact, has been more than once.  The last continental glaciation in this part of the world, the Laurentide, resulted in an ice sheet that buried the spot where I'm now sitting under three hundred meters of ice, and dug out not only the nearby Finger Lakes but the Great Lakes.  The southern edge of the ice sheet created the Elmira Moraine, only thirty miles south of me -- a moraine is basically the debris left behind when a glacier recedes -- and also Long Island, the sand and gravel soils of which were shoved forward as the ice sheet pushed southward then left in place, much like the pile of snow left when a snowplow backs up (explaining its long, narrow shape).

So I shouldn't complain about the cold.  The era of the Laurentide Glaciation was a lot colder.  And in fact, there have been periods in Earth's history where everyone, not just people like who live in the frozen north, would have been in the icebox.

Our knowledge of this rather miserable time in the far distant past has, like so many discoveries, built by accretion.  In the 1870s and 1880s geologists found evidence of widespread glaciation in strata in Scotland -- then, more puzzlingly, in Australia and India.  Any deep understanding of this was hampered by the fact that back then, scientists thought the continents were firmly fixed in place; continental drift wasn't even first proposed until 1912, and then was soundly rejected until magnetometer data proved in 1958 that the tectonic plates were in constant motion.  The first evidence of a worldwide glaciation -- not just a big one, like the Laurentide -- was uncovered in 1964 by Cambridge University geologist W. Brian Harland, who showed that glacial strata in Svalbard and Greenland had been deposited in tropical latitudes.  Thus demonstrating two rather amazing conclusions in one fell swoop; first, that Svalbard and Greenland had moved a long way, and second, that at the time when they were near the equator, the whole world was covered with ice.

This "Snowball Earth" model has since been demonstrated as accurate in multiple ways.  More than once, but most significantly between 720 and 580 million years ago (i.e. the end of the Precambrian Era), the whole planet was covered with a kilometers-thick sheet of ice.  Picturing what this was like is a little mind-boggling.  The glaciers covered not only the land, but the entire ocean.  Because the liquid water underneath was moving, the ice sheets broke up and ground together, much like the rocky tectonic plates do today, floating on the liquid mantle of the Earth.  Any organisms caught in the cracks of the ice sheet, or between the glaciers and the seafloor, would have been pulverized.  "It’s basically like having a giant bulldozer," said Huw Griffiths, of the British Antarctic Survey, in an interview with Eos.  "The next glacial expansion would have just erased all [traces of life] and turned it into mush, basically."

Griffiths is the reason the topic comes up, actually; he, Rowan Whittle (also of the British Antarctic Survey), and Emily Mitchell (of the University of Cambridge) are the authors of a paper in The Journal of Geophysical Research that looked at the rare fossils that have survived since that time, and have drawn some fascinating parallels to species who survive today in similar conditions -- on the seafloor beneath the Antarctic Ice Sheets:

The timing of the first appearance of animals is of crucial importance for understanding the evolution of life on Earth.  Although the fossil record places the earliest metazoans at 572–602 Ma, molecular clock studies suggest a far earlier origination, as far back as ~850 Ma.  The difference in these dates would place the rise of animal life into a time period punctuated by multiple colossal, potentially global, glacial events...  The history of recent polar biota shows that organisms have found ways of persisting on and around the ice of the Antarctic continent throughout the Last Glacial Maximum (33–14 Ka), with some endemic species present before the breakup of Gondwana (180–23 Ma)...  [D]espite the apparent harshness of many ice covered, sub-zero, Antarctic marine habitats, animal life thrives on, in and under the ice.  Ice dominated systems and processes make some local environments more habitable through water circulation, oxygenation, terrigenous nutrient input and novel habitats...  The recent glacial cycle has driven the evolution of Antarctica's unique fauna by acting as a “diversity pump,” and the same could be true for the late Proterozoic and the evolution of animal life on Earth, and the existence of life elsewhere in the universe on icy worlds or moons.

One group of weird animals they looked at, which apparently thrived in these harsh conditions, were frondomorphs (Phylum Petalonamae), which are thought to have left no descendants whatsoever, and whose alliances to other animals are uncertain at best.

Fossil of a Precambrian frondomorph, Charniodiscus arboreus, from the Flinders Range in Australia [Image licensed under the Creative Commons tina negus from UK, Charniodiscus arboreus, CC BY 2.0]

These peculiar beasts were apparently anchored to the seafloor and absorbed nutrients and oxygen from the frigid waters through the feathery bits, but honestly, we know barely anything about how they made a living.  Some may have -- as many Antarctic sponges and sea anemones do today -- been affixed upside-down from the underside of the ice sheet.

