June 19, 2013
For nearly two centuries, biologists have been struck by a mystery of geography and biodiversity peculiar to Europe. As Edward Forbes pointed out as far back as 1846, there are a number of life forms (including the Kerry slug, a particular species of strawberry tree and the Pyrenean glass snail) that are found in two specific distant places—Ireland and the Iberian Peninsula—but few areas in between.
Recently, Adele Grindon and Angus Davidson, a pair of scientists at the University of Nottingham in the UK, decided to come at the question with one of the tools of modern biology: DNA sequencing. By closely examining the genetic diversity of one of the species shared by these two locales, the grove snail, they thought they’d be able to trace the migratory history of the creatures and better understand their present-day distribution.
When they sequenced the mitochondrial DNA of hundreds of these snails scattered across Europe, the data pointed them towards an unexpected explanation for the snails’ unusual range. As they suggest in a paper published today in PLOS ONE, the snails likely hitched a boat ride from Spain to Ireland some 8,000 years ago along with migrating bands of Stone Age humans.
Grove snails as a whole are distributed all over Europe, but a specific variety of the snail, with a distinctive white-lipped shell, is found exclusively in Ireland and in the Pyrenees mountains that lie on the border between France and Spain. The researchers sampled a total of 423 snail specimens from 36 sites distributed across Europe, with an emphasis on gathering large numbers of the white-lipped variety.
When they sequenced genes from the mitochondrial DNA of each of these snails and used algorithms to analyze the genetic diversity between them, they found that the snails fell into one of 7 different evolutionary lineages. And as indicated by the snails’ outward appearance, a distinct lineage (the snails with the white-lipped shells) was indeed endemic to the two very specific and distant places in question:
Explaining this is tricky. Previously, some had speculated that the strange distributions of creatures such as the white-lipped grove snails could be explained by convergent evolution—in which two populations evolve the same trait by coincidence—but the underlying genetic similarities between the two groups rules that out. Alternately, some scientists had suggested that the white-lipped variety had simply spread over the whole continent, then been wiped out everywhere besides Ireland and the Pyrenees, but the researchers say their sampling and subsequent DNA analysis eliminate that possibility too.
“If the snails naturally colonized Ireland, you would expect to find some of the same genetic type in other areas of Europe, especially Britain. We just don’t find them,” Davidson, the lead author, said in a press statement.
Moreover, if they’d gradually spread across the continent, there would be some genetic variation within the white-lipped type, because evolution would introduce variety over the thousands of years it would have taken them to spread from the Pyrenees to Ireland. That variation doesn’t exist, at least in the genes sampled. This means that rather than the organism gradually expanding its range, large populations instead were somehow moved en mass to the other location within the space of a few dozen generations, ensuring a lack of genetic variety.
“There is a very clear pattern, which is difficult to explain except by involving humans,” Davidson said. Humans, after all, colonized Ireland roughly 9,000 years ago, and the oldest fossil evidence of grove snails in Ireland dates to roughly the same era. Additionally, there is archaeological evidence of early sea trade between the ancient peoples of Spain and Ireland via the Atlantic and even evidence that humans routinely ate these types of snails (pdf) before the advent of agriculture, as their burnt shells have been found in Stone Age trash heaps.
The simplest explanation, then? Boats. These snails may have inadvertently traveled on the floor of the small, coast-hugging skiffs these early humans used for travel, or they may have been intentionally carried to Ireland by the seafarers as a food source. “The highways of the past were rivers and the ocean–as the river that flanks the Pyrenees was an ancient trade route to the Atlantic, what we’re actually seeing might be the long lasting legacy of snails that hitched a ride…as humans travelled from the South of France to Ireland 8,000 years ago,” Davidson said.
All this analysis might help biologists solve the bigger mystery: why so many other species share this strange distribution pattern. More research could reveal that the Kerry slug, strawberry tree and others were carried from Iberia to Ireland by prehistoric humans too—and that, as a species, we were impacting the Earth’s biodiversity long before we could’ve possibly realized it.
June 12, 2013
You likely don’t give a ton of thought to the sounds and patterns that make up the language you speak everyday. But the human voice is capable making of a tremendous variety of noises, and no language includes all of them.
About 20 percent of the world’s languages, for example, make use of a type of sound called an ejective consonant, in which an intense burst of air is released suddenly. (Listen to all the ejectives here.) English, however—along with most European languages—does not include this noise.
