November 12, 2013
In the summer of 2010, husband-and-wife paleobiologist team Z. Jack Tseng and Juan Liu traveled to the Zanda Basin in western Tibet with a group of colleagues. The remote area, a week’s drive from Beijing and near the border of Pakistan and China, is “basically badlands everywhere, with deeply cut valleys throughout,” Tseng says.
To explore the valleys, the team drove up dirt trail after dirt trail before coming upon a dense patch of fossils sticking out of the ground halfway up a hill. “In the little concentration of fossils, there were lots of limb bones from antelopes and horses obscuring everything else,” says Tseng, who was then a graduate student at USC and is now with the American Museum of Natural History. “It wasn’t until we started lifting things up, one by one, that we saw the top of a skull, and we thought, from the shape, that it looked something like a cat.”
After a few years of analysis, Tseng’s team has discovered that the skull doesn’t belong to any old cat. As they’ve documented in a study published today in Proceedings of the Royal Society B, the skull and six associated fossilized jawbone fragments are the first evidence of a newly discovered species, which they’ve called Panthera blytheae. The discovery represents the oldest “big cat” (a group that includes large predatory cats like lions, jaguars, tigers and leopards) ever found by a wide margin.
The sediments that make up the basin as a whole range from 6 million to 400,000 years in age, so the group dated the fossil by analyzing the age of the particular rock layers it was buried in. This involved using techniques of magnetostratigraphy, in which scientists analyze the magnetic orientation of the rocks and compare it to known reversals of the Earth’s magnetic field. This method can only provide rough estimates for an item’s age, but it revealed that the skull is between 4.10 and 5.95 million years old. Previously, the oldest known big cat fossils—a number of tooth fragments found in Tanzania—were 3.6 million years old.
The new find fills a gap in the evolutionary record of big cats. By analyzing the DNA of living species, scientists had previously estimated that big cats had split from the Felinae subfamily—which includes smaller wild cats, like cougars, lynxes, along with domestic cats—about 6.37 million years ago. The very existence of P. blytheae confirms that the split happened prior to when this big cat roamed.
But how much earlier? The find could suggest, Tsang says, that big cats branched off from smaller cats much farther back than thought. By comparing the skull’s characteristics with fossils from other extinct big cats, the anatomy of living cat species, and DNA samples taken from both living cats and a few recently extinct, Ice Age-era species (known as cave lions), the researchers assembled a new evolutionary family tree for all big cats. Using known rates of anatomical changes over time and the observed anatomy of P. blytheae, they projected backwards, and estimated that the earliest big cats likely branched off from the Felinae subfamily between 10 and 11 million years ago.
The new fossil also solves a geological mystery. Previously, using DNA analysis of all living big cats and mapping the the fossils excavated from various sites around the world, researchers had determined it was most likely that their common ancestor had lived in Asia. The oldest known specimens, however, were found in Africa. The new species provides the first direct evidence that central Asia was indeed the big cats’ ancestral home, at least as far back as the current fossil record currently goes.
From the fragmented fossils, it’s hard to know much about the extinct species’ behavior and lifestyle, but the researchers were able to make some basic extrapolations from the skull’s anatomy. “It’s not a huge cat, like a lion or a tiger, but closer to a leopard,” Tsang says. The creature’s habitat was likely similar to the current Tibetan plateau, so Tseng speculates that, like the snow leopards that currently live in the area, this species did not hunt on the open plains, but rather cliffs and valleys. Tooth wear patterns also suggest similarities with current snow leopards—the rear teeth, likely used for cutting soft tissue, remain sharp, whereas the front teeth are heavily worn, perhaps reflecting their use in prying open carcasses and picking meat off bones.
Tseng says that he and colleagues plan to return to the area to search for more fossils that could help enlighten us on the evolutionary history of big cats. “The gap still isn’t completely filled yet,” he says. “We need to find older big cats to put the picture together.”
