December 4, 2013
Since its discovery in 1990, La Sima de los Huesos, an underground cave in Northern Spain’s Atapuerca Mountains, has yielded more than 6,000 fossils from 28 individual ancient human ancestors, making it Europe’s most significant site for the study of ancient humans. But despite years of analysis, the exact age and even the species to which these individuals belonged has been in doubt.
Now, though, an international group of scientists has extracted and sequenced DNA from the fossilized femur of one of these individuals for the first time. The resulting data—which represent the oldest genetic material ever sequenced from a hominin, or ancient human ancestor—finally give us an idea of the age and lineage of these mysterious individuals, and it’s not what many scientists expected.
The fossilized bone tested, a femur, is roughly 400,000 years old. But the big surprise is that, although scientists had previously believed the fossils belonged to Neanderthals because of their anatomical appearance, the DNA analysis actually shows they’re more closely related to Denisovans, a recently-discovered third lineage of human ancestors known only from DNA isolated from a few fossils found in Siberia in 2010. The findings, published today in Nature, will force anthropologists to further reconsider how the Denisovans, Neanderthals and the direct ancestors of modern-day humans fit together in a complicated family tree.
The analysis was enabled by recent advances in methods for recovering ancient DNA fragments developed at the Max Planck Institute for Evolutionary Anthropology in Germany, previously used to analyze the DNA of a cave bear fossil found in the same cave. “This wouldn’t have been possible just two years ago,” says Juan Luis Arsuaga, a paleontologist at the University of Madrid who led the initial excavations of the cave and collaborated on the new study. “And even given these new methods, we still didn’t expect these bones to preserve DNA, because they’re so old—ten times older than some of the oldest Neanderthals from whom we’ve taken DNA.”
After extracting a two grams of crushed bone from the femur, a group of scientists led by Matthias Meyer isolated the mitochondrial DNA (mtDNA), a pool of genetic material that’s distinct from the DNA in the chromosomes located in our cells’ nuclei. Instead, this mtDNA lives in our cells’ mitochondria—microscopic organelles responsible for cellular respiration—and is much shorter in length than nuclear DNA.
There’s another quirk of mtDNA that makes it especially valuable as a means of studying the evolution of ancient humans: Unlike your nuclear DNA, which is a mix of DNA from both your parents, your mtDNA comes solely from your mother, because most of a sperm’s mitochondria are found in its tail, which it sheds after fertilization. As a result, mtDNA is nearly identical from generation to generation, and a limited number of distinct sequences of mtDNA (called haplogroups) have been observed in both modern humans and ancient human ancestors. Unlike anatomical characteristics and nuclear DNA, which can vary within a group and make it difficult to confidently distinguish one from another, mtDNA is generally consistent, making it easier to link a particular specimen with a lineage.
Which is why, when the researchers compared the femur’s mtDNA to previously sequenced samples from Neanderthals, from a Denisovan finger bone and tooth found in Siberia and from many different modern humans, they found it so surprising that it more closely resembled the Denisovans. “This was really unexpected,” Arsuaga says. “We had to think really hard to come up with a few scenarios that could potentially explain this.”
Anthropologists had already known that all three lineages (humans, Neanderthals and Denisovans) shared a common ancestor, but it’s far from clear how all three groups fit together, and the picture is further clouded by the fact that interbreeding may have occurred between them after they diverged. Helpfully, comparing the femur’s mtDNA to the Neanderthal, Denisovan and modern human samples allowed the researchers to estimate its age—based upon known rates of mtDNA mutation, the previously established ages of the other samples, and the degree of difference between them—leading to the 400,000 year figure.
To explain how a Neanderthal-looking individual could come to have Denisovan mtDNA during this time period, the scientists present several different hypothetical scenarios. It’s possible, for instance, that the fossil in question belongs to a lineage that served as ancestors of both Neanderthals and Denisovans, or more likely, one that came after the split between the two groups (estimated to be around 1 million years ago) and was closely related to the latter but not the former. It’s also a possibility that the femur belongs to a third, different group, and that its similarities to Denisovan mtDNA are explained by either interbreeding with the Denisovans or the existence of yet another hominin lineage that bred with both Denisovans and the La Sima de los Huesos population and introduced the same mtDNA to both groups.
