Three vaccines offer complete protection against Zika virus in monkeys.

The results are the latest step in the quest to create a Zika vaccine that’s safe and effective for humans (SN Online: 6/28/16).

One vaccine, made with a purified, inactivated form of the virus — designated PIV — helped rhesus monkeys fend off infection from both a Zika strain from Brazil and one circulating in Puerto Rico, study coauthor Nelson Michael and colleagues report August 4 in Science. A second DNA-based vaccine that uses snippets of Zika’s genetic material to rev up the immune system was tested against a Brazilian strain. So was a third type of vaccine that relies on a virus called adenovirus to carry Zika genes into the monkeys’ bodies.
In recent days, the U.S. government and Inovio Pharmaceuticals have both started human safety trials for two other DNA-based candidates. But Michael, a vaccine researcher at the Walter Reed Army Institute of Research in Silver Spring, Md., thinks the PIV vaccine may have the best shot.

“It’s the one that’s probably going to go the distance,” he says.

DNA-based vaccines have never before been licensed for use in humans, he notes. The technique to make a PIV vaccine “goes back to Jonas Salk and polio,” Michael says. Essentially, researchers grow Zika in a lab, kill it and then purify it. “It’s a classic way to make a vaccine,” he says. “And you know what? It works.”

Human testing of the PIV vaccine will start in October. Still, Michael says evaluating many vaccine candidates is important. Any number of factors, from a bad reaction to a bankrupt manufacturer, can knock a vaccine out of the running.

“You definitely want to bet on more than one horse,” he says.

The latest in birthday science proposes that the vertebrate with the longest life span yet measured is the mysterious Greenland shark.

Dating based on forms of carbon found in sharks’ eye lenses suggests that a large female Somniosus microcephalus was about 392 years old (give or take 120 years) when she died, says marine biologist Julius Nielsen of University of Copenhagen. Even with that uncertainty, the shark outdoes what Nielsen considers the previous record holder: a bowhead whale estimated to have lived 211 years.
The dating comes from the first use of eye-lens dating for a fish, Nielsen says. An analysis that produced the date, involving 27 other Greenland shark specimens, suggests that females don’t reach sexual maturity until they’re about 156 years old, Nielsen and his colleagues report August 12 in Science. Remarkably little basic biology is known for the Greenland shark, though.

And figuring out the age of these sharks has “stymied all solution attempts,” says Steven Campana of the University of Iceland in Reykjavik. ”Given that the Greenland shark is one of the largest carnivores in the world and the king of the food chain [in northern waters], it is almost unbelievable that we don’t know if this shark lives to 20 years or to 1,000,” says Campana, who has long studied shark aging but was not part of this research. Both extremes have been suggested.

Unlike familiar bony fish, such as salmon and cod, sharks don’t have ear bones that build up calcified rings that reveal age. Some sharks, such as the great whites, have some calcified vertebrae that serve, but the Greenland species is “a soft shark,” Nielsen says. And it’s an odd-looking one. He finds that some people are disappointed with their first sight of the big, dark, ponderous beasts because they’re a long way from the stereotype of the great white sharks’ streamlined killer look. “Definitely plump,” Nielsen says.

Working with 28 Greenland sharks of different sizes that were accidentally caught during fisheries surveys, Nielsen and his colleagues examined eye lenses. The highly specialized clear proteins in lenses start with a nugget formed in utero, and studies in mammals have scrutinized that small bit for clues to a creature’s birth date.

Nielsen’s team looked for anomalies in carbon created by the pulse of radioactivity from the 1950s bomb testing in the Pacific Ocean. Radiocarbons worked their way into, and lingered in, all the food webs on the planet. The pulse first reached the sharks’ realms in the North Atlantic in the 1960s, the scientific literature indicates. Nielsen was startled to discover that only three specimens in his collection had the carbon anomalies — and they were the smaller sharks.
He and colleagues used the size of a shark that appeared to have been born just as the bomb pulse was arriving in the ocean food system as a kind of calibration marker. Then, in an elaborate statistical analysis, they used size and growth rates to work out ages for the rest.

