Who Was Horse Eve?

Her story doesn't involve any borrowed ribs or knowledge-bestowing apples, but she was the female forbear of all horses alive today. Researchers say the Eve of horses lived about 140,000 years ago. Her family tree contains some revealing gossip about when, and where, horses began their relationship with humans.

To understand the story of Horse Eve, you'll have first convince yourself that any group of living organisms has a most recent common ancestor. Think of yourself and a friend. Unless he or she is descended from an unknown, second branch of life that happened to evolve exactly the same way ours did--without ever interbreeding--then at some point in your family tree you must share an ancestor. It might only take a few generations. If you pick someone on the other side of the globe, you'll probably have to go further back to find how you're related. If you pick a gorilla, you'll have to go back about 7 million years. But you'll get there eventually.

We can also find the most recent female common ancestor between two individuals by looking at something called mitochondrial DNA. Nearly all your DNA is packaged inside the nuclei of your cells. But your mitochondria, the engines that power your cells, have their own miniature set of DNA. And since sperm are essentially a nucleus with a tail, they don't carry any mitochondria. This means all your mitochondrial DNA was passed down, intact, from your mother's egg cell. Your mitochondria are clones of hers, as hers are of your maternal grandmother, and so on.

The one female ancestor who passed down her mitochondrial DNA to every human alive today is called Mitochondrial Eve. She lived about 200,000 years ago in Africa. Her male counterpart can be found by tracing Y chromosomes, which are only passed between men.

That's not to say this Eve and Adam were the first humans, or the only humans of their generation, or even lived at the same time (they didn't). But over the millennia, the lineages of their peers have dead-ended.

And now we can get back to the horses. Italian researcher Antonion Torroni and a large group of collaborators sequenced the mitochondrial DNA of 83 horses. These represented a wide range of horse breeds across Asia, Europe, the Middle East, and the Americas.

The researchers found that horse mitochondrial DNA was diverse, falling into 18 major groups. Based on the DNA mutations that had occurred in each of these groups, they could create a family tree of all horses, including the rare Przewalski's horse (a subspecies from central Asia that was never domesticated). At the base of this tree is the mitochondrial Eve of horses.

When did she live? The researchers attached a time scale to their tree by calculating the rate at which DNA mutations accumulate in horses. Working backward, they placed the so-called Ancestral Mare between 130,000 and 160,000 years ago.

It was only about 6,000 years ago that we domesticated horses, breeding them to carry around humans and our stuff. Some other domesticated species--such as cattle, sheep and goats--have low genetic diversity, indicating that a small population was initially used for breeding. But the many genetic groups found in this study all predate the domestication of horses. In other words, the diversity didn't come from breeding; it came before breeding. Many different types of horses from different locations were incorporated into the domestic horse's gene pool.

But remember that, since we're looking at mitochondrial DNA passed down by female horses, we're only seeing half the story. And in fact a previous study of Y chromosomes in horses found the opposite result: There's almost no diversity among the DNA passed down through males.

To get the whole picture (just as to get the whole horse) we need to combine the male and female donations to the story. When horse breeding began in Asia, it seems that only a few male horses were used. "The modern Y chromosomes derive only from the one or few [male] animals which were domesticated first," Torroni explained in an email. "You could imagine that early horse breeders continued to domesticate wild females while they spread geographically with their animals, but not males."

Those early breeders, spreading across Eurasia, must have assumed that only the male contribution was important to maintaining the quality of their stock. They pulled in new breeding mares from the wild, but kept their male lines pure. It would be another several millennia before people were acquainted with the science of sperm and eggs. But those breeders unknowingly worked a lot of genetic diversity into the domestic horse.

And that diversity might have implications for how we breed and take care of horses today. It's possible, for example, that different categories of mitochondrial DNA make horses more or less successful at racing. Such a finding would be big news for the people who make their money breeding racehorses.

Genetic diversity might also help explain the success of feral horse populations around the world. Though these "wild" groups are descended from domestic horses, they're totally fine living, once again, without us.