Linguists have long assumed that the incorporation of different sounds into various languages is an entirely random process—that the fact that English includes no ejectives, for instance, is an accident of history, simply a result of the sounds arbitrarily incorporated into the language that would evolve into German, English and most other European languages. But recently, Caleb Everett, a linguist at the University of Miami, made a surprising discovery that suggests the assortment of sounds in human languages is not so random after all.
When Everett analyzed hundreds of different languages from around the world, as part of a study published today in PLOS ONE, he found that those that originally developed at higher elevations are significantly more likely to include ejective consonants. Moreover, he suggests an explanation that, at least intuitively, makes a lot of sense: The lower air pressure present at higher elevations enables speakers to make these ejective sounds with much less effort.
The finding—if it holds up when all languages are analyzed—would be the first instance in which geography is found to influence the sound patterns present in spoken words. It could open up many new avenues of inquiry for researchers seeking to understand the evolution of language throughout human history.
Everett started out by pulling a geographically diverse sampling of 567 languages from the pool of an estimated 6,909 that are currently spoken worldwide. For each language, he used one location that most accurately represented its point of origin, according to the World Atlas of Linguistic Structures. English, for example, was plotted as originating in England, even though it’s spread widely in the years since. But for most of the languages, making this determination is much less difficult than for English, since they’re typically pretty restricted in terms of geographic scope (the average number of speakers of each languageanalyzedis just 7,000).
He then compared the traits of the 475 languages that do not contain ejective consonants with the 92 that do. The ejective languages were clustered in eight geographic groups that roughly corresponded with five regions of high elevation—the North American Cordillera (which include the Cascades and the Sierra Nevadas), the Andes and the Andean altiplano, the southern African plateau, the plateau of the east African rift and the Caucasus range.
When Everett broke things down statistically, he found that 87 percent of the languages with ejectives were located in or near high altitude regions (defined as places with elevations 1500 meters or greater), compared to just 43 precent of the languages without the sound. Of all languages located far from regions with high elevation, just 4 percent contained ejectives. And when he sliced the elevation criteria more finely—rather than just high altitude versus. low altitude—he found that the odds of a given language containing ejectives kept increasing as the elevation of its origin point also increased:
Everett’s explanation for this phenomenon is fairly simple: Making ejective sounds requires effort, but slightly less effort when the air is thinner, as is the case at high altitudes. This is because the sound depends upon the speaker compressing a breath of air and releasing it in a sudden burst that accompanies the sound, and compressing air is easier when it’s less dense to begin with. As a result, over the thousands of years and countless random events that shape the evolution of a language, those that developed at high altitudes became gradually more and more likely to incorporate and retain ejectives. Noticeably absent, however, are ejectives in languages that originate close to the Tibetean and Iranian plateaus, a region known colloquially as the roof of the world.
The finding could prompt linguists to look for other geographically-driven trends in the languages spoken around the world. For instance, there might be sounds that are easier to make at lower elevations, or perhaps drier air could make certain sounds trip off the tongue more readily.
June 6, 2013
Thirteen years after the release of On the Origin of Species, Charles Darwin published another report on the evolution of mankind. In the 1872 book The Expression of the Emotions in Man and Animals, the naturalist argued that people from different cultures exhibit any given emotion through the same facial expression. This hypothesis didn’t quite pan out—last year, researchers poked a hole in the idea by showing that the expression of emotions such as anger, happiness and fear wasn’t universal (PDF). Nonetheless, certain basic things—such as the urge to cry out in pain, an increase in blood pressure when feeling anger, even shrugging when we don’t understand something—cross cultures.
A new study, published today in the journal Frontiers in Psychology, compares such involuntary responses, but with an added twist: Some observable behaviors aren’t only universal to the human species, but to our closest relatives too—chimpanzees and bonobos.
Using video analysis, a team of UCLA researchers found that human, chimpanzee and bonobo babies make similar gestures when interacting with caregivers. Members of all three species reach with their arms and hands for objects or people, and point with their fingers or heads. They also raise their arms up, a motion indicating that they want to be picked up, in the same manner. Such gestures, which seemed to be innate in all three species, precede and eventually lead to the development of language in humans, the researchers say.
