Tooth and claw

The earliest living things, way back in the Precambrian Era, were almost certainly either autotrophs (those that could produce their own nutrients from inorganic chemicals) or else scavengers.  One of the reasons for this inference is that these early life forms had few in the way of hard, fossilizable parts, of the kind you might use to protect yourself from predators.  Most of the fossils from that era are casts and impressions, and suggest soft-bodied organisms that, all things considered, had life fairly easy.

But the Cambrian Explosion saw the rather sudden evolution of exoskeletons, scales, spines... and big, nasty, pointy teeth.  There's credible evidence that one of the main reasons behind that rapid diversification was the evolution of carnivory.  Rather than waiting for your neighbor to die before you can have a snack, you hasten the process yourself -- and create strong selection for adaptations involving self-defense and speed.

After that, life became a much dicier business.  I was discussing this just a couple of days ago with the amazing paleontologist and writer Riley Black (you should definitely check out her books at the link provided).  She'd posted on Bluesky about the terrifying Cretaceous mosasaur Tylosaurus proriger, which got to be a mind-blowing twelve meters long (around the length of a school bus).  This species lived in the Western Interior Seaway, which back then covered the entire middle of the North American continent.  I commented to her what a difficult place that must have been even to survive in.  "We always describe the Western Interior Seaway as 'a warm, shallow sea,'" Riley responded.  "Ahh, soothing -- and not like 'holy shit these waters are full of TEETH!'"

What's interesting, though, is that even though we think of predators as mostly being macroscopic carnivores, this practice goes all the way down to the microscopic.  The topic comes up because of a paper this week in Science about some research at ETH Zürich about a species of predatory marine bacteria called Aureispira.  These little things are downright terrifying.  They slither about on the ocean floor looking for prey -- other bacteria, especially those of the genus Vibrio -- and when they encounter one, they throw out structures that look like grappling hooks.  The hooks get tangled in the victim's flagella, and at that point it's game over.  The prey is pulled toward the predator, and when it's close enough, it shoots the prey with a microscopic bolt gun, and then chows down.

Aureispira isn't a one-off.  The soil bacterium Myxococcus xanthus forms what have been called "wolf packs" -- biofilms of millions of bacteria that can be up to several centimeters wide, that glide along soil particles, digesting any other bacteria or fungi they happen to run across. 

A "wolf pack" of Myxococcus xanthus [Image licensed under the Creative Commons Trance Gemini, M. xanthus development, CC BY-SA 3.0]

This one immediately put me in mind of one of the most terrifying episodes of The X Files; "Field Trip."  In this freaky story, people are put into a series of powerful hallucinations after inhaling spores of a microorganism.  The hallucinations keep the victim quiet -- while (s)he is then slowly digested.

Of course, the microbe in "Field Trip" isn't real (thank heaven), but there are plenty of little horrors in the world of the tiny that are just as scary.  Take, for example, the aptly-named Vampirococcus, which is an anaerobic aquatic genus that latches onto other bacterial cells and sucks out their cytoplasm.

But the weirdest one of all is the bizarre Bdellovibrio, which is a free-swimming aquatic bacterium that launches itself at other single-celled organisms, moving at about a hundred times its own body length per second, then uses its flagella to spin at an unimaginable one hundred revolutions per second, turning itself into a living drill.  The prey's cell membrane is punctured in short order, and the Bdellovibrio burrows inside to feast on the innards.

So.  Yeah.  When Alfred, Lord Tennyson said that nature is "red in tooth and claw," I doubt he was thinking of bacteria.  But some of them are as scary as the mosasaurs I was discussing with Riley Black.  The world is a dangerous place -- even on the scale of the very, very small.

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The stowaways

Aficionados of the Star Trek universe undoubtedly recall the iconic character Jadzia Dax.  Dax was a Trill -- a fusion of a humanoid host and a strange-looking brain symbiont.  The union of the two blended their personalities, resulting in what was truly a new, composite life form.