If this sounds like a complicated family tree to you, you’re not alone. This analysis, along with earlier work, adds further mystery to an already puzzling situation. Initial testing on the Denisovan finger bone found in Siberia, for instance, found that it shared mtDNA with modern humans living in New Guinea, but nowhere else. Meanwhile, it was previously thought that Neanderthals had settled in Europe and Denisovans further east, on the other side of the Ural Mountains. The new analysis complicates that idea.
For now, the researchers believe the most plausible scenario (illustrated below) is the femur belongs to a lineage that split off from Denisovans sometime after they diverged from the common ancestor of both Neanderthals and modern humans. But perhaps the most exciting conclusion to come out of this work is that it proves that genetic material can survive for at least 400,000 years, and can be analyzed even after that amount of degradation. Armed with this knowledge and the new techniques, anthropologists can now attempt to genetically survey many other ancient specimens in hopes of better understanding our family tree.
In 1917, British artist Norman Wilkinson experienced a eureka moment while serving in the Royal Naval Volunteer Reserve. Throughout the month of April, German U-boats had been mercilessly torpedoing British ships, sending around eight of those vessels per day into the watery abyss. Hiding a ship traveling on the open ocean from plain sight was impossible, Wilkinson knew, but a bit of artistic trickery may be able to muddle the Germans’ ability to accurately judge the exact location of that ship, he realized.
From that idea, Wilkinson devised a type of camouflage called “razzle dazzle” (its slightly more serious name is dazzle camouflage). The technique consists of squashing together contrasting geometric patterns, shapes and colors to create a pattern of optics that would confuse enemies by distorting the object’s dimensions and boundaries. All in all, more than 2,000 ships received such a makeover, although the scheme’s effectiveness seemed to produce mixed results.
By World War II, razzle dazzle had largely fallen out of favor, but as it turns out, this technique lives on in the natural world. High contrast patterns–nature’s equivalent to dazzle camouflage–are used by animals ranging from snakes to zebra to fish. Like those hidden World War I ships, many creatures seem to use dazzle patterns to conceal themselves from predators. Until now, however, researchers hadn’t considered the flipside of this relationship: could predators use razzle dazzle to sneak up on prey as they mounted an attack?
To investigate this possibility, biologist Roger Santer of Aberystwyth University in the U.K. turned to locusts. These insects are particularly well suited for vision studies due to something called a single lobula giant movement detector neuron, a unique cell that specializes in detecting looming objects (think of a car speeding towards you, or a hand reaching for your face). Researchers think this neuron works by measuring the shape and movement of patterns of light and dark across the eye. Whatever the mechanism, as looming objects approach a locust, its detector neuron fires away, alerting the insect to imminent potential danger and triggering it to flee.
To see how the locusts responded to dazzle camouflage, Santer created an array of visual patterns using a graphics software. He situated the locusts just in front of the computer monitor, and then projected a simulated approach of those objects from about 10 meters away to about 0.07 meters from the cowering insects. The objects varied in contrast: black, grey or white on a grey background. Around 20 locusts took part in the experiment, and Santer measured their cellular reactions to the various shapes through copper wires inserted into the locusts’ neck.
The locusts’ neurological responses to the looming objects depended on which patterns they saw, Santer reports in Biology Letters. Squares with both a darker-than-background top and bottom half elicited the strongest panic response, followed by squares with a dark upper half, but a bottom half that was the same color as the background. Squares that had an upper half that was dark but a bottom half that was bright (in other words, the razzle dazzle ones) produced a significantly weaker panic response, as did squares that were brighter than the background. Finally, squares that were the same color as the background produced no response at all.
These results are interesting in that they correlate with similar dazzle tests performed on humans, who also had trouble quickly registering dazzle patterns. However, at this point, whether or not locust predators actually use dazzle to catch their unsuspecting insect prey remains a matter of speculation. Though lab tests confirm this strategy might work, Santer did not investigate whether or not a dazzle dance of death is carried out in the real world.