Achilli, A., Olivieri, A., Soares, P., Lancioni, H., Kashani, B., Perego, U., Nergadze, S., Carossa, V., Santagostino, M., Capomaccio, S., Felicetti, M., Al-Achkar, W., Penedo, M., Verini-Supplizi, A., Houshmand, M., Woodward, S., Semino, O., Silvestrelli, M., Giulotto, E., Pereira, L., Bandelt, H., & Torroni, A. (2012). Mitochondrial genomes from modern horses reveal the major haplogroups that underwent domestication Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1111637109


Image: Wikimedia Commons/David

When Psychologists Take Things Too Literally

Thankfully, in brainstorming meetings where I'm asked to "think outside the box," no one has ever put me in an actual box. That's not true of the undergrads who volunteered for a recent psychology study.

Angela Leung, a researcher in Singapore, and her colleagues in the United States were studying a phenomenon called "embodied cognition." The idea is that a brain can't help being influenced by the body it's stuck inside. Feelings can run backward: We might be smiling because we're happy, or we might feel happy because our face is being forced into a smile.

Researchers have extended this idea to figurative language. They've found that people who hold a hot cup of coffee judge others as having warmer personalities, compared to when they're holding an iced coffee. When people hold a heavy clipboard, they judge certain matters as weightier. After tasting a sweet food, people feel more agreeable and helpful--that is, sweeter.

The new study looked at a few different metaphors for creative thinking and problem solving. One experiment involved a 5-foot-by-5-foot box made out of cardboard and PVC piping. Subjects completed a word association test while sitting inside the box, outside of the box, or with the box nowhere in sight. The test presented sets of three words and asked for a fourth word that went with all of them. For example, the word that goes with the set measure, worm, and video is tape. (Subjects also filled out a claustrophobia questionnaire.)

Subjects who sat in the box performed as well on the test as those who never saw the box at all. But subjects who sat outside the box--witnessing themselves acting out the metaphor--did significantly better.

In another task, subjects were asked to raise their right hands out in front of them palm-up, as if declaring something to an audience. They held their left hands behind their backs. Then they were asked to think up creative new uses for a university building complex, continuing until they ran out of ideas.

Then subjects were asked either to switch hands, or to raise their right hands again. And surprise! They had to answer the same question about the university buildings. Those who got to switch hands came up with more ideas, and a greater variety of ideas, than those who kept their right hands raised the whole time.

Did you guess the metaphor? That one was on the one hand...on the other hand.

For a third metaphor, subjects were presented with stacks of round paper coasters cut in half. Some of the subjects had to pull pieces from both stacks and recombine them, while the other subjects just moved around the pieces of paper. Afterward, subjects did the same word-matching test from the box experiment, as well as a Lego creativity test.

The subjects all performed the same on the creativity task. But those who had recombined the coaster pieces did better on the word-association test, successfully drawing connections between the sets of words. The researchers say that's because the task got them to--did you figure this one out?--put two and two together.

I'm not sure I would have guessed that the halves of a paper circle signified "two and two." But the theory behind embodied cognition would say it doesn't matter whether you can tell what metaphor you're acting out. The connection exists, the authors say, because the metaphor comes from how we actually carry out that cognitive act in our brains. Maybe we call it "thinking outside the box" because, at some fundamental level, that's what our brains are really doing. We call it a metaphor, but maybe it's not.

This was an international study, but it would be interesting to see cross-cultural studies of embodied cognition. Is it universal to consider other people as "sweet" or "warm" or "icy"? Do people in every country really "think outside the box" and consider arguments "on the other hand"? If there are figures of speech that can be interpreted physically and that also vary between cultures, how do these play out in embodied cognition experiments?

On the other hand (!), if common metaphors do come from some deep cerebral place, then they should be universal. It would mean that whatever language our figures of speech come in, they're really translations of the same literal actions in our brains. At least that's how I'll explain it the next time I need to escape the four walls of a conference room.


Angela Leung, Suntae Kim, Evan Polman, See Lay, Link Qiu, Jack Goncalo, & Jeffrey Sanchez-Burks (2012). Embodied Metaphors and Creative "Acts" Psychological Science


Photo: Flickr/Ben McLeod 

If You Give an Opossum a Mozzarella Stick

When the bus driver pulled away and I was left standing with a dozen near-strangers in the lobby of Durham's Museum of Life and Science, I admit I felt some doubt.

It was the second day of Science Online 2012, a conference-meets-mothership for bloggers, researchers, and other science communicators. We'd been promised a tour, but that promise had come electronically from yet another person I didn't know. I began to wonder how much time we could kill in the gift shop if needed, and where exactly in North Carolina I was. Then Keeper Mikey appeared, wearing a baseball cap and a big grin, and told us we were going to see some bears today.