To pick up on these behaviors, the team studied
hree babies of differing species through videos taken over a number of months. The child stars of these videos included a chimpanzee named Panpanzee, a bonobo called Panbanisha and a human girl, identified as GN. The apes were raised together at the Georgia State University Language Research Center in Atlanta, where researchers study language and cognitive processes in chimps, monkeys and humans. There, Panpanzee and Panbanisha were taught to communicate with their human caregivers using gestures, noises and lexigrams, abstract symbols that represent words. The human child grew up in her family’s home, where her parents facilitated her learning.
Researchers filmed the child’s development for seven months, starting when she was 11 months old, while the apes were taped from 12 months of age to 26 months. In the early stages of the study, the observed gestures were of a communicative nature: all three infants engaged in the behavior with the intention of conveying how their emotions and needs. They made eye contact with their caregivers, added non-verbal vocalizations to their movements or exerted physical effort to elicit a response.
By the second half of the experiment, the production of communicative symbols—visual ones for the apes, vocal ones for the human—increased. As she grew older, the human child began using more spoken words, while the chimpanzee and bonobo learned and used more lexigrams. Eventually, the child began speaking to convey what she felt, rather than only gesturing. The apes, on the other hand, continued to rely on gestures. The study calls this divergence in behavior “the first indication of a distinctive human pathway to language.”
The researchers speculate that the matching behaviors can be traced to the last shared ancestor of humans, chimps and bobonos, who lived between four and seven million years ago. That ancestor probably exhibited the same early gestures, which all three species then inherited. When the species
diverged, humans managed to build on this communicative capacity by eventually graduating to speech.
Hints of this can be seen in how the human child paired her gestures with non-speech vocalizations, the precursors to words, far more than the apes did. It’s this successful combination of gestures and words that may have led to the birth of human language.
Take a close look at nearly any male land bird—say, a rooster, hawk or even a bald eagle—and you’ll notice that they lack something present in most male animals that have sex via internal fertilization. Namely, a penis.
With a few exceptions (such as ostriches, ducks, and geese), male land fowl have no external sex organs. Instead of using a penis to fertilize a female’s eggs during mating, they eject sperm out of their cloaca—an orifice also used to excrete urine and feces—directly into the cloaca of a female (the maneuver is known by the touchingly romantic name “cloacal kiss”).
The evolutionary reason why these birds don’t have penises remains a mystery. But new research has finally shed light on the genetic factors that prevent male land birds from growing penises as they mature.
As described in an article published today in Current Biology, researchers from the University of Florida and elsewhere determined that most types of land fowl actually do have penises while in an early embryonic state. Then, as they develop, a gene called Bmp4 triggers a cascade of chemical signals that causes the cells in the developing penis to die off and wither away.
The team, led by Martin Cohn and graduate student Anna Herrera, compared the embryonic development of two types of land birds that lack penises (chickens and quail) with two species of waterfowl that have coiled penises that can be elongated (geese and ducks). Using an electron microscope, they found that in the early stages of development, male embryos from both of these groups had penis precursors.
But soon afterward, for the chickens and quail, the Bmp4 gene activates in the cells at the tips of the developing penises. This gene triggers the synthesis of a particular protein called Bmp4 (bone morphogenetic protein 4), which leads to the controlled death of the cells in this area. As the rest of the bird embryo develops, the penis shrinks away, ultimately producing the modest proto-phallus found on the birds as adults.
To confirm the role of the Bmp4 gene, the researchers artificially blocked the chemical signaling pathway through which it triggers cell death, and found that the chicken embryos went on to develop full-fledged penises. Additionally, the researchers performed the opposite experiment with duck embryos, artificially activating the Bmp4 signal in the cells at the tip of the developing penis, and found that doing so caused the penis to stop growing and whither away as it usually does in chickens.
Knowing the genetics behind these birds’ lack of penises doesn’t explain what evolutionary benefit it might confer, but the researchers do have some ideas. Male ducks, for instance, are notorious for having sex with females by force; by contrast, the fact that most land birds have no penis means that females have more control over their reproductive destiny. This could theoretically allow them to be more choosy over their mates, and select higher quality males overall.
Of course, all this might make you wonder: Is there really a point to studying the missing penises of birds? Well, as noted after the brouhaha that erupted a few months ago over federally-funded research into duck penises, research into seemingly esoteric aspects of the biological world—and, really, the natural world as a whole—can provide very real benefits to humanity in the long-term.