Star Trek is amazing in a lot of ways, not least because of their attention to current science and an uncanny prescience about where science is heading.  It turns out that we're all composite life forms.  We carry around something like 39 trillion bacterial cells in and on our own bodies -- the vast majority of which are either commensals (neither helpful nor harmful) or are actually beneficial -- a number that is higher than the number of human cells we have.  Each of our cells also contains mitochondria, which are the descendants of endosymbiotic bacteria that have inhabited the cells of eukaryotes for billions of years, and without which we couldn't release energy from our food molecules.  Plants have not only mitochondria but chloroplasts, yet another species of bacteria that like mitochondria, have their own DNA, took up residence in their hosts billions of years ago, and have been there ever since.

But the rabbit hole goes a hell of a lot deeper than that.  By some estimates, between five and eight percent of our genomes are endogenous retroviruses -- genetic fragments left behind by viruses that spliced their DNA into ours.  Like our bacterial hitchhikers, a good many of these are either neutral or beneficial; for example, the production of bile, estrogen, and several proteins essential for the formation of the placenta are all directly affected by endogenous retroviral genes.  A few do seem to be deleterious, and have roles in certain cancers, autoimmune diseases, and neurological disorders like ALS and schizophrenia.

What brings this topic up is an astonishing study led by Tyler Coale, of the University of California - Santa Cruz, that came out in the journal Science this week.  Coale's study found there's yet another example of endosymbiosis -- this one a lot more recently evolved -- which turned a formerly free-living nitrogen-fixing bacterium into a true cellular organelle.

Nitrogen is critical for the production of both proteins and DNA.  Although 78% of the air we breathe is nitrogen, it's completely useless to us; we breathe it right back out.  All the nitrogen in our bodies' proteins and nucleic acids had to pass through a food chain that started with nitrogen-fixing bacteria, the only known organisms that can absorb nitrogen from the air and convert it to an organic compound.  Leguminous plants like beans, peas, alfalfa, and clover have a nifty symbiotic arrangement with nitrogen-fixing bacteria; they create nodules in their roots where the bacteria live, and the bacteria provide the plants with a ready source of nitrogen.

But in legumes, the two remain independent organisms.  What Coale and his colleague discovered is a species of algae (Braarudosphaera bigelowii) in which the bacteria (UCYN-A) have evolved to become inseparable from the host cells.  In other words, they became an organelle, just like mitochondria and chloroplasts.

Although there's no canonical definition of organelle, most biologists include two must-haves: (1) coordinated division of the organelle within the cell; and (2) the evolution of a transport system that allows for specific tagging and importation of proteins into the organelle.  By those standards, UCYN-A is definitely an organelle.  

"Both boxes are checked by Coale," said Jeff Elhai, microbiologist at Virginia Commonwealth University.  "Even to the semantic purists, UCYN-A must be counted as an organelle, joining mitochondria, chloroplasts and chromatophores."

All these stowaways, in the cells of just about every living thing on Earth, call into question what exactly we mean not only by the word organelle but by the word organism.  The high-school-biology-class definition of an organism is "an individual life form of a species."  But is there any such thing?  The ostensibly individual life form called Gordon who is currently writing this post is made of (at least) equal numbers of human cells and cells from different species of bacteria, without many of which I'd be sick as hell, or possibly even dead.  Remove the symbiotic mitochondria from within my cells, and I'd definitely be dead -- within minutes.  Deeper still, at a minimum, one in twenty of the genes in my "human DNA" comes from viruses and bacteria.

Looked at closely, I'm as put together of spare parts as the Junk Man in Lost in Space.  Fortunately, I appear to run a bit more smoothly most days than he did.

In any case, calling me "a single organism" is so far from accurate it's almost laughable.

Honestly, it's kind of cool how interconnected everything is.  Back in the days of the first serious taxonomist, Swedish biologist Carl Linnaeus, scientists had the idea that all living things were categorizable into neat little cubbyholes.  Not only is that incorrect on the species level (something I wrote about in detail a couple of years ago), it's not even true on the individual level or on the level of genomes.  Life on Earth is a huge, tangled skein of threads.  The whole thing puts me in mind of a quote from John Muir: "Tug at a single thing in nature, and you find that it is hitched to everything else in the universe."

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Life in the shadows

In Michael Ray Taylor's brilliant1999 book Dark Life, the author looks at some of the strangest forms of life on Earth -- extremophiles, organisms (mainly bacteria) that thrive in places where nothing else does.  Surrounding hydrothermal vents under crushing pressures and temperatures over 100 C, buried underground below the deepest mines, frozen in Antarctic ice, floating in boiling, acidic hot springs.  Taylor himself is a veteran spelunker and got interested in the topic after running into the aptly-named snottites -- biofilms found in caves that hang downward from the ceiling and are the consistency of, well, snot.