I learned several things that afternoon. Outside, on the trail, we ran into three boys who ranged from knee-height to thigh-height. I asked them if they'd seen any animals they liked. "I saw a snake!" the biggest brother said. "But it would be cool if there was a tiger." In the spirit of the museum's tinier visitors, I'm going to present our adventures in modified children's book titles.

Make Way for Alligators

The first thing Mike did after leading us into a staff-only hallway was disappear behind another door. "You stay put," he called behind him, "It's venomous down here!" Without leaving us much time to wonder what that meant, Mike reappeared holding Phoebe.

I learned from Phoebe that baby alligators are pretty irresistible. Mike wanted us to take turns petting Phoebe on her back, but we convinced him to let us hold the alligator ourselves. "Just promise you won't yell at me if she gets nippy," he said. Phoebe was a good sport, looking around and blinking with her upside-down eyelids while a succession of humans held her around the belly. (Later, on the bus, one woman confessed that she thought Phoebe felt like a purse. "Technically," someone else countered, "a purse feels like Phoebe.")

I had also never figured alligators for intelligent animals. But Mike told stories of his work at other alligator facilities, where the animals knew their names and would come when called. He said he could yell out the name of an alligator, see its wake approaching him in the water, and pass it a meatball on a stick when it stopped near the shore. Summoning alligators isn't a talent I'd be quite so excited about.

If You Give an Opossum a Mozzarella Stick

Most of the animals at the museum are rescues, used for education because they're too tame or injured to go back to the wild. While juggling an extraordinarily squirmy opossum, Mike let us in on a secret: Opossums go nuts for mozzarella sticks. In fact, he seemed to know the favorite food of every animal under his care. Lemurs love Craisins. Black bears don't care too much for raw sweet potato, but they love it after it's been cooked with a little vanilla. Doesn't sound too bad to me, either.


Everybody Bites

No matter how cuddly the animals look, sometimes they want to chew on your fingers. Mike gave this potbellied pig a stern "No nipping!" command as it stood on its hind legs to greet us. Potbellied pigs are highly intelligent ("Smarter than your dog!") and often kept as pets.

This pig's companion was named, Mike told us regretfully, Miss Piggy. Any animals that arrives at the museum with a name gets to keep it--no matter how much that name pains the staff. The center receives plenty of cast-off pet reptiles, for example: "People say, 'Oh, this is my iguana, his name's Iggy,'" Mike said. "Of course it is." We met a lizard named Godzilla, a donkey named Lightning, and a gecko named Gordon.

Go, Wolf, Go!

I'd never seen a red wolf before, and that's probably because there are only a few hundred of them alive in the world. The two at the Museum of Science and Nature belong to the government--they have numbers, not names.

Mike told us they're hoping for cubs this year, but so far the male and female wolf haven't hit it off. He described watching video footage of the wolves one night: The male lay down a safe distance from where the female was sleeping and closed his eyes. Later in the night, he opened his eyes, crept a little closer to the female, and slept again. He kept sneaking closer to her throughout the night until she finally woke up and growled, sending the male slinking away.

Since the red wolf population at one time dipped to only a handful of breeding individuals, today's population isn't healthy by genetic standards. When a whole population is closely related, there aren't likely to be individuals with genetic mutations that give them different strengths. This means a virus or a shift in climate could wipe out the whole population in one go.

One member of our tour suggested solving this problem by irradiating the animals before releasing them in the wild. "I'm joking, by the way," she added for the benefit of everyone live-tweeting.


Where the Wild Nerds Are
We'd been on our tour for at least an hour when two members of the group suddenly burst out with, "Oh! It's so nice to meet you!" and shook each other's hands. Mike looked totally baffled. I told him that plenty of people at the conference knew each other by their Twitter handles but hadn't met face-to-face before. He looked only slightly less baffled.


One Lemur, Two Lemur, Red Lemur, Ringtail

While Mike went into the ring-tailed lemur enclosure and fed them Craisins, we called down to him with questions: What's their social structure like? (Matriarchal.) Why do they carry their tails up in the air when they walk? (To signal to each other in tall grass.) What are those collars for? (They're radio collars so the lemurs can be tracked through the woods if they escape.)

"Are they soft?" someone yelled down.

Mikey looked up at us and nodded, grinning.