Hypothetically speaking, dazzle camouflage, Santer concludes, would help a predator but would not be the most effective way of snagging a locust lunch. Instead, classic camouflage–blending in with the background rather than creating an optical illusion–seems to be the most effective means of tricking would-be prey. However, in the case that other selection pressures favor high-contrast patterns (such as if females of a predator species prefer bold stripes in males), Santer thinks that predators could indeed evolve to give ‘em the old razzle dazzle.
November 29, 2013
A few years ago, University of Maryland PhD student Nathan Jud was routinely examining a batch of ancient plant fossils in the Smithsonian Natural History Museum‘s collections when one in particular caught his eye.
“It looked sort of like a little piece of fern, so I tried to remove a bit of the rock that was covering it to get a sense of what type of fern it was,” he says. “But the more of the rock I would lift off the surface, the more fossil I found buried. What I thought had been one little piece of a leaf actually turned out to be two, connected to each other.”
As he labored to carefully flake the rock without defacing the fossil, he noticed a series of curious characteristics that suggested the preserved plant was no ordinary fern: It had a closed network of veins, rather than a series of branching ones that split off from each other without coming back together, and at its tips, there were tiny structures called glandular teeth, used to shed excess water.
“Eventually, I realized this wasn’t a fern at all, but some kind of early flowering plant,” he says. Its features wouldn’t be at all out of the ordinary in a plant growing outside today. The fact that they occur in a fossil from the Early Cretaceous period, though, is remarkable. At somewhere between 125 and 115 million years old, this fossil, described in a paper Jud published today in the American Journal of Botany, is among the oldest flowering plants ever found in North America.
Flowering plants—which replicate with sexual structures (i.e. flowers) to produce seeds—now dominate the planet, but for the first 300 million years or so of plant existence, beginning around 450 million years ago, the only types of vegetation belonged to older, more primitive families, such as algae, mosses and ferns, which all reproduce with spores rather than seeds, or gymnosperms, which produce seeds but not flowers.
During the Early Cretaceous, some of the first primitive flowering plants began to evolve. Researchers know that the layer in which this new fossil was found dates to this time period due to a few factors: Pollen analysis (which considers the chemical makeup of pollen embedded in the surrounding rock) and as well as study of the surrounding sediment itself. The same layer has previously produced several other flowering plant fossils of a similar age—together, they’re the oldest ever discovered in North America—but this is the oldest example of a eudicot, a group that includes roughly 70 percent of flowering plants worldwide today that share a distinctively-shaped pollen structure.
Compared to the other fossils found in the same layer, this one is especially remarkable for its derived traits, anatomical characteristics that were previously thought to have developed much more recently in flowers. Their existence so long ago suggests that some early plants were actually quite complex.
“When I compared it to living plants, I realized it was remarkably similar to the leaves of a certain group of modern poppies,” Jud says. “I didn’t expect to see a group that seemingly modern in a collection that old.” The fact that these features existed so long ago, both in this plant and other ancient fossils recently excavated in China, tells us that the evolution of flowering plants (which Charles Darwin famously called an “abominable mystery“) did not happen gradually, but instead occurred very rapidly during a narrow time interval in the Early Cretaceous period between when flowering plants first emerged and the date of this fossil.
There’s also a much more recent history of this fossil that’s just as fascinating. Jud did a bit of research and found that it’d been excavated in 1971 by a former Smithsonian curator, Leo Hickey, who went on to Yale and died in February before working with Jud to re-analyze the fossil after all these years. Hickey had found it during a dig at the Dutch Gap, in Virginia, in sediments that were exposed over a century earlier, by freed slaves who were forcibly taken from the Roanoke Island Freedmen’s Colony by Union troops and forced to dig a canal in August 1864.
While digging, they exposed ancient fossil-filled rocks, and a few decades later, in the 1870s and 1880s, scientists worked there to collect fossils and create some of the Smithsonian’s first fossil collections. Later, Hickey and other researchers returned to collect remaining specimens.
Jud honored this recent history in naming the ancient species that this specimen represents. “Potomac refers to the Potomac Group beds where the fossil was found, capnos is a reference to living poppies that are quite similar to the fossil and apeleutheron is the Greek word for freedmen,” he says. “So the new plant will be named Potomacapnos apeleutheron: roughly, ‘freedmen’s poppy of the Potomac.’”