The brilliant colors of Grand Prismatic Spring in Yellowstone National Park are due, in part, to extremophilic bacteria [Image is in the Public Domain]

Taylor's contention -- that such bizarre creatures are so numerous that they outnumber all other life forms on Earth put together -- got a boost from a piece of research published in the Journal of Geomicrobiology.  Written by a team from the University of Toronto -- Garnet S. Lollar, Oliver Warr, Jon Telling, Magdalena R. Osburn, and Barbara Sherwood Lollar -- it describes the discovery, 7,900 meters underground, of a thriving ecosystem of microbes in a mine 350 kilometers north of Toronto.

The life forms are odd in a number of respects.  The first is that they're anaerobic -- they don't need oxygen to survive.  The second is that they metabolize sulfur, primarily in the form of iron sulfate, better known as pyrite or fool's gold.  It's a food chain completely unhooked from light -- for nearly every other organism on Earth, the energy they contain and utilize can ultimately be traced back to sunlight.  Here, if you follow the energy backwards, you arrive at the geothermal heat from the mantle of the Earth producing reduced (high energy) compounds that can support a food web, similar to what you see in deep-sea hydrothermal vents.

"It's a fascinating system where the organisms are literally eating fool's gold to survive," team member Barbara Sherwood Lollar said in an interview with NBC News.  "What we are finding is so exciting — like ‘being a kid again’ level exciting."  The ecosystem is in the Laurentian Shield, one of the oldest and most geologically-stable places on Earth, so it's likely that this thriving community deep underground has been there for a billion years or more.  "The number of systems we've looked at so far really is limited, but they probably had a single origin at some point in life’s four-billion-year history."  As far as their discovery, she added, "We see only what we look for.  If we don't look for something, we miss it."

And it's a lot to miss.  The current research springboards off a 2018 report sponsored by the Deep Carbon Observatory conducted by a team led by Cara Magnabosco, a geobiologist at the Swiss technical university ETH Zurich, which estimated that some 5 x 10^29 cells live in the deep Earth.

For those you who don't like scientific notation, that's five hundred thousand trillion trillion organisms.  Put succinctly, it's a really freakin' huge number.

Considering the (to us) inhospitable conditions a lot of these organisms live under, it raises hopes of finding life in other, perhaps unexpected, places in the universe.  Astronomers talk about the "Goldilocks zone," the region around a star that has temperatures where water is a liquid, and that to host life a planet would have to have a similar mass to Earth and be orbiting a star relatively similar to the Sun.  The University of Toronto research suggests that may be placing unnecessary and inaccurate strictures on where life can exist, and that we may have to rethink our definition of what we mean by "hospitable conditions."

"We're finding we really don't understand the limits to life," Sherwood Lollar said.

Which also raises the question of whether we'd recognize alien life if we saw it.  Star Trek may have been prescient; they expanded the boundaries of what we think of as life by featuring aliens that were gaseous, crystalline, thrived at searing temperatures, could tolerate the chill dark vacuum of space, or were composed of pure energy.  While some of these -- at least at first glance -- seem pretty far-fetched, what the current research suggests is that we shouldn't be too hasty to say, "Okay, that's out of the question."

"We've literally only scratched the surface of the deep biosphere," said Robert Hazen, mineralogist at the Carnegie Institution’s Geophysical Laboratory in Washington, and co-founder of Deep Carbon Observatory.  "Might there be entire domains that are not dependent on the DNA, RNA and protein basis of life as we know it?  Perhaps we just haven’t found them yet."

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Microborgs

One of the most terrifying alien species in the Star Trek universe is the Borg, a hive-mind collective of interlinked cyborgs that reproduce by assimilating individuals from other species, not the old-fashioned way (although they may do that, too, judging by how taken the Borg Queen was with Captain Picard in Star Trek: First Contact).  

