Darwin’s Rathtars

In honor of Star Wars week at the Athens Science Observer, I wrote a guest post on Rathtars and all they can teach you about evolution. Enjoy!

A rathtar roams free aboard Han Solo’s ship the Eravana. Image: Wookieepedia

When Kanjiklub and the Guavian Death Gang boarded the Eravana, the three most beloved characters in The Force Awakens suddenly found themselves in a tight spot. Thanks to a con that was risky even by Han Solo’s standards, our trio was left stranded while the film’s more charismatic heroes made a narrow escape on the Millennium Falcon. Yes, after eating a large number of the galaxy’s most dangerous gangsters and scaring off the rest, those poor, adorable rathtars were left to fend for themselves.

Thanks to the ship’s presumably heavy infestation with Corellian scavenge rats combined with a rathtar’s ability to eat literally anything, the marooned rathtars would have been able to feed and reproduce without a problem. But how would their newfound isolation affect them? Would they continue to be a colony of rathtars just like the ones on their home planet?

When a new population is founded by a few individuals from a larger group — like, for example, when three rathtars are stranded on a Baleen-class heavy freighter — a phenomenon called the “founder effect” is observed. The founder effect is a loss in genetic diversity in this new colony resulting from a random sampling of the individuals from the original population that isn’t necessarily representative of that original population. What does that mean in Galactic Basic?

Say, for example, that most rathtars have tentacles ranging in length from eight to 11 horrifying feet long and an appalling number of blood-red eyes. However, by sheer chance, or maybe because they are easier to catch, the rathtars Han Solo happened to capture had tentacles that were a merely terrifying eight and a half feet long and puce-colored eyes.

The growing rathtar colony on the Eravana was derived from those select few individuals Han Solo captured. Therefore, even though rathtars throughout the galaxy have a much wider variety of tentacle lengths and eye colors, it can be expected that after several generations, all of the Eravana rathtars will have eight and a half foot long tentacles and puce-colored eyes. There was a loss in genetic diversity in the new group, meaning that fewer versions of the genes that determine tentacle length and eye color are available to choose from when a new rathtar is produced.


Galapagos finches sketched by Darwin during his 1845 journey on the H.M.S. Beagle. Finches have very little in common with rathtars, as rathtars are horrifying. Image: Wikimedia Commons

This founder effect was famously first described by Darwin in the context of finches living on the Galapagos Islands. He noted that finches living on different Galapagos Islands appeared to have a common ancestry, but had grown different characteristics once isolated on their separate islands. Like with the Eravana rathtars, the finch populations on the different islands were established by a small number of individuals from the larger, mainland finch populations.

Over time, the differences in genetic diversity between the finch populations weren’t the only reason why they no longer resembled each other. Darwin noted that the finches’ beaks were different sizes and shapes, based on whichever type of beak was best suited to eating the specific seeds and nuts available on their islands. Something very similar would have happened to the Eravana rathtars.

Not much is known about the rathtars’ home world, but it is likely quite different from the metallic confines of a freighter. As a result, the Eravana rathtars would have also gone through the same process as Darwin’s finches, known as adaptive radiation, in order to fit into their new environment.

With successive generations, rathtar traits would naturally be selected to help them thrive on the Eravana, like smaller body size to fit through the narrow corridors and more nimble tentacles to better snack on unsuspecting Corellian scavenge rats. Given enough time, the Eravana rathtars might even become so distinct from rathtars in general that they would form a completely new species!

So if you, like me, are concerned about the marooned rathtars, worry no more! Thanks to natural selection and a presumed rat infestation, they aren’t lost and lonely, drifting aimlessly in space; the Eravana rathtars are the proud founding fathers of a new species in a galaxy far, far away.

Is This a Kissing Blog?

wide the-princess-bride-kiss

Buttercup and Westley’s final kiss in Rob Reiner’s 1987 adaptation of William Goldman’s classic, The Princess Bride

“Since the invention of the kiss, there have only been five kisses that were rated the most passionate, the most pure. This one left them all behind.” –The Princess Bride

Ah, wuv. Twue wuv. With that record-shattering kiss, Buttercup and Westley traded vows of everlasting love, the promise of a lifetime’s “As you wish”es, and approximately 80 million bacteria.