November 7, 2013
A lot has changed in the last 165 million years. A cluster of islands coalesced to form Europe, the Atlantic Ocean opened up and India collided with the rest of Asia as the world’s tectonic plates gradually drifted to form the continents we know today. Dinosaurs largely went extinct, felled mostly by a mass extinction event some 65 million years ago that was likely caused by an enormous meteorite impact. Eventually, apes came down from the trees, evolved into humans, and migrated out of Africa to cover the planet.
During all this time, though, one thing has stayed exactly the same: The anatomical structure of froghopper genitals.
We know that from a remarkable new fossil excavated from the Jiulongshan Formation in Northeastern China and described in the journal PLOS ONE. The 165-million-year-old fossil, analyzed by researchers at the Capital Normal University in China, captures two of the small insects in the act of copulation—the oldest record of insect sex ever discovered. And though the insects themselves belong to a particular species that went extinct long ago, their genital structures appear to be identical to those of the modern-day froghoppers that are distributed all around the world.
There are only 33 known fossils that show insects mating worldwide, and the previous oldest—which contains fossilized midges in the act of copulation—dates to 135 million years ago. These sorts of fossils are so rare because of the nature of fossilization: an organism is buried in dirt or sediment sometime after its death and its skeletal remains are slowly replaced by minerals or converted into new compounds over time.
So, for the act of copulation to be fossilized, it means that two insects would have to die while mating, then fall to the ground locked in the mating position. The excavation site, in Ningcheng County, may help explain how this happened: Most of the fossils there were created as the result of a volcanic eruption, which appears to have knocked all sorts of creatures into a lake, so it’s quite possible that the insects died, their mating bodies stuck to the plant stem they were perched on, then the entire plant settled to the lake bottom with them on it.
The researchers are sure that the fossil depicts the act of mating, rather than two insects lying next to each other, because of their inspection of the insects’ reproductive parts. The fossil shows the male’s aedeagus—roughly, the insect equivalent of a penis—inserted into the female’s bursa copulatrix, the organ that receives sperm for fertilization.
Although the creatures differ slightly from known froghoppers, the anatomical structure of their genitals—even down to the number of segments in the male’s aedeagus—is the same as those of modern-day froghoppers. For that reason, the researchers named the new species Anthoscytina perpetua: “From the Latin perpet, eternal love, in reference to this everlasting copulation,” they write in the paper.
One thing that may have changed, though, is the position these insects assume while they’re mating. From the fossil, it seems that ancient froghoppers faced each other, belly-to-belly, in order to mate. Currently, froghoppers usually do it rather differently, laying side-by-side on their stomachs, with the male’s aedeagus twisting underneath to enter the female:
November 6, 2013
New species of insects, worms and other creepy-crawlers are announced on a monthly basis. Similarly, just last week, two new humpback dolphin species splashed into the headlines. And in October, news broke that early humans may have included fewer species than previously thought. This forces the question: what does it take to be a distinct species?
More than 70 official species definitions exist, of which 48 are widely accepted and used by scientists. And there’s no hard rule that scientists must stick to just one definition; some apply a handful of species definitions when approaching the topic. “I personally go to my lab every day and use five species definitions to conduct research,” says Sergios-Orestis Kolokotronis, a molecular ecologist at Fordham University, and co-author of the new dolphin study, published in Molecular Ecology. “And I sleep just fine amidst this uncertainty.”
Species definitions oftentimes do not translate from one organism to another. Dolphins may become isolated by distance and behavior that prevents them from reproducing, but in other cases–such as bacteria, which reproduce asexually–these distinguishing markers do not apply. Thus, the definition of what constitutes a species varies depending on whether scientists are studying dolphins, monkeys, insects, jellyfish, plants, fungi, bacteria, viruses or other organisms, Kolokotronis explains. And likewise, methods for investigating those species also vary. “Whoever figures out THE unifying species definition across the Domains of Life gets the Crafoord Prize!” Kolokotronis jokes.