November 26, 2013
Seahorses belong to the genus Hippocampus, which gets its name from the Greek words for “horse” and “sea monster.” With their extreme snouts, weirdly coiled bodies and sluggish movements produced by to two puny little fins, these oddly shaped fish seem like an example of evolution gone terribly awry. And yet, new research published today in Nature Communications shows that it is precisely the seahorse’s uncanny looks and slow motions that allow it to act as one of the most stealthy predators under the sea.
Seahorses, like their close relatives the pipefish and sea dragons, sustain themselves by feasting on elusive, spastic little crustaceans called copepods. To do this, they use a method called pivot feeding: they sneak up on a copepod and then rapidly strike before the animal can escape, much like a person wielding a bug swatter tries to do to take out an irritating but otherwise impossible-to-catch fly. But like that lumbering human, the seahorse will only be successful if it is able to get near enough to its prey to strike at very close range. In the water, however, this is an even greater feat than on land because creatures like copepods are extremely sensitive to any slight hydrodynamic change in the currents around them.
So how do those ungainly little guys manage to feed themselves? As it turns out, the seahorse is a more sophisticated predator than appearance might suggest. In fact, it is precisely its looks that make it an ace in the stealth department. To arrive at this surprising conclusion, researchers from the University of Texas at Austin and the University of Minnesota used holographic and particle image velocimetry–fancy ways of visualizing 3D movements and water flow, respectively–to monitor the hunting patterns of dwarf seahorses in the lab.
In dozens of trials, they found that 84 percent of the seahorses’ approaches successfully managed not to sound the copepod’s retreat alarms. The closer the seahorse could get to its unsuspecting prey and the faster it struck, the greater its odds of success, they observed. Once in range of the copepod, seahorses managed to capture those crustaceans 94 percent of the time. Here, you can see that method of attack, in which the seahorse’s giant head looks like a floating bit of marine sludge drifting toward the blissfully ignorant copepod:
The way the seahorse’s movements and morphology–especially its head–interact with the water particles, the researchers found, likely take the credit for its exceptional hunting skill. The animal’s arched neck acts like a spring for generating an explosive strike, they describe, while the shape of its snout–a thin tube with the mouth positioned at the very end–allows it to drift through the water while creating minimal disturbance.
To emphasize this pinnacle of engineering, the team compared water disruptions caused by seahorses with those of sticklebacks, a relative of the seahorse but with a more traditional fishy look. Thanks to the shape and contours of the seahorse’s head, that predator produced significantly less fluid deformation in the surrounding water than the stickleback. The poor stickleback possesses neither the morphology nor posture to generate “a hydrodynamically quiet zone where strikes occur,” the authors describe. In other words, while the seahorse may appear a bit odd so far as fishes go, evolution was obviously looking out for that funny but deadly animal’s best interests.
November 12, 2013
When it comes to sex, males and females are not always equal in their desires. No, you haven’t stepped into a couples therapy class.
Welcome to the animal kingdom, where what’s good for one gender could in fact be detrimental for the other. Similar to the struggle between a parasite and its host, some species are locked in an evolutionary arms race between the sexes, with each gender battling to put its best interests forth. Although male and female sexual preferences and tactics are as variable as the thousands of species they represent, a particular species of snake provides an interesting example of conflict that can occur during mating itself, researchers describe in the journal Proceedings of the Royal Society B.
The authors focused their paper on an intimate discussion of red-sided garter snake behaviors. When red-sided garter snakes are ready to mate, several dozen males find their way to a female. Just as she is emerging from hibernation into the warm spring air, the males–which slithered forth days earlier–swarm over her, forming a “mating ball.” Here’s one, from thamnophis14 on YouTube–it’s mesmerizing to watch:
Rather than pick the nicest looking or most impressive male, mating is more of a crapshoot for the female, with the closest male latching on as soon as the female presents herself by opening her cloaca, an orifice that leads into the vagina. But sometimes, things get a bit ugly: males may go so far as to cut the female’s oxygen supply off, which triggers a panic reaction in the female, who releases feces and musk. In doing so, however, she opens up her cloaca, effectively allowing the males to sneak in and get what they want.