Campana is skeptical that Greenland sharks can live nearly 400 years. Other sharks typically live for 10 to 80 years, he says. “I certainly accept that it grows for more than a century.” But to crown the Greenland shark a record holder, he is waiting for future research.

Extreme life spans evolve just like polar bear white fur or long giraffe necks to fit into the sum of ways an organism feeds, dodges its predators and reproduces in its environment. Says James R. Carey of the University of California, Davis, who studies demography across the tree of life, “the really deeper question is once you identify a species that’s long-lived — why?”

A debate over when the gap between North and South America closed has opened a rift in the scientific community.

Analyzing existing data from ancient rocks, fossils and genetic studies, a group of researchers has assembled a defense of the conventional view that the Isthmus of Panama formed around 3 million years ago. That work rebuts papers published last year that concluded that the continental connection started millions of years earlier (SN: 5/2/15, p. 10). The authors of the new paper, published August 17 in Science Advances, caution against the “uncritical acceptance” of the older formation date.
“Those of us who are advocating the traditional view are in danger of being seen as old fuddy-duddy conservatives,” says study coauthor Harilaos Lessios, a molecular evolutionist at the Smithsonian Tropical Research Institute in Panama City. “But sometimes the traditional view is the correct one.”
The American continents drifted apart following the breakup of the Pangaea supercontinent around 200 million years ago. Eventually, the landmasses slid back together. As they reconnected, a volcanic mound on the Caribbean tectonic plate collided with South America and rose above the ocean. This new land closed a seaway between the Pacific and Atlantic oceans, rerouted ocean currents and sparked animal migrations, leaving clues that scientists on both sides of the debate are using to determine the age of the Isthmus of Panama.

Aaron O’Dea, a paleontologist at the Smithsonian Tropical Research Institute, Lessios and colleagues revisited several of those lines of evidence to date the seaway closure. For instance, fossil records reveal that land animals began migrating more frequently between the Americas around 2.7 million years ago, possible evidence of a newly available land route, O’Dea’s team concludes. Critics, though, counter that those migrations were instead driven by climate and ecosystem changes that allowed animals to migrate.
In the oceans, the closed seaway divided populations of marine organisms such as sand dollars. Over time, these populations’ genetic makeups diverged. Based on the degree of genetic change between the groups as well as fossil evidence, O’Dea’s team estimates that the seaway closed roughly 3 million years ago.

Christine Bacon, an evolutionary biologist at the University of Gothenburg in Sweden, and colleagues analyzed similar evidence last year but came to a different conclusion. The seaway closed between 23 million and 7 million years ago, Bacon and colleagues estimated in the Proceedings of the National Academy of Sciences. That study assumed a different rate of genetic divergence and looked at more species than the work by O’Dea and colleagues, Bacon says.

Rocks also trace the isthmus’s rise from the sea. Chemical traces from ancient ocean sediments record when seawater stopped mixing between the Atlantic and Pacific. Analyzing those traces, O’Dea and colleagues estimate that the seaway became relatively shallow around 12 million to 9.2 million years ago and completely shut around 2.7 million years ago.

Other rocky evidence tells a different story, proponents of the older age claim. Volcanically-forged crystals, known as zircons, found in South America date back to around 13 million to 15 million years ago. The only possible source of those crystals was in Panama, suggesting that a river washed the crystals down a land connection between Panama and South Americaaround that time, geologist Camilo Montes of the Universidad de los Andes in Bogotá, Colombia, and colleagues concluded last year in Science.
Those South American crystals may have formed closer to home, O’Dea and colleagues argue in the new paper. Similar crystals have been found elsewhere in South America, so the crystals reported by Montes and colleagues may have originated from a source in South America, not Panama, O’Dea says.

Some of the disagreement between the two sides stems from the fact that the seaway closure wasn’t a single event, says Carlos Jaramillo, a paleontologist at the Smithsonian Tropical Research Institute who coauthored the studies by Montes and Bacon. The seaway would have closed in stages, with various segments shortened and closed off over millions of years, Jaramillo says. “You can’t just use one date for everything, it depends on what you’re looking at,”he says.

Bacon is holding her ground. “They basically rehashed a mishmash of old papers,” she says of the new work. “We need to gather new data and collaborate rather than hold on to old ideas bitterly.”