Yes, a research group at TMO in the Netherlands investigated how kissing affects a couple’s oral bacteria and found that Valentines swap more than just spit. Just like in your intestines, stomach and skin, your mouth and airways are home to a whole host of bacteria that help keep you healthy, known as your microflora.

Oral bacteria were almost certainly the first to be viewed by humans when back in 1683, Antonie van Leeuwenhoek (the father of microbiology) scraped some tarter off his own teeth and stuck it under a microscope. As many as 600 different species of bacteria can be found in the human mouth, some of which are responsible for common problems like gum disease and tooth decay. However, keeping the bacteria living in your mouth happy and balanced helps keep out disease-causing bacteria, stops bad breath, and may even help prevent more serious diseases elsewhere in your body, including stroke, cardiovascular disease, and diabetes.

While the makeup of a person’s microflora is important, it isn’t static. The mouth is an open system, meaning that the composition of the bacteria living there can be changed by things like your genes, age, diet and who you…interact with.

princess bride grandson

INTIMATE kissing? Are you trying to kill me?


The Dutch research group that performed this study swabbed the mouths of random visitors at the Artis Royal Zoo in Amsterdam. Then, the researchers had them make out for ten seconds before re-sampling. You can’t make this stuff up.

The researchers found that among their amorous zoo-goers, romantic partners had more similar oral microflora composition pre-kiss than two people not in a relationship. In particular, partners showed the most striking similarities in the microflora associated with their tongue. The long-term couples’ microflora didn’t change much post-kiss, but this is most likely because they are in like with each other and already swap bacteria fairly regularly.


The scientists conducting the study then wanted to know precisely how many bacteria are passed around per make-out sesh. To answer that question, they gave one partner of each couple probiotic yogurt containing what they referred to as “marker bacteria” and, again, had them snog away.

They then resampled everyone’s mouths to see how much bacteria had transferred from the person who ate the yogurt to the person who did not. It turns out that on average, 80 million bacteria were swapped from one mouth to the other per 10-second make-out.

aaaaaas youuuuu wiiiiiiiiiish

Aaaaaaaaassss youuuuuuuuu wiiiiiiiiiish!


True love is not as easy as one simple kiss, however; it requires effort and real commitment. In fact, the researchers found that in order for a couples’ microfloras to really change to resemble each other, they need to kiss — intimate kissing, they specified — nine times per day.

So, friends, to have the sort of love that cannot be tracked, even with a thousand bloodhounds, that cannot be broken, even with a thousand swords, that is second only to a good MLT — remember to say those three little words every time your Valentine wants a smooch: As you wish.

Finding Vibrio

anglerfish scene gif

“…Good feeling’s gone.” Image: Disney Enterprises, Inc./Pixar Animation Studios

“Come on back here! I’m gonna getcha! I’m gonna swim with you…I’m gonna be your best friend!”

Little did Marlin and Dory know that the light — and the hungry anglerfish attached to it — already had millions of best friends without whom it couldn’t glow. Thanks to bioluminescent bacteria like Vibrio fischeri and Photobacterium phosphoreum, many species have found a way to shine in the most unlikely of habitats.


Bioluminescence is the production and emission of light by a living thing. This light has a wide variety of uses, from confusing prey, to startling predators, to attracting a mate. Bioluminescent organisms can be found on most of the branches of the tree of life, from bacteria all the way up to fish.

Bioluminescence probably originated in the ocean, and is more commonly found in marine species than in ones on land. Bioluminescent marine species live across all of the ocean depths; the greatest number of bioluminescent species can be found in the ocean’s dimly lit twilight zone, but of the organisms that have adapted to life in the ocean’s deep, dark midnight zone, about 90% are bioluminescent!


A bioluminescent jelly. Image: Joshua Lambus, Flickr

Unlike the light generated by an incandescent light bulb, bioluminescent light comes not from heat, but from energy released by a chemical reaction in the organism. All bioluminescent reactions involve a “luciferin” compound and a “luciferase” protein, both from the Latin word “lucis,” meaning “of light.” When a luciferin and a luciferase come together with oxygen and ATP (a cell’s fuel source) the reaction releases energy that we see as colored light.