Turns out the assimilate-your-neighbor approach isn't limited to the world of science fiction.  There are terrestrial species who seem to follow the Borg's mantra of "We will add your biological and technological distinctiveness to our own."  Bacteria are especially good at doing this; they have small, mobile pieces of DNA called plasmids that are capable of being exchanged between cells, allowing gene flow without (strictly speaking) sexual reproduction.  Unfortunately for us, these plasmids frequently contain such human-unfriendly gene constructs as antibiotic resistance sequences and "pathogenicity islands" -- genes that code for a virulent attack on the host, such as the ones in the nasty strains of E. coli that can land you in the hospital.

Recently, however, scientists discovered a species of bacteria that has an assemblage of chunks of assimilated DNA (they actually called these strands "Borgs" after the Star Trek villains) that might prove useful to humans rather than harmful.  A species of Archaea (an odd clade of bacteria relatively unrelated to other, more common species, which includes groups that specialize in living in acidic thermal springs, anaerobic mud, and extremely salty water) called Methanoperedens was discovered in lake mud in western North America, and it was found to consume methane -- and has Borgs that allow it to do so at a spectacular rate.

Methanoperedens is odd even without the superlatives.  Most of the Archaea that metabolize methane don't consume it, they create it.  Methanogens -- Archaea that live primarily in deep ocean sediments -- produce methane as a byproduct of their metabolism, secreting it in the form of methane clathrate (frozen methane hydrate) at such a rate that the abyssal plains are covered with the stuff.  (Some ecologists believe that methanogens are, individual for individual, the commonest organisms on Earth, outnumbering all other species put together.)  

Burning methane clathrate -- "flammable snow" [Image is in the Public Domain courtesy of the United States Geological Service]

Methanoperedens, though, is a different sort of beast.  It lives by breaking down methane.  More interesting still, this ability comes from the fact that it has Borgs almost a third the size of its ordinary complement of DNA, made up of gene fragments assimilated from a dozen different species.

What has sparked interest in this bizarre species is the potential for using it to combat climate change.  Methane is a powerful greenhouse gas -- it has thirty times the heat-trapping capacity that carbon dioxide does -- and there's a significant concern that as the Earth warms, decomposing organic matter in the tundra will trigger a positive feedback loop, releasing more methane and warming the planet further.  If this methane-eating bacteria could consume some of the excess methane, it's possible that it could be turned into a tool for bringing the climate back into equilibrium.

I'm a little dubious, however.  It seems unlikely that any kind of attempt to culture Methanoperedens would be possible on a big enough scale to make a difference.  It'd be nice if we'd just face up to the fact that there is, and always has been, one obvious solution; stop burning so damn much fossil fuel.  We're so desperate to cling to our conspicuous-consumption lifestyle that we can't face the reality of what we're doing to the long-term habitability of the Earth.  (It doesn't help, of course, that a great many of our politicians here in the United States are being funded by the fossil fuel industry.)

Whether or not this bacteria species turns out to have any practical applications, the whole phenomenon of evolution by assimilation of DNA from other species is absolutely fascinating.  We ourselves contain "foreign genes" -- most notably endogenous retroviruses, pieces of viral DNA that have taken up permanent residence in our DNA and which might comprise as much as five percent of our total genome.  (There is good evidence that the activation of certain endogenous retroviruses is connected to the development of multiple sclerosis and some forms of schizophrenia.)

Walt Whitman didn't know how true his words were when he said, "I contain multitudes."

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Space germs

I'm fully in support of pure research, which should be obvious to anyone who is a regular reader of Skeptophilia.  But sometimes I run into a paper that leaves me scratching my head.

This happened this past weekend when I stumbled upon a press release from the University of Exeter entitled, "Mammals Could Struggle to Fight Space Germs."  The gist was that a team led by microbiologist Neil Gow did a series of experiments exposing mammalian cells to lab-synthesized peptides containing two amino acids that have been detected in space but not found in terrestrial proteins (isovaline and α-aminoisobutyric acid), and they found that the cell cultures had a "weak immune response."  From this, they concluded that if we're exposed to extraterrestrial microbes, we might really suck at fighting them off.