Forget garlic. In real life, a tomato can defeat a vampire. And researchers have now figured out the first step to vegetable triumph.

The vampires are slim, tangling vines that look like splats of orange or yellow-green spaghetti after a toddler’s dinnertime tantrum. Botanically, the 200 or so Cuscuta species are morning glories gone bad. In the same family as the heavenly blue garden trumpets, the dodders, as they’re sometimes called, lose their roots about a week after sprouting and never grow real leaves. Why bother when you can drain food and water from the neighbors?
A dodder seedling, basically a bare stem, finds that first neighbor by writhing and groping (in slow plant time) toward attractive plant odors. “The Cuscuta can smell its victims,” says Markus Albert of the University of Tübingen in Germany.

Depending on the dodder species, victims include asparagus, melons, sugar beets, petunias, garlic, chrysanthemums and oak trees. Even worse for civilization as we know it, some Cuscuta species vampirize coffee plants and grapevines.
Certain dodders do kill tomato plants. But not the C. reflexa from Asia that Albert studies; instead, it gets its skinny little haustoria whipped. Haustoria are the organs that make plant parasitism possible. When a dodder seedling brushes against tasty prey, a haustorium disk forms and pushes out from the dodder stem with a fast-growing point. “It really looks like a vampire tooth,” Albert says.

If the prey is, say, a soybean plant, it’s doomed. The growing dodder haustorium not only exerts force but also releases enzymes that weaken the bean’s tissue. Haustorium tip cells send out projections that grasp the bean’s inner ducts for water and nutrients, diverting so much that the bean starves.

A tomato plant poked by a haustorium, however, panics. A patch of cells on the stem elongate and burst, forming a scab that stops the intruder. The haustorium stalls and eventually dies.

A gene called CuRe1 lets the tomato recognize the dodder as a dire threat, Albert and colleagues report in the July 29 Science. They transferred the gene to a normally susceptible relative and — Ha! Bite that, vampire! Albert predicts additional biochemistry could be needed to dodder-proof other crops. But for starters, researchers now know the first step in protection: A tomato’s rare power to survive a scary vampire is the ability to get really scared itself.

Female mosquitoes carrying the Zika virus can pass the infection to the next generation, lab tests show.

Among Aedes aegypti mosquitoes, thought to be the main species spreading Zika in the Americas, at least one out of every 290 lab offspring catches the virus from its mother, Texas researchers say August 29 in the Journal of Tropical Medicine and Hygiene. Infected eggs, which can survive for months on dry surfaces, could keep the virus circulating even after dry or cold spells, when adult mosquitoes die off, warns Robert Tesh of University of Texas Medical Branch in Galveston.

Earlier research had already shown that youngsters of this species can inherit related viruses, such as those causing dengue, West Nile and yellow fever. Mom-to-egg transmission though is not a given: The same research project also reported no evidence so far of this vertical transmission in 803 offspring of another possible Zika spreader, Ae. albopictus.

It’s not known how likely mosquito moms are to infect their young outside of the lab. Doing a reliable test with wild mosquitoes outdoors is a much more difficult project, the researchers say.

Contrary to many adorable children’s stories, hibernation is so not sleeping. And most animals can’t do both at the same time.

So what’s with Madagascar’s dwarf lemurs? The fat-tailed dwarf lemur slows its metabolism into true hibernation, and stays there even when brain monitoring shows it’s also sleeping. But two lemur cousins, scientists have just learned, don’t multitask. Like other animals, they have to rev their metabolisms out of hibernation if they want a nap.
Hibernating animals, in the strictest sense, stop regulating body temperature, says Peter Klopfer, cofounder of the Duke Lemur Center in Durham, N.C. “They become totally cold-blooded, like snakes.” By this definition, bears don’t hibernate; they downregulate, dropping their body temperatures only modestly, even when winter den temperatures sink lower. And real hibernation lasts months, disqualifying short-termers such as subtropical hummingbirds. The darting fliers cease temperature regulation and go truly torpid at night. “You can pick them out of the trees,” Klopfer says.