In this case, a better understanding of the genetics and chemical signals responsible for the development of the organ could have applications that stretch much farther than even the duck’s penis. Many of the particulars of embryonic development—including the Bmp4 gene and associated protein—are highly conserved, evolutionarily, meaning that they’re shared between many diverse species, including both birds and humans. So researching the embryonic development of even animals that are only distantly related to us, like birds, could one day help us better understand what goes on when human fetuses are in the womb and perhaps enable us to address congenital defects and other deformities.
And if that doesn’t do it for you, there’s also just the astounding weirdness of watching duck penises unfurl in slow motion. Brace yourself:
May 30, 2013
Think of an ecosystem as a Jenga pillar. Each piece–microbes, birds, trees, insects, animals, fungi–comes together to form the larger, intertwined structure. Maybe you can knock out a Jenga block or two, but tamper with those components enough and the system will collapse. As ecologists well know, small changes in the environment–cutting down a few patches of forest, causing a local species to go extinct–can create cascading and potentially disastrous effects on the broader environment.
Like a teetering Jenga tower, predicting which of those changes will most significantly reverberate in the complex natural world is nearly impossible. So we wait to see the consequences. Today, an international team of researchers just identified a pointed example of one such fatal tinkering. In Brazil’s damaged Atlantic forest, the absence of large birds has caused seeds to shrink and become weaker, in turn threatening the forest’s future.
The story began more than a century ago, they found. Local people began hacking away at the Atlantic forest, which once covered more than 400,000 square miles of Brazil’s coast. Agricultural and livestock fields, as well as growing urban centers, divided swaths of jungle, creating isolated patches of green. By the time people realized there was value in keeping the forest around, nearly 90 percent of it had been lost. Much of what remains today occurs in isolated, random pockets, though those patches still represent some of the world’s most biodiverse forests.
When a forest becomes divided, like the Atlantic forest did, wildlife often loses its ability to disperse from one patch of trees to another. Larger species may not be able to survive in some of the smaller, resource-scarce patches, and hunters can more easily track down animals if they’re confined to a smaller area. This turned out to be the case for some large birds that once made their home throughout the Atlantic forest, including toucans and toucanets–prized for their brilliant plumage, the birds are a favorite of hunters.
Significantly, these birds’ big beaks–which open up more than half an inch, on average–make them key players in distributing larger seeds throughout the jungle. Smaller birds can’t swallow or fit those big seeds into their beaks, meaning the toucans and toucanets carry nearly sole responsibility for regenerating the jungle with new seedlings of several plant species.
The authors of this new study, published in Science, compared the size of more than 9,000 seeds from 22 palm plant populations–a major tree type in the Atlantic forest, several species of which are threatened. Some of the seeds came from robust patches of forest with lots of large birds, while others came from smaller patches where those birds have long been missing. In those smaller patches, they found, seeds of palm plants were significantly smaller.
The researchers also used statistical models to independently evaluate 13 different environmental variables, including soil type and climate, to find out whether they could have driven the size difference seen in the seeds instead. None of the other factors could explain the difference, suggesting birds–the primary transport mechanism for large seeds–as the most likely culprit. Additional genetic analyses indicated that, in the smaller forest patches, seeds most likely began shrinking about 100 years ago, or right around the time that coffee and sugar cane plantations began to boom. Human activities a century ago, the authors conclude, likely drove a rapid evolutionary change in the forest palms’ seed size.
When the birds disappear, the larger seeds do not get distributed throughout the forest. Only the smaller ones wind up in new plots of earth, which in turn sprout into more trees that produce smaller seeds. Gradually, the forest becomes dominated by smaller seed-producing trees.
Shrinking seed size is no small detail for forest palms. The larger the seed, the more nutrients that are packed in to give the seedling the best possible chances of survival in the tough jungle ecosystem. Prior research has found that forest palms that began life as smaller seeds are smaller on average after a year of growth than those that came from larger seeds, meaning that the runty plants are more likely to lose out to competition with other species. Smaller seeds are also more prone to drying out. Given that climate models predict hotter temperatures and longer periods of drought for South America in the coming years, this could be a serious problem for smaller-seeded forest palms’ survival.
If palms start dying out throughout the Atlantic forest, researchers have no idea what will happen to the tens of thousands of species that take shelter in the ecological web the plants help to maintain–a web that includes more than 11,000 threatened plants and animals. For those smaller jungle patches, the authors speculate, the choices long-dead humans made may lead to complete collapse of some of the world’s most diverse sections of rainforest. Like a real-life game of Jenga, those birds could prove to be the key piece that causes the entire jungle system to fall down.