Luciferins and luciferases come in many different flavors — over a dozen chemical luminescence systems are known — suggesting that bioluminescence has evolved many different times under different conditions. The luciferins and luciferases usually seen in marine bioluminescent species react to produce green or blue light, as these wavelengths of light travel well through seawater without getting absorbed or scattered.


Many bioluminescent species are able to produce light on their own, but others, like our toothy anglerfish friend, have had to get more creative. A large number of bioluminescent species are not bioluminescent in their own right, but have evolved a symbiotic relationship with bioluminescent bacteria.

A symbiotic relationship is one in which two living organisms live in or on each other in a close, physical way (from the Greek sym, “together,” and bio, “life”). Such relationships aren’t always good ones; the luminous bacteria that live symbiotically with the Tanner crab, for example, are parasitic and damage the crab’s legs. In the case of the deep-sea anglerfish and its photobacteria, however, the relationship is a mutualistic one, meaning both the fish and its glowy roommates benefit from their partnership.

Luminous bacteria living in the seawater, like Vibrio fischeri or Photobacterium phosphorum, float through pores in the bulbous sac, or esca, on the end of an anglerfish’s lure and colonize it. This lure is a completely mobile appendage whose luminescence helps female anglerfish lure charming, anthropomorphized prey to their mouths and attract their unfortunate potential mates.


Bioluminescent bacteria and algae can cause glowing red tides, as seen here in Black Point, Anglesey. Image: Kris Williams, Flickr


While photobacteria can glow on their own, they just…don’t. Before they’ll start to glow, photobacteria like V. fischeri need a little help from their friends.

Bacteria are surprisingly social organisms; instead of communicating using words or howls, bacteria send signals to each other by releasing chemicals into their environment. The more bacteria there are in an area, the more signaling molecules will be floating around them. Many bacteria regulate the production of some proteins — like those that make them glow — in response to changes in levels of these signaling molecules in their surrounding environment.

Vibrio Fischeri

Vibrio fischeri, seen plated here, are quorum-sensing bacteria that regulate their bioluminescence in response to changes in population density.

Photobacteria use this kind of communication, known as quorum sensing, as a sort of light switch. Once the number of other photobacteria reaches a certain level, the bacteria detect the resulting elevated levels of signaling molecules around them. In response, the genes that encode the luciferin and luciferase proteins get turned on and the bacteria begin to glow. If the numbers of bacteria drop, however, those genes get switched off again and the glowing stops.

Quorum sensing was first observed in V. fischeri — the glowing made it easy to see the correlation between bacterial density and regulation of the genes responsible for light production. Eventually, it became clear that the process wasn’t specific to V. fischeri and its bioluminescent abilities, but was common to the regulation of many different types of proteins across the bacterial family tree.


Whether glowing bacteria or luminescent fish, the ocean’s bioluminescent organisms are stunning, sometimes literally. The next time you’re on a deep sea adventure and see a pretty light, perhaps don’t stand in awe too long—you might just go from ardent admirer to snack in a flash.

Life on Mars

M. Dudouyt’s creepy Martians from the 1917 edition of H.G. Wells’ classic novel, The War of the Worlds

The idea that extraterrestrial life may be out there is one that has captured our species’ collective imagination for centuries. It’s possible that being able to share this incomprehensibly vast universe with someone else makes us feel a little less alone. It’s equally possible that we just want advanced alien technology without the effort and expense associated with inventing it ourselves. Perhaps that’s why the theories suggesting that there is, or ever was, bacterial life on Mars is such an attractive one to Terrestrial scientists.

The presence of even unintelligent life on a planet as nearby as Mars would mean that, however improbable it may be, life somehow found a way to exist twice in the same tiny corner of the galaxy. If that’s the case, what’s to say it’s as rare as we think it is? If life could be found just next-door, we reason, the void stretching between us and another intelligent species might not be as expansive as we previously thought.