The fat-tailed dwarf lemur, Cheirogaleus medius, was the first primate hibernator discovered, snuggling deep into the softly rotting wood of dead trees. “You’d think they’d suffocate,” he says. But their oxygen demands plunge to somewhere around 1 percent of usual. As trees warm during the day and cool at night, so do these lemurs. When both a tree and its inner lemur heat up, the lemur’s brain activity reflects mammalian REM sleep.

Klopfer expected much the same from two other dwarf lemurs from an upland forest with cold, wet winters. There, C. crossleyi and C. sibreei spend three to seven months curled up underground, below a thick cushion of fallen leaves. “If you didn’t know better, you might think they were dead because they’re cold to the touch,” Klopfer says.

Unlike the tree-hibernators, the upland lemurs take periodic breaks from hibernating to sleep, Klopfer, the Lemur Center’s Marina Blanco and colleagues report in the August Royal Society Open Science. The lemurs generated some body heat of their own about once a week, which is when their brains showed signs of sleep (REM-like and slow-wave). “My suspicion is that sleep during torpor is only possible at relatively high temperatures, above 20º Celsius,” Klopfer says. Sleep may be important enough for cold-winter lemurs to come out of the storybook “long winter’s nap.”

By sneakily influencing brain activity, scientists changed people’s opinions of faces. This covert neural sculpting relied on a sophisticated brain training technique in which people learn to direct their thoughts in specific ways.

The results, published September 8 in PLOS Biology, support the idea that neurofeedback methods could help reveal how the brain’s behavior gives rise to perceptions and emotions. What’s more, the technique may ultimately prove useful for easing traumatic memories and treating disorders such as depression. The research is still at an early stage, says neurofeedback researcher Michelle Hampson of Yale University, but, she notes, “I think it has great promise.”
Takeo Watanabe of Brown University and colleagues used functional MRI to measure people’s brain activity in an area called the cingulate cortex as participants saw pictures of faces. After participants had rated each face, a computer algorithm sorted their brain responses into patterns that corresponded to faces they liked and faces they disliked. With this knowledge in hand, the researchers then attempted to change people’s face preferences by subtly nudging brain activity in the cingulate cortex.

In step 2 of the experiment, returning to the fMRI scanner, participants saw an image of a face that they had previously rated as neutral. Just after that, they were shown a disk. The goal, the participants were told, was simple: make the disk bigger by using their brains. They had no idea that the only way to make the disk grow was to think in a very particular way.

For 12 people, the researchers made the disk grow when the participants’ brain activity looked like the activity that corresponded to faces they had liked in the first round. For 12 other people, the disk grew when their brain activity mirrored activity elicited by previously unliked faces. Another six people saw the faces, but didn’t do any disk training. This training lasted an hour each day for three days.

At the end of the training, people induced to call up brain activity similar to positive responses rated previously neutral faces as slightly more positive. “By doing this again and again, subjects began to like what was neutral before,” Watanabe says. And people who had called up activity associated with negative responses rated previously neutral faces as slightly more negative. People who hadn’t trained on the disk didn’t change their ratings. These opinion shifts lasted at least three months, later experiments showed.

Participants were simply told to make the disk bigger; they had no idea what the disk actually represented. “These results are fascinating in showing how nonconscious brain activity can be utilized to modify brain function and behavior in a targeted way,” says neuroscientist Rafi Malach of the Weizmann Institute of Science in Israel.

By showing that neurofeedback can influence complex mental processes, this study and others raise the possibility that similar methods could change the brain in desirable ways. Perhaps this sort of neural training could get rid of problematic patterns of thinking, such as those that come with abnormal fear and depression, Watanabe says.

Galápagos cormorants are the only flightless cormorant species. Their wings are too small to lift their heavy bodies. To trace the genetic changes responsible for the birds’ shrunken wings, Alejandro Burga needed DNA from the grounded bird and from a few related species. For the UCLA evolutionary geneticist, getting the right DNA was a yearlong effort.