In the case of the four dolphin species, each occupy different sections of ocean around the world, including in the Atlantic off West Africa (Sousa teuszii), in the central to western Indo-Pacific (Sousa plumbea), in the eastern Indian and western Pacific (Sousa chinensis) and in northern Australia (researchers are in the process of working on a name for that one–Sousa bazinga, anyone?).
While the humpback dolphins look quite similar, their genetics tells a different story. Researchers collected 235 tissue samples and 180 skulls throughout the animals’ distribution, representing the biggest dataset assembled to date for the animals. The team analyzed mitochondrial and nuclear DNA from the tissue, which revealed significant variations between those four populations. They also compared the skulls for morphological differences.
Although the line between species, sub-species and populations is a blurry one, in this case, the researchers are confident that the four dolphins are divergent enough to warrant the “species” title. The mitochondrial DNA turned up genetic signatures distinct enough to signal a separate species, and likewise, differences in the dolphins skulls supported this divergence. Although the nuclear DNA provided a slightly more confounding picture, it still clearly showed differences between the four species.
“We can confidently say that such strong divergence means these populations are demographically and evolutionarily isolated,” says Martin Mendez, a molecular ecologist at the American Museum of Natural History and lead author of the dolphin paper. “The key is that all the evidence–mitochondrial DNA, nuclear DNA and morphology–exhibited concordant patterns of distinct units,” he continues, which are “usually a must for species proposals.”
The genetic data the team collected does not have enough resolution to reveal how long ago the humpback dolphins diverged, and the team has yet to examine the drivers that fueled those speciation events. But Mendez and his colleagues have found that, in some dolphin populations, environmental factors such as currents and temperature play a role in separating populations and encouraging speciation. Different behaviors can help reinforce that separation, too. Most likely, however, geographic isolation plays a significant role in this case. “For populations living a couple hundred kilometers from one another, it’s perfectly possible for them to meet,” Mendez says. “But the distance from Africa to Australia is so great, it’s difficult to imagine those populations would ever be linked.”
Dolphins, Mendez and his colleagues are finding, evolve relatively quickly once isolated from parent populations. New cryptic–or hidden–species have similarly turned up in waters near South America. There may very well be other species of dolphins–or any type of animal, in fact–lurking undetected within an already-discovered species. ”This really applies to most taxa,” Mendez says. Across the board, “we’re adding many more species by looking at genetic data.”
While cryptic species almost certainly await discovery and will increase the head-counts of some organisms, in the case of ancient human ancestors, on the other hand, researchers now suspect that we’ve been too quick to pull the species card. An extremely well-preserved, approximately 1.8 million year-old Homo erectus skull discovered in Georgia alerted scientists to the potential revision. The skull’s odd proportions–large, but with a small brain case–prompted researchers to analyze variation between modern human and chimpanzee skulls, and compare those variations with other known human ancestor species. As the Guardian reports:
They concluded that the variation among them was no greater than that seen at Dmanisi. Rather than being separate species, the human ancestors found in Africa from the same period may simply be normal variants of H erectus.
If the scientists are right, it would trim the base of the human evolutionary tree and spell the end for names such as H rudolfensis, H gautengensis, H ergaster and possibly H habilis.
Ancient humans, of course, are no longer around for us to study their behaviors and mating tendencies, so anatomy has to do. For now, researchers are calling for more specimens to determine where that line will fall.
The line distinguishing two species may be a fuzzy one, but in the case of the dolphins, it is a big deal in terms of conservation. Australia, for example, is planning to design protective legislation for its new dolphin species, and Mendez hopes other countries will do the same.
Nonetheless, pondering the speciation of humans in dolphins in light of these two findings raises lots of questions: Are we fractally subdividing genetic information and brain cavity size to group and regroup organisms, or is there vast genetic diversity in even familiar species that we’ve yet to uncover? What does it mean for a species to gain or lose members of its family tree? The world and its organisms await more research.