Speculation that life could be found on the Red Planet is not a new one – from Edgar Rice Burrough’s John Carter of Barsoom to the Looney Tunes’ Marvin the Martian, little green men (or huge green men, as the case may be) have played a prevalent role in science fiction since its inception. However, it wasn’t until the mid-1990s that fantasies about Martian life began to bleed into the realm of possibility.


On December 27th, 1984, the scientific community received the best Christmas present it had been given in a long time when a National Science Foundation team found a meteorite in the Allan Hills of Antarctica. The meteorite, now named ALH84001, was described by the team as being grayish-green, highly-shocked, and the rarest find of that season. Additional remarks in the field notes include simply, “Yowza-yowza.”

Nine years of analysis revealed that the meteorite was both of Martian origin and incredibly ancient; radiometric dating determined that ALH84001 is approximately 4.1 billion years old, meaning it formed very shortly after Mars itself came into being. After an impact event launched it from Mars’ surface, ALH84001 had spent nearly 16 million years dancing out in space before crash-landing in Antarctica somewhere around 13,000 years ago.

In 1996, ALH84001 found its way into the spotlight when NASA scientist David McKay reported a shocking discovery: this meteorite may hold evidence of ancient microbial life on Mars.

The carbonate disks found in ALH84001, rimmed with black magnetite rings. Image: Thomas-Keprta et al. (2009)

The carbonate disks found in ALH84001, rimmed with black magnetite rings.
Image: Thomas-Keprta et al. (2009)


While examining the meteorite, McKay’s group found that ALH84001 contains flat carbonate disks rimmed with rings of tiny magnetite crystals. Magnetite is a commonly occurring iron oxide that is a natural magnet. Terrestrial magnetite is known to form in igneous and metamorphic rock when under pressure in high temperatures. However, some magnetite is of biogenic origin – it is created by living organisms.

In 1975, Richard Blakemore discovered a group of magnetite-producing bacteria that he named “magnetotactic bacteria.” Magnetite derived from magnetotactic bacteria is very distinct; they produce very fine (as small as 0.000000005 meters!) magnetite crystals bound by intracellular membranes. These tiny organelles are called “magnetosomes.”

The magnetosomes of magnetotactic bacteria are aligned in chains that act like a bacterial compass needle – they orient the bacteria into perfect alignment with the Earth’s geomagnetic field. The bacteria use this process, called “magnetotaxis,” to narrow their search for an ideal growing environment.

Interestingly, magnetotactic bacteria exist in numerous forms in as many varied locations around the planet. Magnetotactic cocci, rods, vibrios, spiriella, and even multicellular forms have been found in diverse aquatic environments, from oceans to lakes to rice paddies. The only things these tiny, living magnets seem to have in common is that they are all Gram-negative, contain magnetosome chains, and live in watery habitats.

A transmission electron micrograph of a magnetotactic bacterium. The dark spheres are the magnetosomes, lined up in a chain along the bacterium's axis. Image: Nature Education

A transmission electron micrograph of a magnetotactic bacterium. The dark spheres are the magnetosomes, lined up in a chain along the bacterium’s axis.
Image: Nature Education

The magnetite from magnetotactic bacteria is so finely, purely, and consistently produced, that to date, no lab has been able to synthesize magnetite of the quality made by magnetotactic bacteria. However, scientists have been able to use both magnetotactic bacteria and isolated magnetosomes in a variety of medical and scientific applications.


That magnetite was found in ALH84001 is in itself not very exciting; magnetite is made all the time near terrestrial volcanoes and hydrothermal vents. In fact, about 75% of the magnetite crystals ringing ALH84001’s carbonate disks may have been produced by those same processes. What excited the scientific community were the unique chemical and physical properties of the remaining 25% of the crystals.

These magnetite crystals are chemically pure and very fine, measuring a tiny tens of nanometers in size. In terms of size, shape, purity, and magnetic properties, these crystals match the characteristics of magnetite produced by terrestrial magnetotactic bacteria – the same properties that neither humans nor geological processes could imitate.

Many experts therefore believe that these magnetite crystals are a Martian biosignature: a physical or chemical marker of the presence of life. If this is true, these Martian bacteria are the earliest forms of life known to man.