After Galápagos cormorants (Phalacrocorax harrisi) split off from other cormorants, their wings shrunk to 19 centimeters long and their bodies grew to 3.6 kilograms, not a flying-friendly combination. Burga suspected he would have difficulty getting permission to collect DNA from the endangered birds. So he e-mailed “anybody who had ever published anything on cormorants” in the last 20 years, he says.
He found disease ecologist Patricia Parker of the University of Missouri-St. Louis who had collected blood from Galápagos cormorants in 2000 to monitor the spread of pathogens. Getting to the islands takes special permission, long flights and boat trips, but getting DNA from the meter-tall birds wasn’t hard.

“They’re sluggish, and they just sit there and look at you,” Parker says. She shared DNA that had been sitting in her lab refrigerator for more than a decade. Burga used it to reconstruct the cormorants’ genetic instruction book, or genome.

Next he needed comparison DNA from closely related species, such as the double-crested cormorant — a goose-sized waterbird with a broad wingspan. The bird is protected under a migratory bird treaty between the United States and Canada. Since Burga couldn’t just trap one and collect DNA, he got creative. He tried to extract DNA from preserved specimens at the Natural History Museum of Los Angeles County, but the genetic material was unusable. The San Diego Zoo sent samples of a too-distantly related great cormorant. An international bird rescue facility in Los Angeles notified him when someone found a dead cormorant on the beach. Burga rushed over, but the bird was a Brandt’s cormorant — also too far removed in the family tree to be of use.

One e-mail chain led to Paul Wolf, a U.S. Department of Agriculture wildlife disease biologist monitoring Newcastle disease virus in double-crested cormorants in Minnesota. With a special permit, Wolf removed one double-crested cormorant egg from a nest. The egg was at just the right stage of development — when the wings were beginning to grow — to determine which genes are active during wing development. Two down, two to go.

While on Alaska’s Middleton Island studying seabird parasites, Andrew Ramey of the U.S. Geological Survey collected two eggs from pelagic cormorants for Burga.
Burga also enlisted Claudio Verdugo, a molecular epidemiologist at Universidad Austral de Chile in Valdivia. Bird samples can’t be transported between countries because of fears of disease spread. So Burga sent chemicals and protocols to Verdugo, who took DNA from another species, the neotropic cormorant, and sent it to Burga.

With DNA from four cormorant species in hand, Burga and his newfound friends learned that the Galápagos cormorants’ stubby wings result, in part, from mutations in specific genes that encourage limb growth (SN: 6/11/16, p. 11). Burga is now studying how evolution grounded other birds.

It’s a problem that sounds simple, but the best minds in mathematics have puzzled over it for generations: A salesman wants to hawk his wares in several cities and return home when he’s done. If he’s only visiting a handful of places, it’s easy for him to schedule his visits to create the shortest round-trip route. But the task rapidly becomes unwieldy as the number of destinations increases, ballooning the number of possible routes.

Theoretical computer scientist Shayan Oveis Gharan, an assistant professor at the University of Washington in Seattle, has made record-breaking advances on this puzzle, known as the traveling salesman problem. The problem is famous in mathematical circles for being deceptively easy to describe but difficult to solve. But Oveis Gharan has persisted. “He is relentless,” says Amin Saberi of Stanford University, Oveis Gharan’s former Ph.D. adviser. “He just doesn’t give up.”
Oveis Gharan’s unwavering focus has enabled him to identify connections between seemingly unrelated areas of mathematics and computer science. He scrutinizes the work of the fields’ most brilliant minds and adapts those techniques to fit his purposes. This strategy — bringing new tools to old problems — is the basis for leaps he has made on two varieties of the traveling salesman problem.

“If you want to build a house, you need to have a sledgehammer and a level, a wrench, tape measure,” he says. “You need to have a lot of tools and use them one after another.” Oveis Gharan, age 30, stocks his toolkit with the latest advances in fields with obscure-sounding names, including spectral graph theory, polyhedral theory and geometry of polynomials. And in a twist that only Oveis Gharan saw coming, a recent solution to a long-standing problem originating in quantum mechanics turned out to be the missing piece to one aspect of the salesman’s puzzle.
For a salesman’s tour of five cities, there are just 12 possible routes; it’s easy enough to pick the one that will save the most gas. But for 20 cities, there are 60 quadrillion possibilities, and for 80 cities, there are more routes than the number of atoms in the observable universe. Relying on brute force — calculating the distances of all the possible routes — is intractable for all but the easiest cases. Yet no one has found a simple method that can quickly find the shortest path for any number and arrangement of cities. The quandary has real-world importance: Companies like Amazon and Uber, for example, want to ferry goods and people to many destinations in the most efficient way possible.