Cloudy with a Chance of Bears

It began to sprinkle while we headed out to see the animals we'd all been waiting for: the black bears. When we arrived, two were huddled in a cave and the other two were high on a cliff, hidden except for their faces. Mike led us into a shed and through a series of padlocked metal gates. The last gate, which he kept closed, looked directly onto the enclosure. "You stay here," Mike said. "I'm going to go on the roof and throw raisins."

He disappeared, and a few moments later the bears began to trot toward us. A bear that had been on the cliff shimmied backward down an incline, checking behind her, like a toddler descending stairs. Curious to see who was visiting, another bear came right up to the fence we were behind and stood up against it. Cameras flashed. We gaped at the bear's big paw pads, looked it in the eyes. Mike returned and called us around to a fenced-in area at the side of the shed.

The bears have been taught various commands; they stood up, sat down, and followed Mike back and forth (their round teddy-bear bodies bouncing) in exchange for spoonfuls of raisins. This is so that, if the bears need a physical exam or to have blood drawn, a veterinarian will be able to interact with them easily. We witnessed the bears' different personalities: Virginia licked up raisins delicately, while Gus tried to gobble the whole plastic spoon. If Mikey ignored them for too long, they snuffled up behind him to get attention.

We were sorry to leave the bears and the center's other inhabitants, including Keeper Mike, the friendliest animal of all. Virginia was nice enough to pose for a picture with us before we left. 

Meanwhile, Gus turned away and scratched his rear end at length on the corner of the shed. He wasn't feeling quite as sentimental.

Photos by me, except for the final photo provided by @CogSciLibrarian. You can find more photos (better than mine) at this Storify. Thanks to Mikey Romano for a terrific afternoon!

Dances with Dung Beetles

You know that moment when you realize that in every unloved corner of the animal kingdom, there’s an ant or a bee or a beetle standing on its head and pushing a boulder of crap that has a better sense of direction than you do?

Dung beetles are named for their favorite food source. Upon finding, say, a fresh cowpile, the dung beetle cuts off a chunk, shapes the specimen into a ball bigger than its body, and then rolls the ball away to a new location. Unlike Sisyphus, the dung beetle pushes its burden by facing backward and rolling the ball with its hind legs. Once it finds a nice spot to settle down with its ball of dung, the beetle buries the ball for safekeeping and gradually consumes it.

The beetle’s journey away from the poop pile is punctuated by another surprising behavior. Periodically, the dung beetle stops pushing, climbs on top of its dung ball, and spins around. Scientists generously refer to this behavior as “dancing.”

A group of researchers from Sweden and South Africa hypothesized that the dancing behavior is really a way for the dung beetle to get its bearings. The beetles are known to have good orienteering skills; but unlike other animals that use environmental cues to navigate homeward, the dung beetle’s goal is to get far away from that big pile of poop. If it stays too near to the source, or accidentally circles back to where it started, it risks being attacked and robbed of its prize by other dung beetles.

Dung beetles always travel in a very straight line away from where they found their poop. And their ability to navigate relies on clues from the sky, such as the sun, the moon, and polarized light patterns. To find out whether spinning around on top of its dung ball is a way for the beetle to check its bearings and maintain a straight path, the researchers put a few dozen beetles and their balls through an obstacle course.

The first step was simply to film the dung beetles, which had been gathered from the wild in South Africa, as they built and began rolling their balls. A majority of the beetles climbed on top of their dung balls and “danced” before they even started rolling. This might have been when the beetles chose the bearings they would use for the rest of the trip. (Though almost 40 percent of the beetles did not dance at this stage, the researchers note that the beetles spend a lot of time on top of the ball while they’re forming it, and might get their bearings during this stage instead.)

Then the obstacles began. The beetles faced challenges that were meant to simulate the real difficulties of rolling a dung ball along a bumpy surface, say, or a grassy slope. First, beetles were steered into an open-roofed tunnel with a closed door at the end of it. Upon bumping into the solid object, every beetle climbed at once onto its dung ball and spun around as if reassessing its path. (“It should be noted,” the authors write, “that all beetles continued to roll straight into the closed door after performing a dance.” The beetles may have been confirming their bearings, but they apparently weren’t interested in a new course.)

When sent through a different tunnel that veered them off to the left or right of their original course, most beetles performed a dance. When put in a swiveling tunnel and quickly spun 180 degrees, only about half the beetles noticed and did a dance—but almost all of those beetles then turned back in the correct direction.