[Image licensed under the Creative Commons Phoebus87 at English Wikipedia, Symian virus, CC BY-SA 3.0]

This seemed like a rather overblown conclusion, so I went to the original paper (always a good idea; even university press releases are often oversimplifications or miss important points).  In this case, though, the press release was pretty much spot-on.  Here it is, straight from the paper:

The discovery of liquid water at several locations in the solar system raises the possibility that microbial life may have evolved outside Earth and as such could be accidently introduced into the Earth’s ecosystem.  Unusual sugars or amino acids, like non-proteinogenic isovaline and α-aminoisobutyric acid that are vanishingly rare or absent from life forms on Earth, have been found in high abundance on non-terrestrial carbonaceous meteorites.  It is therefore conceivable that exo-microorganisms might contain proteins that include these rare amino acids.  We therefore asked whether the mammalian immune system would be able to recognize and induce appropriate immune responses to putative proteinaceous antigens that include these rare amino acids. To address this, we synthesised peptide antigens based on a backbone of ovalbumin and introduced isovaline and α-aminoisobutyric acid residues and demonstrated that these peptides can promote naïve OT-I cell activation and proliferation, but did so less efficiently than the canonical peptides.  This is relevant to the biosecurity of missions that may retrieve samples from exoplanets and moons that have conditions that may be permissive for life, suggesting that accidental contamination and exposure to exo-microorganisms with such distinct proteomes might pose an immunological challenge.

Okay, I'll admit that this is one possible conclusion you could draw; it certainly has been riffed on often enough in science fiction, starting all the way back in 1969 with The Andromeda Strain.  (You could argue that it goes back further than that, given that at the end of H. G. Wells's 1898 novel The War of the Worlds, the invading Martians are destroyed by terrestrial microbes to which they have no natural immunity.)

The other possibility, however, is that the microbes wouldn't affect us at all.  When pathogens attack our cells, they usually obtain ingress by bonding to receptors on the surface.  Those receptors can be amazingly specific; this is why there are so many strains of flu, some of which only attack birds or pigs... or humans.  The immune species, in this case, lack the surface proteins that can form bonds to the viral proteins, so they don't get in.  The result: no disease.

In fact, it's even more specific than that.  In 2006, an outbreak of H5N1 bird flu generated worries about a pandemic, until it was learned that although highly contagious in birds, it only affects humans if the virus binds deep in the lung tissue -- the receptors in the upper respiratory system aren't able to bind to the virus efficiently (fortunately for us).  The only ones who became ill were poultry workers who were exposed to dust and debris in poultry houses.  No cases of human-to-human transmission were recorded.

So my suspicion is that extraterrestrial microbes probably wouldn't be able to attack us at all.  And given that our tissues would lack the two oddball amino acids the researchers used in their experiments, it seems pretty likely that if the microbes did get in, they'd starve to death.  (Put more scientifically, our proteins would lack two amino acids they need, so we wouldn't be of much use to them as a food source.)

Of course, it's possible that Gow et al. are right, and extraterrestrial microorganisms would consider the Earth an all-you-can-eat buffet.  But given that (1) the number of extraterrestrial microorganisms we've actually studied is zero, and (2) there are equally persuasive arguments to the contrary, it might be a little bit of a premature conclusion.

Now, that doesn't mean we should be bringing outer space debris to Earth, sans quarantine.  Hell, I've read The Colour Out of Space, and last thing I want is to have a gaseous entity from a meteorite cause my limbs to crumble and fall off.  COVID-19 is bad enough, thanks.  We really don't need any more reasons to panic, however.  So for now, let's confine ourselves to dealing with threats that currently exist.

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Being in the middle of a pandemic, we're constantly being urged to wash our hands and/or use hand sanitizer.  It's not a bad idea, of course; multiple studies have shown that communicable diseases spread far less readily if people take the simple precaution of a thirty-second hand-washing with soap.

But as a culture, we're pretty obsessed with cleanliness.  Consider how many commercial products -- soaps, shampoos, body washes, and so on -- are dedicated solely to cleaning our skin.  Then there are all the products intended to return back to our skin and hair what the first set of products removed; the whole range of conditioners, softeners, lotions, and oils.

How much of this is necessary, or even beneficial?  That's the topic of the new book Clean: The New Science of Skin by doctor and journalist James Hamblin, who considers all of this and more -- the role of hyper-cleanliness in allergies, asthma, and eczema, and fascinating and recently-discovered information about our skin microbiome, the bacteria that colonize our skin and which are actually beneficial to our overall health.  Along the way, he questions things a lot of us take for granted... such as whether we should be showering daily.