A side-by-side comparison showing the similarities between magnetite crystals found in ALH84001 and those produced by the Earth magnetotactic bacterium strain MV-1. Image: Kathie Thomas-Keprta, NASA Johnson Space Center

A side-by-side comparison showing the similarities between magnetite crystals found in ALH84001 and those produced by the Earth magnetotactic bacterium strain MV-1.
Image: Kathie Thomas-Keprta, NASA Johnson Space Center


Whether or not these magnetite crystals were actually made by ancient Martian magnetotactic bacteria is a subject of hot debate. Other potential Martian biosignatures exist, though none so definitive as a mineral of biogenic origin. Methane found in Martian rock samples may imply active biological processes are taking place on Mars, perhaps just below the planet’s surface.

Further, analysis of the carbonate disks in ALH84001 showed that they were formed during what is known as the Noachian epoch on Mars, when high numbers of asteroid and meteorite impacts formed the oldest Martian surfaces that exist today, and water was possibly an abundant resource on Mars. These disks precipitated 3.9 billion years ago in a shallow, sub-surface watery environment near a temperature of 18°C. This means that this magnetite was produced in an environment similar to ones where some terrestrial magnetotactic bacteria are found.

Some experts believe that, rather than being made by bacteria, the magnetite may have been produced by the geological process of thermal decomposition, as often seen on Earth. However, experiments have shown that none of the currently proposed scenarios for geological production of these crystals could have resulted in magnetite crystals with these properties. This doesn’t prove that the crystals definitely resulted from a biological process; it just means that we still can’t rule out the possibility that they did.


It may not be a giant face on the surface of Mars, but ALH84001 has brought us closer to finding extraterrestrial life than ever before. Will our search for neighbors somewhere in the cosmos prove fruitless, or miraculously reveal that we are not alone in this vast universe? We may never know for sure, but at least the clues hidden in ALH84001 have given mankind a real reason to hold on to hope, and Congress a real reason to give NASA funding. Perhaps that’s a big enough miracle in itself.

Parasite Lost

Ernst Haeckel's illustration of parasitic worms from the 1904 edition of Kunstformen der Natur. A Taenia solium rectangular proglottid and round scolex can be seen in the middle right of this image.

Ernst Haeckel’s illustration of parasitic worms from the 1904 edition of Kunstformen der Natur. A Taenia solium rectangular proglottid and round scolex can be seen in the middle right of this image.

While parasites are fantastically well-adapted to life in their specific hosts, that usually means they are somewhat lacking in the “living literally anywhere else” department. Thus, when natural selection has nixed the ability to read a map and a parasite finds itself in unfamiliar territory, it is rarely very good at improvising.

Years of co-evolution with their hosts have helped many parasites find a happy medium between harming its host and causing just enough damage to happily feed, reproduce, and spread to a new host. After all, it’s ill-advised to destroy your meal ticket. However, if a parasite finds itself in a host that is not a usual part of its lifecycle – an incidental infection – the usual rules no longer apply.


As a parasite’s attempts to MacGyver its way out of an incidental host to a more familiar one often aren’t pretty, incidental infections can be more severe, or simply more bizarre, than the normal course of infection.

Take, for example, the case of cutaneous larval migrans. Cutaneous larval migrans, also known by the appetizing names “creeping eruption” and “sandworm,” occurs when a human becomes infected with canine hookworm larvae. These parasites have evolved to infect the intestines of dogs and other canids, and so become very confused when they find themselves in a two-legged, hairless ape instead of a loveably dopey quadruped.

A leg with the red, itchy trails characteristic of cutaneous larval migrans. Image: Grook Da Oger

A leg with the red, itchy trails characteristic of cutaneous larval migrans.
Image: Grook Da Oger

Rather than stopping to ask for directions, the now very lost larvae short-circuit and meander in the upper layers of the skin, trying to find their way to the gut. This aimless rambling gives the disease its name – though it uses more SAT words than necessary to say so, “cutaneous larval migrans” just means “worm larvae that wander in the skin.”

The result of the larvae’s migration through the skin is an angry, red, very itchy rash. The little worms actually leave behind tiny, visible trails on the skin in their search for a familiar environment (someone call Kevin Bacon.)