However, when the swiveling tunnel was covered so the beetles couldn’t see the sky, almost none of them danced. They seemed to be responding only to visual cues, not to the feeling of being spun around. To further test the beetles’ reliance on sight, the researchers used a mirror to make it seem like the sun had shifted 180 degrees. In response, more than half of the beetles stopped and did a dance, and most of those beetles switched direction: changing the angle of the sun had convinced them to go the other way.

The authors think there’s more to be learned about dung beetle dancing by studying the non-dancers in various situations. Are those individuals relying on different cues, such as light polarization instead of the location of the sun? Or are they just not the sharpest pooper-scoopers in the shed? Either way, the dung beetles that make it to safety with their pre-digested treasures have a skill set that should make us humans feel like, well, number two.

Image: Wikimedia Commons/Dewet 


Baird, E., Byrne, M., Smolka, J., Warrant, E., & Dacke, M. (2012). The Dung Beetle Dance: An Orientation Behaviour? PLoS ONE, 7 (1) DOI: 10.1371/journal.pone.0030211

Photographic Memory? Depends Where You're Looking

Even if you like to brag about your visual memory, your recall may really depend on the direction of your gaze. That's what two MIT scientists say after testing people's recollection of items on a screen. We're OK at remembering where an object was in space. But we're better at remembering where it was in our eyeballs.

Brain researchers Julie Golomb and Nancy Kanwisher were interested in two types of visual memory. The first is called "spatiotopic": related to space. This would seem at first glance (ahem) to be the type of visual memory we use most often. We reach for a coffee cup while scanning the computer screen; we recall where in the kitchen we set down our keys. The second type of memory is "retinotopic": related to the retina of the eye. In this case, we recall an object's position relative to the direction of our gaze.

The researchers performed a series of tests on their subjects. In all the tests, subjects first stared at a gray computer screen and saw a white dot appear. Subjects had to focus their gaze on the dot; an eye-tracking device made sure there was no cheating. Then a small black box briefly appeared somewhere else on the screen, while subjects kept their eyes on the white target.

Next, the black box disappeared, and the white dot either stayed in place or began to travel around the screen. It moved to zero, one, or two other spots, requiring subjects to follow it with their eyeballs, before landing at its final position. Then subjects were asked one of two questions: Do you remember where the black box appeared on the screen? or, Do you remember where the black box was in relation to where you're looking now? The first question tested subjects' spatiotopic memory; the correct answer was simply where the black box originally appeared. But the second question tested retinotopic memory. So, for example, if the black box had originally been to the lower left of the white target, the correct answer was to the lower left of wherever that white dot ended up.

As the white dot made more movements around the screen, subjects made more errors in their spatiotopic memory (Where was the black box on the screen?) This isn't too surprising. Following the white dot to more locations made subjects lose track of where the black box had originally appeared.

But while people's performance on the spatiotopic test got worse, their performance on the retinotopic test (Where was the black box in relation to where you're looking right now?) stayed exactly the same.

To make sure the subjects weren't just using the white target dot to anchor themselves, the researchers tried leaving a ghost of the original target on the screen while the white dot traveled around. This ghost dot could have helped subjects to anchor their spatiotopic answers too. But the results stayed the same, suggesting that subjects were really succeeding in the retinotopic task by anchoring their answers to their eyeballs.

This seems like a non-optimal way for our species to remember where things are. Since our gaze is always moving around, it would make sense for our memories to ignore that variable. But the authors suggest that this might just be the best evolution could do with the tools available. Keeping an accurate map in our heads would be hard. Perhaps instead, we record our visual memories with reference to our retinas, and try to update them as our gaze moves around. The more movements our eyes make, the more we lose track of where we saw something. It's not perfect, but we do a pretty good job of finding things in everyday life.

The study was small, so it would take more evidence to show for certain that our gaze-related memory is stronger than our space-related memory. If true, there would have to be some handy applications. Maybe gaze-related memorization tricks would be the new mnemonics. Or maybe, after we've seen something we want to remember clearly, we should freeze our gaze in one direction and not move. Of course, that wouldn't help with the coffee cup, but it might make you a champ in a game of Memory.

Image: Wikimedia Commons 


Julie D. Golomb, & Nancy Kanwisher (2012). Retinotopic memory is more precise than spatiotopic memory Proceedings of the National Academy of Sciences

How to Decode a Monkey Face

Look closely: The black-capped squirrel monkey is trying to tell you something. No, staring deep into its eyes won't help. Actually, forget looking closely. Maybe squint.