These animals, nicknamed "extremophiles" for obvious reasons, just about all died out when things warmed up and the ice finally melted.  But it bears mentioning how long the Snowball Earth conditions persisted -- around 140 million years.  In other words, about the same amount of time as between the end of the Jurassic Period and now.   During that time, there were minor ups and downs, temperature-wise, but that's still a huge expanse of time during which the Earth was an ice-covered wasteland.

When the snowball finally did melt, and the cold-loving extremophiles such as Charniodiscus went extinct, it opened the door for one of the major events in the history of life on Earth -- the Cambrian explosion, when all of the main phyla of animals evolved in a relative flash.  But even when conditions were at their worst, life still survived, somehow.  The fact that life can thrive in apparently hostile conditions improves our chances of finding it elsewhere in the universe, and cheers me up significantly with regards to the weather we're currently having here.

It's also further support for the famous line from the inimitable Ian Malcolm in Jurassic Park: "Life, uh, finds a way."

****************************************

Primordial soup dwellers

A paper in Nature last week blew my mind from several different perspectives.

Entitled, "Evidence for Early Life in Earth's Oldest Hydrothermal Vent Precipitates," it sounds at first like something that could only possibly interest paleontology and/or geology geeks.  But as soon as you start looking closely, you find that what this paper describes is groundbreaking.

*rimshot*

The group, led by Matthew Dodd of University College London, thin-sliced rock excavated from a piece of the Nuvvuagittuq Supracrustal Belt in Québec, one of the oldest intact rock formations on Earth.  And I do mean thin; the rock slices were, on average, 100 microns thick, or about the thickness of a sheet of printer paper.  And "old" is no exaggeration, either.  The rock is estimated at four billion years old, only three hundred million or so years after the crust solidified from molten magma.

The rock is an iron-rich sedimentary rock that formed at a hydrothermal vent -- a fissure on the deep ocean floor that is spitting out geothermally-heated, mineral-rich water.  We still have these around, mostly in places where the tectonic plates are moving apart, like the Mid-Atlantic Ridge, and even today they host a biome that is unlike any other on Earth.  There are species of shrimp, tube worm, sponges, and bacteria found nowhere else.  Not only that, they are one of only a handful of communities that is disconnected, energetically, from the Sun.  Everything else -- so, almost all life on Earth -- can trace the energy that makes it go back down the food chain and ultimately to a photosynthesizer (usually plants or phytoplankton), which are powered by sunlight.  The hydrothermal vent organisms, on the other hand, are powered by chemical reactions between the seawater and the hot stone of the upper mantle.

And when the scientists looked at the thin slices of the four-billion-year-old rock from Québec, they found...

... fossils.

The formation where the fossil-bearing rock was found [Photograph by Dominic Papineau]

The fossil traces are almost certainly from thermophilic bacteria, but form a colonial structure nearly a centimeter long.  It includes tubes, branching filaments, and spheres that are (the researchers claim) too complex to be explainable by inorganic chemical reactions.  This pushes the earliest life forms back by almost a third of a billion years earlier than the previous estimate, so we're not talking about a small shift, here.

"Using many different lines of evidence, our study strongly suggests a number of different types of bacteria existed on Earth between 3.75 and 4.28 billion years ago," said study co-author Dominic Papineau, in an interview with GeologyIn.  "This means life could have begun as little as 300 million years after Earth formed.  In geological terms, this is quick – about one spin of the Sun around the galaxy."

What this immediately brought to my mind is that it is increasingly looking as if the development of life is much faster and easier than anyone thought, and this bodes well for finding it elsewhere.  Probably lots of elsewheres, considering the billions of extrasolar planets there undoubtedly are in the Milky Way.  Perhaps, too, we might look closer to home; there may even be life in tectonically-active moons in our own Solar System such as Titan and Europa.

I'm not the only one who had this reaction.  "These findings have implications for the possibility of extraterrestrial life," Papineau added.  "If life is relatively quick to emerge, given the right conditions, this increases the chance that life exists on other planets."

Now, bear in mind that still is talking about microscopic life.  Even if the start of life turns out to be common on any sufficiently hospitable planet, that still leaves us with four variables in the Drake equation that are relatively poorly understood -- the fraction of life in the universe that becomes multicellular, the fraction of multicellular life that becomes intelligent/sentient, the fraction of intelligent life that advances in technology enough to send signals into space, and the average length of time such high-tech civilizations last.  So while the current study is encouraging to exobiology aficionados like myself, it may not have a lot of impact on our search for signs of extraterrestrial intelligence.

But no matter how you slice it (*rimshot* again), the Nature paper is amazingly cool.  It's hard to believe that such a short time after the Earth's crust solidified, there were already tiny living things building homes in the oceans.  And it boggles the imagination to think about where else similar life forms might exist -- on some other planet, perhaps, circling one of the stars we see in the night sky.

**************************************

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.

*****************************

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.

*****************************************

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!]

Thawing the snowball

One of the frightening things about a system in equilibrium is what happens when you perturb it.