October 24, 2013
If you were stung by a bark scorpion, the most venomous scorpion in North America, you’d feel something like the intense, painful jolt of being electrocuted. Moments after the creature flips its tail and injects venom into your skin, the intense pain would be joined by a numbness or tingling in the body part that was stung, and you might experience a shortness of breath. The effect of this venom on some people—small children, the elderly or adults with compromised immune systems—can even trigger frothing at the mouth, seizure-like symptoms, paralysis and potentially death.
Based solely on its body size, the four-inch-long furry grasshopper mouse should die within minutes of being stung—thanks to the scorpion’s venom, which causes temporary paralysis, the muscles that allow the mouse to breathe should shut down, leading to asphyxiation—so you’d think the rodent would avoid the scorpions at all costs. But if you put a mouse and a scorpion in the same place, the rodent’s reaction is strikingly brazen.
If stung, the four-inch-long rodent might jump back for a moment in surprise. Then, after a brief pause, it’ll go in for the kill and devour the scorpion piece by piece:
This predatory behavior isn’t the result of remarkable toughness. As scientists recently discovered, the mouse has evolved a particularly useful adaptation: It’s immune to both the pain and paralytic effects that make the scorpion’s venom so toxic.
Although scientists long knew that the mouse, native to the deserts of the American Southwest, preys upon a range of non-toxic scorpions, “no one had ever really asked whether they attack and kill really toxic scorpions,” says Ashlee Rowe of Michigan State University, who led the new study published today in Science.
To investigate, Rowe visited the desert nearby Arizona’s Santa Rita Mountains and collected a number of mice and scorpions. Back at her lab, when she and colleagues put the two animals together in the same tank, they saw that the mice devoured the scorpions with gusto and were seemingly impervious to their toxic strings, showing no signs of inflammation or paralysis afterward. They even directly injected the venom into other mouse specimens to further confirm that it didn’t affect them physiologically.
The question remained, though, whether the mice were merely immune to the venom’s paralyzing effects, or were also unable to feel pain as a result of a sting. “I’d see the mice get stung, and they’d just groom a little bit and blow it off,” Rowe says. After she talked to people who’d been stung and heard how badly it hurt, she hypothesized that the mild reaction in the mice indicated that they were resistant to the pain itself.
Working with Yucheng Xiao and Theodore Cummins of Indiana University, she closely examined the physical structures that connect the sensory neurons (which convey external stimuli, such as pain) to the central nervous system (where pain is experienced). “There are big, long neurons that extend from the hands and feet all the way to the spinal cord, and they’re responsible for taking information from the environment and sending it to the brain,” she says.
Incredibly, the nerve cells associated with the interface between these two systems can continue functioning normally when they’ve been removed from the mice, if they’ve been properly preserved and cultured in a medium. As a result, her team was able to look at the mechanisms that control the flow of signals between the sensory neurons and the spinal cord—structures known as ion channels—and see if those present in grasshopper mice functioned differently than those in house mice when exposed to scorpion venom.
They found, in house mice, the venom caused a channel known as Nav1.7 to pass along a signal, causing the perception of pain. In grasshopper mice, though, something unexpected happened: The arrival of venom caused no change in the activity of Nav1.7, because proteins produced by a different ion channel, known as Nav1.8, bound to venom molecules and rendered them futile. In fact, this reaction produced an overall numbing effect on the entire mouse pain transmission system, leaving the animals temporary incapable of feeling all sorts of pain, including those unrelated to scorpion venom.
The researchers also looked at the underlying genetics, sequencing the genes that correspond to these alternatively-structured ion channels, which will allow them to investigate the specific evolutionary background of this remarkable adaptation. In theory, the incentives for the mouse species evolving an immunity to scorpion toxins seem obvious: The nocturnal rodent feeds on all sorts of scorpions, so unless it can visually distinguish between those that are benign and toxic, it will face severe consequences if it’s sensitive to the venom. “Death, after all, is a pretty strong selection pressure,” Rowe notes.