This South American monkey's face is a patchwork of coloration: dark on the crown, cheekbones, and mouth; gray on the sides of the face; white on the ears, chin, and around the eyes. The pattern holds a hint to how the species lives in the wild. The length of its hair is a clue about where it lives--and so's that black cap.

Researchers at the University of California, Los Angeles, mapped the faces of 129 monkey species native to South and Central America. Studying several representative photos for each species, they subdivided the monkeys' faces into more than a dozen regions. In each region, they recorded the color of the monkeys' hair or visibile skin, as well as hair length. (They didn't make any assessment of attitude, but this white-faced capuchin looks a little surly, no?)

Each face was scored for its overall complexity. A monkey with a mostly monochromatic face, like the capuchin above, would be low in complexity. The squirrel monkeys at the top of the page have more distinctive markings, or higher complexity.

The researchers wanted to know why evolution has driven some monkeys to develop complicated facial markings, while the faces of others are simple. Is the intricacy of monkeys' markings, or the color or length of their hair, related to how and where they live?

The team discovered a clear correlation between monkey species' facial complexity and the kind of social groups they live in. Species with complicated markings are more likely to live in small groups. The researchers think that the monkeys' distinctive faces help them to find each other when there aren't that many of them around.

Further supporting this theory, the team found that monkeys with complex faces are likely to live in the same area as a lot of closely related species. It's in the monkeys' best interest if members of the same species can easily spot each other in the crowd--especially for mating purposes. (As the emperor tamarin knows, a mustache is also a good way to attract mates.)

Monkeys with more monochrome faces, on the other hand, are likely to live in larger social groups. The researchers speculate that when members of a species don't have to worry as much about finding each other, evolution selects for a face that can clearly display expressions. Without a lot of markings on their faces, monochromatic monkeys might be able to communicate more easily. As further evidence for this theory, the authors point out that more social species are known to have a greater number of muscles in their faces. With more flexible features, animals can better convey friendliness, aggression, or fear to one another.

The researchers also found ties between a monkey's coloration and where it lives. Species in the tropics are more likely to have dark patches on the tops of their heads and around their eyes. The eye masks may help reduce glare, like the face paint worn by a football player. Dark hair on the top of the head or elsewhere on the body might help carry body heat away, or keep the monkeys hidden from predators in the dim rainforest.

As for hair length, the authors found that species living farther from the tropics tend to have longer hair, like the golden lion tamarin above. Long, thick hair may help animals in temperate climates regulate their body temperature.

You may never be able to communicate with a squirrel monkey. No matter how cryptic an animal's expression is, though, you'll find its evolutionary history written all over its face.

Santana, S., Lynch Alfaro, J., & Alfaro, M. (2012). Adaptive evolution of facial colour patterns in Neotropical primates Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2011.2326


Images: S. boliviensis Joachim S. Müller/Flickr; S. boliviensis Brian Gratwicke/Wikimedia Commons; C. capuchinus Santana et al.; S. imperator Mila Zinkova/Wikimedia Commons; L. rosalia KRO-Media/Flickr

Regenerating Worm Ignores Rules of Biology

Meet Schmidt. Schmidtea mediterranea is a hermaphroditic, googly-eye-spotted flatworm that sometimes chooses to reproduce by breaking in half and regenerating two new bodies, but that's not the most interesting thing about her/him. This tiny animal has just demonstrated to biologists that a part of the cell crucial to all animal life on Earth is, in fact, optional.

You might remember the centrosome from high school biology as a pair of perpendicular, hot-dog-shaped objects inside a cell. Just before cell division, this structure duplicates itself, and the two centrosomes travel to opposite ends of the nucleus. Then each centrosome sends a cascade of delicate fibers toward the center of the nucleus, where the chromosomes have already copied themselves and paired off in an orderly manner. The fibers latch on to the chromosomes and drag them back to the poles of the nucleus, one copy on each side. After this, the cell can divide cleanly into two identical daughter cells.

Clearly the centrosome, which shuttles around chromosomes like a chaperone on a school field trip, is important to cell division. That's why evolution has held on to centrosomes in every animal biologists ever examined--until the planarian Schmidtea mediterranea.