Within limits, most systems can recover from perturbation through some combination of negative feedbacks.  An example is your body temperature.  If something makes it goes up -- exercise, for example, or being outside on a hot, humid day -- you sweat, bringing your temperature back down.  If your body temperature goes down too much, you increase your rate of burning calories, and also have responses like shivering -- which brings it back up.  Those combine to keep your temperature in a narrow range (what the biologists call homeostasis).

Push it too much, though, and the whole thing falls apart.  If your temperature rises beyond about 105 F, you can experience seizures, convulsions, brain damage -- or death.  Your feedback mechanisms are simply not able to cope.

This, in a nutshell, is why climate scientists are so concerned about the effects of anthropogenic carbon dioxide.  Within limits -- as with your body temperature -- an increase in carbon dioxide results in an increase in processes that remove the carbon dioxide from the atmosphere, and the whole system stays in equilibrium.  There is a tipping point, however.

The problem is that no one knows where it is -- and whether we may have already passed it.

A new piece of research from the Virginia Polytechnic Institute has indicated that this flip from stability to instability may be fast and unpredictable.  A paper authored by a team led by paleobiologist Shuhai Xiao, that came out last month in Geology, looks at one of the main destabilization events that the Earth has ever experienced -- when the "Snowball Earth" thawed out in the late Precambrian Period,  635 million years ago.

Artist's conception of the Precambrian Snowball Earth [Image is in the Public Domain, courtesy of NASA/JPL]

Xiao and his team studied rocks from Yunnan and Guizhou, China, that are called cap carbonates.  They are made of limestone and dolomite and are deposited quickly in marine environments when the carbon dioxide content of the atmosphere spikes, leading to a dramatic temperature increase and a subsequent increase in absorption of carbonates into seawater (and ultimately deposition of those carbonates on the seafloor).  The cap carbonates Xiao et al. studied were dated to between 634.6 and 635.2 million years old, which means that the entire jump in both temperature and carbon dioxide content took less than 800,000 years.

So in less than a million years, the Earth went from being completely covered in ice to being subtropical.  The jump in global average temperature is estimated at 7 C -- conditions that then persisted for the next hundred million years.

Xiao et al. describe this as "the most severe paleoclimatic [event] in Earth history," and that the resulting deglaciations worldwide were "globally synchronous, rapid, and catastrophic."

Carol Dehler, a geologist at Utah State University, is unequivocal about the implications.  "I think one of the biggest messages that Snowball Earth can send humanity is that it shows the Earth’s capabilities to change in extreme ways on short and longer time scales."

What frustrates me most about today's climate change deniers is that they are entirely unwilling to admit that the changes we are seeing are happening at an unprecedented rate.  "It's all natural," they say.  "There have been climatic ups and downs throughout history."  Which is true -- as far as it goes.  But the speed with which the Earth is currently warming is faster than what the planet experienced when it flipped between an ice-covered frozen wasteland and a subtropical jungle.  It took 800,000 years to see an increase of the Earth's average temperature by 7 degrees C.

The best climate models predict that's what we'll see in two hundred years.

And that is why we're alarmed.

It's unknown what kind of effect that climate change in the Precambrian had on the existing life forms.  The fossil record just isn't that complete.  But whatever effect it had, the living creatures that were around when it happened had 800,000 years to adapt to the changing conditions.  What's certain is that an equivalent change in two centuries will cause massive extinctions.  Evolution simply doesn't happen that quickly.  Organisms that can't tolerate the temperature fluctuation will die.

We can only speculate on the effects this would have on humanity.

This is clearly the biggest threat we face, and yet the politicians still sit on their hands, claim it's not happening, that remediation would be too costly, that we can't prevent it, that short-term profits are more important than the long-term habitability of the Earth.  Our descendants five hundred years from now will look upon the leaders from this century as having completely abdicated their responsibility of care for the people they represent.

Presuming we still have descendants at that point.

*****************************

This week's Skeptophilia book recommendation combines science with biography and high drama.  It's the story of the discovery of oxygen, through the work of the sometimes friends, sometimes bitter rivals Joseph Priestley and Antoine Lavoisier.   A World on Fire: A Heretic, an Aristocrat, and the Race to Discover Oxygen is a fascinating read, both for the science and for the very different personalities of the two men involved.  Priestley was determined, serious, and a bit of a recluse; Lavoisier a pampered nobleman who was as often making the rounds of the social upper-crust in 18th century Paris as he was in his laboratory.  But despite their differences, their contributions were both essential -- and each of them ended up running afoul of the conventional powers-that-be, with tragic results.

The story of how their combined efforts led to a complete overturning of our understanding of that most ubiquitous of substances -- air -- will keep you engaged until the very last page.

[Note:  If you purchase this book by clicking on the image/link below, part of the proceeds will go to support Skeptophilia!]