On the bright side, since the parasites are not well-adapted to life in a human host, they cannot complete their life cycle and survive in people. As a result, the larvae die within a few weeks and the rash clears up on its own, though treatment with standard anthelmintic drugs can speed up the process.

An 1831 illustration of Taenia solium by Johann Gottfried Bremser

An 1831 illustration of Taenia solium by Johann Gottfried Bremser


Sometimes meeting a familiar host at an unfamiliar time can be equally as disastrous for the parasite and the host.

The pork tapeworm, Taenia solium, is a very common human intestinal worm. If a person eats undercooked pork that is infected with hard, spherical cysticerci – encysted T. solium larvae – the digestive proteins in the intestines propel the worm into the next stage of its lifecycle.

During this stage, the worm latches onto the intestinal lining, feeds itself on the nutrients in its surroundings, and sheds eggs in the feces of their human host. Pigs, who are not particularly known for their discerning palates, become infected upon eating these eggs, completing the worm’s lifecycle.

While not pretty, a normal Taenia infection is not severe; if any symptoms show up at all, the worst of them include indigestion and mild anemia. However, when it comes to parasites, timing is everything. Even a relatively harmless parasite can be thrown for a loop if a host enters the mix at the wrong stage of its lifecycle.

If a human ingests T. solium eggs instead of its larvae, that person effectively takes the place of the pig in the lifecycle. This means that cysticerci form in the tissues of the human instead of the pig.

This MRI image shows the brain of a patient suffering from Neurocysticercosis. Each dark round spot is an encysted Taenia solium larva.

This MRI image shows the brain of a patient suffering from Neurocysticercosis. Each dark, round spot is an encysted Taenia solium larva.

This isn’t ideal for the parasite; as cannibalism is typically frowned upon in polite society, a human host at this stage of the lifecycle is a dead-end for the worm. As a result, pork tapeworms haven’t had a chance to evolve to become as good at forming cysticerci in humans as they are in pigs.

These misplaced, inexperienced tapeworms have a penchant for forming cysts in brain tissue in people, as opposed to muscle tissue in pigs. The resulting disease – called neurocysticercosis – causes severe neurological problems.

Though neurocysticercosis is preventable through proper sanitation and hygiene practices, the lack of basic sanitation infrastructure and sufficient medical care has allowed this disease to persist as one of the leading causes of epileptic seizures in the developing world.


West Nile Virus is an arbovirus of simple tastes – give it a warm environment populated by birds and mosquitoes, and it will be happy to cycle back and forth between those two hosts. It doesn’t feel the need to make life difficult with an overly complex lifecycle like some pathogens (…we’re looking at you, Dicrocoelium dendriticum.)

Sadly, you know what they say about the best-laid plans of mice and Flaviviridae. If a West Nile Virus-infected mosquito takes a blood meal from a human instead of a bird, the virus enters this strange host and finds itself trapped.

Though it passes into this new host easily enough, it cannot reproduce efficiently in it, and so only achieves low levels of virus in the bloodstream. This means that even if another mosquito feeds on one of these incidental hosts, it cannot pick up enough of the virus to become infected.

A transmission electron micrograph of West Nile Virus. This image has been colored - the original would have been in black and white. Image: Cynthia Goldsmith (CDC)

A transmission electron micrograph of West Nile Virus. This image has been colored – the original would have been in black and white.
Image: Cynthia Goldsmith (CDC)

Despite the fact that West Nile Virus does not replicate well in people, the virus still causes disease in about 20% of infected human hosts. Most people who develop West Nile fever experience mild symptoms that soon resolve themselves, such as headache and fever.

However, in some rare cases the virus causes encephalitis – inflammation of the brain – or meningitis – inflammation of the brain, spinal cord, and surrounding tissues. Encephalitis and meningitis cause serious neurological problems, including paralysis in the limbs or, worse, the muscles needed to breathe.


In the end, things rarely go well for a parasite that has been cast out of its intended host and into unfamiliar territory. Whether they end up in the right place at the wrong time, or simply go where they never should have gone in the first place, parasites that have lost their way are far from paradise.