Researchers led by Wallace Marshall at the University of California, San Francisco, wanted to know how centrosomes were involved in the amazing regenerative abilities of this planarian. Tearing itself in half isn't its only trick--even a tiny piece of a planarian's body can regenerate into an entire individual. The researchers used interfering bits of RNA to block the planarian from manufacturing centrioles, the rod-shaped bricks that make up centrosomes.

In addition to building centrosomes, these rods also make up cilia, the wavy little arms on the outside of some single-celled organisms (or on the cells of your trachea, brushing debris out of your airway). When the researchers prevented S. mediterranea from making centrioles, the worms had trouble moving around. This was unsurprising: without an adequate supply of centrioles, they couldn't build cilia to keep them gliding as usual. (Instead, they inched.)

Since the worms weren't making centriole bricks, they shouldn't be able to build centrosomes, either. The researchers sliced up their subjects, expecting that a lack of centrosomes would prevent the planarians from regenerating.

The worms responded by regenerating exactly as usual.

The researchers went back and looked at normal S. mediterranea individuals whose genes they hadn't messed with. Their cells didn't have centrosomes either. Marshall and his team were forced to conclude that planarians pull off their amazing feats of regeneration without centrosomes--and that the centrosome isn't crucial to animal life, after all. Centrosomes may also be missing from schistosomes, the planarians' parasitic relatives.

So how did the the planarian lose its centrosomes? The team speculates that this just-so story might hinge on the unusual pattern of cell division in a planarian or schistosome embryo. Now that we know centrosomes aren't required for animal life--and that animals without them can perform some incredible stunts of cell division--we'll have to rethink what exactly they're doing in our own bodies.


Azimzadeh, J., Wong, M., Downhour, D., Alvarado, A., & Marshall, W. (2012). Centrosome Loss in the Evolution of Planarians Science DOI: 10.1126/science.1214457


Image: UCSF/J.Azimzadeh

Weightless Flies Have Wanderlust

To astronauts, science fiction writers, and entrepreneurs selling tickets on private space flights, the question of how weightlessness affects an organism is crucial. Our cells and organs are fine-tuned for life within the comfortable harness of Earth's gravity, so what happens to them when we're cut loose? There's at least one way to study this question without the prohibitive price tag of sending something all the way to space. A group of magnetically levitated fruit flies, though they couldn't report on their experience, seemed to find it just as good as the real thing.

University of Nottingham researcher Richard Hill and his colleagues used a powerful magnetic field to create a small, zero-gravity "arena" for fruit flies. Though magnetic fields attract magnetic substances such as iron, they also weakly repel certain other materials that are called "diamagnetic." These include water and organic matter--in other words, most of what's in a fruit fly. (Or in you. But there isn't a magnet big enough to try this trick on a person.)

By carefully aligning their disc-shaped fly arenas inside a superconducting solenoid magnet, the researchers were able to create environments of roughly 1g (equal to Earth's gravity), 2g (twice Earth's gravity), and 0g (whee!). They also left one fly dish outside the magnet, so they could compare the 1g environments and make sure the magnetic field didn't just make all the flies crazy.

Though the researchers provide many mathematical descriptions of their result, you can see it easily and immediately in this video. The 0g flies are on the top left.

Fruit flies' normal behavior is to roam, but the 0g flies are tearing around their dish.

Unlike human astronauts, who don't have much choice but to float, fruit flies have grippy little feet and the power of flight. So the weightless flies spent most of their time walking on the floor, walls, and ceiling as usual. (You might spot a few of them floating dazedly in the center of the dish, though.) What was unusual was the speed and amount that they traveled.

This might be simply because it's easier for flies walk without gravity. If it takes less energy than usual to walk, a fruit fly that's putting the normal amount of effort into moving around will find itself at a near-sprint. The 2g flies supported this theory by walking more sluggishly than usual.

Another possibility is that the flies' altered perception of gravity affected their behavior. Like human astronauts who go ricocheting off the walls for fun, the fruit flies might have noticed something was different and reacted to that feeling.

The finding that weightless flies speed-walk isn't new: Experiments done on the International Space Station and on the space shuttle Columbia found the same result. But replicating the finding here on Earth shows that it wasn't a fluke caused by some other factor, such as the trauma of takeoff. The flies' altered behavior was directly due to their low-gravity environment, making it relevant to humans and any other organisms we might carry into space.