Female red-sided garter snakes, not surprisingly, prefer to get copulation over and done with. They attempt to bid their mate goodbye as soon as he has handed over his sperm, and sometimes, even sooner than that. This way, females can get on with their business–which oftentimes entails finding another mate of their choosing. To shake the males off, the female may perform a “body roll,” essentially flipping around until the male detaches.
The males, however, prefer to stick around. The longer they hold on, the more sperm they can transfer and the less chance that another male will snag their female. Sometimes, males take their mate guarding to extremes. Red-sided garter snake males, like some other snake species, may physically plug up the female’s genitals with a ”gelatinous copulatory plug,” preventing her from mating with other males even if he is not around, and stopping her from potentially ejecting his sperm after mating. Over the next few days, however, the plug will dissolve, giving the female a second chance at selecting a mate of her choice under less frantic circumstances.
Researchers aren’t sure what triggers the males to plug up the females. They suspect the female’s “body roll” behavior–essentially a “Get off me!” signal–may have something to do with it. Powerful muscular movements within the female’s vagina may also help to push the male out, but at the same time increase the chances that he attempts to issue a plug.
Finally, to further aid in mating, males of red-sided garter snakes and some other species evolved a special organ whose name and appearance resembles something from a medieval torture chamber: the basal spine. A blunt apparatus covered in small spikes, the basal spine acts as a “grappling hook” for allowing the male to hold the female in place during mating (a process that often makes the females bleed, by the way), some researchers suspect. Overall, however, the basal spine’s adaptive role is a bit of a mystery.
To find out how the snakes’ genital traits influence sexual conflict and behaviors, the researchers caught 42 wild red-sided garter males in Manitoba, Canada, during the spring mating season. They also scooped up newly emerged females, and put two of those females into a small outdoor enclosure with the males. They allowed the snakes to mate naturally while they monitored the duration of copulation, the behaviors involved and whether or not the males left a mating plug behind. Males that copulated for five minutes or more were more likely to leave a plug behind, they found, and the longer the copulation period, the larger the plug.
Afterwards, they divided the males into two groups. Unlucky males in the experimental group suffered a bit of genital mutilation: the researchers clipping off the animals’ basal spines (they did use anesthesia). Males in the other group were left intact. After a four day recovery period, the males were again introduced to two new, unmated females.
This time, the researchers found, the males without a basal spine mated for a significantly shorter duration than the control group. Eight out of 14 of the males lacking basal spines copulated for less than one minute (they were usually shaken off by female body rolls) and did not leave a plug in the female. Moreover, five of them did not manage to eject any sperm.
Next, it was the females’ turn. The researchers collected 24 unmated females. They anesthetized the lady parts of half the females, and used a placebo injection for the others. Females that lost feeling down south, they found, mated for significantly longer than females that were not anesthetized. However, the anesthetized females, compared to the natural ones, received smaller mating plugs even though the copulation period was longer. This may be because those numb females did not struggle, the researchers write, or it could be that the plugs adhere better to engaged vaginal muscles.
Although more experimentation is needed to work out some of the specifics, genital features clearly play significant roles in sexual conflict in this species, the researchers write. In other words, males and females are out for themselves. The males’ strategy increases the chance that they will inseminate a female and thus pass on their own genes, while the females’ strategy increases the chance of insemination from a male they actually want. “The evolution of the basal spine allows males to gain more control over copulation duration, forcing females to evolve some counter trait to regain some control, leading to sexually antagonistic coevolution,” the authors write.
While these tactics may sound brutal to a human reader, the fact that the snakes have evolved these traits prove that they work for the species. And as a small comfort for the snakes, this battle of the sexes is nowhere near the level of brutality seen in the mating behavior of bed bugs–perhaps one of the most graphic example of sexual conflict in the animal kingdom. For that species, males impale the female’s abdomens in a process called traumatic insemination. Compared to being stabbed in the gut, mating plugs may not seem so extreme after all.