It's a fascinating read, and looks at the question from a data-based, scientific standpoint.  Hamblin has put together the most recent evidence on how we should treat the surfaces of our own bodies -- and asks questions that are sure to generate a wealth of discussion.

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

Life in the shadows

In Michael Ray Taylor's brilliant1999 book Dark Life, the author looks at some of the strangest forms of life on Earth -- extremophiles, organisms (mainly bacteria) that thrive in places where nothing else does.  Surrounding hydrothermal vents under crushing pressures and temperatures over 100 C, buried underground below the deepest mines, frozen in Antarctic ice, floating in boiling, acidic hot springs.  Taylor himself is a veteran spelunker and got interested in the topic after running into the aptly-named snottites -- biofilms found in caves that hang downward from the ceiling and are the consistency of, well, snot.

The brilliant colors of Grand Prismatic Spring in Yellowstone National Park are due, in part, to extremophilic bacteria [Image is in the Public Domain]

Taylor's contention -- that such bizarre creatures are so numerous that they outnumber all other life forms on Earth put together -- just got a boost last week from a piece of research published in the Journal of Geomicrobiology.  Written by a team from the University of Toronto -- Garnet S. Lollar, Oliver Warr, Jon Telling, Magdalena R. Osburn, and Barbara Sherwood Lollar -- it describes the discovery, 7,900 meters underground, of a thriving ecosystem of microbes in a mine 350 kilometers north of Toronto.

The life forms are odd in a number of respects.  The first is that they're anaerobic -- they don't need oxygen to survive.  The second is that they metabolize sulfur, primarily in the form of iron sulfate, better known as pyrite or fool's gold.  It's a food chain completely unhooked from light -- for nearly every other organism on Earth, the energy they contain and utilize can ultimately be traced back to sunlight.  Here, if you follow the energy backwards, you arrive at the geothermal heat from the mantle of the Earth producing reduced (high energy) compounds that can support a food web, similar to what you see in deep-sea hydrothermal vents.

"It's a fascinating system where the organisms are literally eating fool's gold to survive," team member Barbara Sherwood Lollar said in an interview with NBC News. "What we are finding is so exciting — like ‘being a kid again’ level exciting."  The ecosystem is in the Laurentian Shield, one of the oldest and most geologically-stable places on Earth, so it's likely that this thriving community deep underground has been there for a billion years or more.  "The number of systems we've looked at so far really is limited, but they probably had a single origin at some point in life’s four-billion-year history."  As far as their discovery, she added, "We see only what we look for.  If we don't look for something, we miss it."

And it's a lot to miss.  The current research springboards off a 2018 report sponsored by the Deep Carbon Observatory conducted by a team led by Cara Magnabosco, a geobiologist at the Swiss technical university ETH Zurich, which estimated that some 5 x 10^29 cells live in the deep Earth.

For those you who don't like scientific notation, that's five hundred thousand trillion trillion organisms.  Put succinctly, it's a really freakin' huge number.

Considering the (to us) inhospitable conditions a lot of these organisms live under, it raises hopes of finding life in other, perhaps unexpected, places in the universe.  Astronomers talk about the "Goldilocks zone," the region around a star that has temperatures where water is a liquid, and that to host life a planet would have to have a similar mass to Earth and be orbiting a star relatively similar to the Sun.  The University of Toronto research suggests that may be placing unnecessary and inaccurate strictures on where life can exist, and that we may have to rethink our definition of what we mean by "hospitable conditions."

"We're finding we really don't understand the limits to life," Sherwood Lollar said.

Which also raises the question of whether we'd recognize alien life if we saw it.  Star Trek may have been prescient; they expanded the boundaries of what we think of as life by featuring aliens that were gaseous, crystalline, thrived at searing temperatures, could tolerate the chill dark vacuum of space, or were composed of pure energy.  While some of these -- at least at first glance -- seem pretty far-fetched, what the current research suggests is that we shouldn't be too hasty to say, "Okay, that's out of the question."

"We've literally only scratched the surface of the deep biosphere," said Robert Hazen, mineralogist at the Carnegie Institution’s Geophysical Laboratory in Washington, and co-founder of Deep Carbon Observatory.  "Might there be entire domains that are not dependent on the DNA, RNA and protein basis of life as we know it?  Perhaps we just haven’t found them yet."

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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!]