Hill's study also shows that zero-gravity experiments, at least on very small organisms, don't have to be done in space. Studies done inexpensively here on Earth can provide real insights into life in outer space, and help create safer technologies for the lucky humans who get to go.

Hill, R., Larkin, O., Dijkstra, C., Manzano, A., de Juan, E., Davey, M., Anthony, P., Eaves, L., Medina, F., Marco, R., & Herranz, R. (2012). Effect of magnetically simulated zero-gravity and enhanced gravity on the walk of the common fruitfly Journal of The Royal Society Interface DOI: 10.1098/rsif.2011.0715 


Movie: Hill et al., data supplement; Photo: NASA

Chimps Prefer the 2-Point Conversion

If non-human great apes were coaching more football games, you could expect to see fewer extra points being kicked. We risk-averse humans usually prefer kicking an easy extra point after a touchdown, rather than attempting a more difficult 2-point conversion. But chimps and other great apes, after considering their odds, usually opt for the greater risk and the bigger reward.

By "reward," I mean banana.

Researchers at the Max Planck Institute in Germany tested a group of chimpanzees, bonobos, gorillas and orangutans on their risk-taking strategies using chunks of banana. They wanted to know whether the apes' likelihood to go hunting for banana pieces hidden under cups, rather than taking a smaller banana piece already in front of them, depended on the "expected value" of their choices. Expected value is simply an item's worth, multiplied by your odds of getting it. If a 2-point conversion attempt is successful exactly half the time, then its expected value is 1 point.

The 22 apes each sat through a series of experiments involving banana bits in cups. On one side of a table, they saw a small piece of banana placed under a yellow cup. Next to that was a row of blue cups,  anywhere from one to four of them. Under one of the blue cups was a larger piece of banana.

The apes knew the larger piece of banana was hidden under one of the blue cups, but unless there was only one blue cup, they didn't know exactly where the banana was. (They understood the setup because there was also a series of trials in which the apes watched the banana being placed under one of the blue cups.) In each trial, an ape could point to just one cup and get the reward--if there was any--underneath.

The yellow cup was a guaranteed small reward. The blue cups were a gamble. And the size of the gamble (in other words, its expected value) depended on how many blue cups were on the table. It also depended on the difference in size between the two banana chunks. The "safe" piece of banana in the yellow cup ranged from one-sixth to two-thirds the size of the large piece.

The researchers found that the apes' decisions did correlate to the expected value of their options. Overall, as the expected value of picking a blue cup increased--there were fewer blue cups on the table, or the safe piece of banana was small and untempting--apes opted more often to try a blue cup. When the expected value of the gamble was lower--because there were a lot of blue cups to choose between, or the safe banana piece was large to begin with--they were more likely to stick with the yellow cup.

Adjusting choices based on the expected value of each option is similar to how humans would decide. But the apes were less human-like in their general propensity for risk. Even at the lowest possible expected values, apes chose to gamble on a blue cup more than 50% of the time.

In other words, apes acted more like humans playing the lottery than humans kicking an extra point after a touchdown. These apes, of course, didn't have their coaching jobs on the line. They might have just enjoyed playing the cup game. And in a human football game, there are plenty of situations in which a kicked extra point is better than going for 2--even though its expected value, with a success rate of about 50%, is the same.

But even outside of football games, humans are known by psychologists for being risk averse, especially when it comes to potential gains. We'd rather take a small guaranteed reward than a larger and riskier one. (For losses, though, we tend to feel the opposite way.)

When the researchers broke down their results by species, they found that while all four species were risk prone, bonobos were a little more conservative in their choices than chimps were. With only a small number of ape subjects, it's hard to draw any serious conclusions. But it's interesting to speculate about the differences between us and our two closest living relatives. Have chimps evolved to take more risks, always gambling on finding something better, because in the wild they must search for fresh fruit year-round? Can bonobos afford to be more conservative because their diet in the wild is more flexible? What factor in our past put risk-averse humans at an evolutionary advantage?

Next time your favorite football team takes an overly conservative extra point, don't blame the coach for his evolutionary history. You could always call up the owners, though, and suggest they hire a chimpanzee instead.









Photo: Flickr/Mat_the_W 


Haun, D., Nawroth, C., & Call, J. (2011). Great Apes' Risk-Taking Strategies in a Decision Making Task PLoS ONE, 6 (12) DOI: 10.1371/journal.pone.0028801