But on the other hand, pain serves a crucial evolutionary role, informing an organism when it’s in danger. Some other species have been know to evolve resistance to particular toxins (garter snakes, for instance, are resistant to the toxin produced by rough-skinned newts), but these examples all involve resistance to toxins that can kill, but don’t actually cause pain.
So the fact that grasshopper mice have evolved resistance to pain itself is novel—and likely a result of a very specific set of evolutionary circumstances. One important aspect is that bark scorpions are a significant proportion of the mouse diet, leading to frequent interactions between the two organisms. Additionally, says Rowe, “the mechanism is specific to the venom itself, so it doesn’t compromise the mouse’s overall pain pathways.” As a result, the mouse is still able to detect other sources of pain (just not right after it’s been bitten by by the scorpion), and thus will know when its faced with unrelated painful perils.
October 23, 2013
We tend to think of giraffes as a single species, but in Kenya not one but three types of giraffe occupy the same scruffy grasslands. These three species–the Masai, Reticulated and Rothschild’s giraffe–often encounter one another in the wild and look similar, but they each maintain a unique genetic makeup and do not interbreed. And yet, throw a male Masai and a female Rothschild’s giraffe, a male Rothschild or a female Reticulated–or any combination thereof–together in a zoo enclosure, and those different species will happily devote themselves to making hybrid giraffe babies.
What is it, then, that keeps these species apart in the wild?
Researchers from the University of California, Los Angeles, may be close to an answer. In nature, at least one of four potential barriers typically keeps similar-looking and similar-acting but distinct species from becoming intimate: distance, physical blocks, disparate habitats or seasonal differences, like rainfall. In the case of the Kenyan giraffes, the researchers could simply look at the habitat and know that physical barriers could probably be ruled out; no mountains, canyons or great bodies of water prevent the giraffes from finding one another. Likewise, giraffes sometimes have home ranges of up to 380 square miles, and those ranges may overlap. Distance alone, therefore, was probably not stopping the animals from meeting.
Either habitat or seasonal differences, they suspected, was the likely firewall preventing species from getting up close and personal with one another. To tease out the roles of these potential drivers, the authors built computer models that took into account a range of factors, including climate, habitat, human presence and genotypes from 429 giraffes that they sampled from 51 sites around Kenya. Just to make sure they weren’t unfairly excluding distance and physical obstacles from the list of possible dividers, they also included elevation values–some giraffes were found in the steep Rift Valley–and the distance between populations of giraffes sampled.
According to their statistical model, regional differences in rain–and the subsequent greening of the plains that it triggers–best explain genetic divergence between giraffe species, the researchers write in the journal PLoS One. East Africa experiences three different regional peaks in rain per year–April and May, July and August and December through March–and those distinct weather envelopes trisect Kenya.
So, although the trio of giraffe species sometimes overlap in range, the authors samples as well as previous studies revealed that they tend to each live and mate in one of those three geographic rain pockets, both within Kenya and throughout the greater East Africa region.
Giraffe species sync their pregnancies up with rain patterns to ensure enough vegetation is around to support the energetically taxing processes of gestation, birth and lactation for mother giraffes, the authors think. Not much information is available on giraffe births, but the few observations on this topic do confirm that giraffe species tend to have their babies during the local wet season, they report.
And while the models indicate that rain is the primary divider keeping giraffes apart, the authors point out that the animals also may be recognizing differences in one another’s coat patterns, for example. But scientists do not know enough about how giraffes chose mates or whether they can distinguish potential mates between species to give the species possible due credit for recognizing one another.
Whether rain alone or some combination of rain and recognition trigger mating, in the wild, at least, those mechanisms seem to work well for keeping giraffe species apart. It will be interesting to see whether this separation is maintained as climate changes.