Growing up in his home country of Iran, Oveis Gharan discovered a natural appreciation for challenging puzzles. In middle school, he acquired a book of problems from mathematics Olympiad competitions in the Soviet Union. As a student, “I tend to be one of the slower ones,” Oveis Gharan says, noting that he was usually not the first to grasp a new theorem. But within a few years, he had doggedly plowed through the 200-page book.

The effort also provided Oveis Gharan with his first taste of tool collecting, through collaboration with classmates who joined him in working through the math problems. Oveis Gharan found that solutions come easier when many minds contribute. “Each person thinks and solves problems differently,” he says. “Once someone is exposed to many different ideas and ways of thinking on a problem, that will help a lot to increase the breadth of problem-attacking directions.”

Oveis Gharan attended Sharif University of Technology in Tehran before making his first breakthroughs on the traveling salesman problem as a graduate student at Stanford University. He spent over a year cracking just one thorny facet, before moving on to a postdoctoral fellowship at the University of California, Berkeley.
Rather than attacking the problem head-on, Oveis Gharan works on approximate solutions — routes that are slightly longer than the optimal path but can be calculated in a reasonable amount of time. Since the 1970s, computer scientists have known of a strategy for quickly finding a route that is at most 50 percent longer than the shortest possible path. That record held for decades, until Oveis Gharan tackled it along with Saberi and Mohit Singh, then of McGill University in Montreal.

In a paper published in 2011, the team made what might sound like an infinitesimal improvement, shrinking the 50-percent figure by four hundredths of a trillionth of a trillionth of a trillionth of a trillionth of a percentage point. “People make fun of our paper because of that small improvement,” says Oveis Gharan, “but the thing is that in our area, the actual number is not the major question.”
Instead, the goal is to develop new ideas that can begin to crack the problem open, says Luca Trevisan, a computer scientist at Berkeley. “What’s so important is not the specific algorithm that he has devised, but that there is a whole new set of techniques that can potentially be applied to other problems.” Following the advance, other scientists revisited the traveling salesman problem, and decreased the number significantly; the selected route is now at most 40 percent longer than optimal.

To make his breakthroughs, Oveis Gharan keeps tabs on the scientific literature across a variety of mathematical fields. “Every time new papers or new techniques come out, he’s one of the first people who will pick up the paper and read it,” says Saberi. To discover tools outside his areas of expertise, Oveis Gharan poses pieces of the problem to researchers in other fields.

In 2015, Oveis Gharan and computer scientist Nima Anari, then at Berkeley, made further progress on an approximate solution for a more general, and more challenging, version of the traveling salesman problem. In this version, the distance to go from point A to point B might not be the same as going the opposite direction — a plausible situation in cities with many one-way streets. Researchers had a way to estimate the optimum tour length, but they didn’t understand how good the estimate was. Oveis Gharan and Anari showed it was exponentially better than known previously.

To make this advance, Oveis Gharan teased out connections to a seemingly unrelated problem in mathematics and quantum mechanics, known as the Kadison-Singer problem. “That was really surprising,” says computer scientist Daniel Spielman of Yale University, part of a team that solved the Kadison-Singer problem in 2013. “There was no obvious connection,” he says. “Shayan is incredibly brilliant and incredibly creative.”

Oveis Gharan is now focused on a furtherconquest of this version of the traveling salesman problem. Though his new advance helps approximate the optimal tour length, it can’t identify the corresponding route. Next, Oveis Gharan would like to produce an algorithm that can navigate the correct course.

You can bet he’ll continue to add to his tool collection by sampling from related mathematical and computational fields. “The grand plan is: Try to better understand how these different areas are connected to one another,” Oveis Gharan says. “There are many big open problems lying in this intersection.”

Methane wasn’t the cozy blanket that kept Earth warm hundreds of millions of years ago when the sun was dim, new research suggests.

By simulating the ancient environment, researchers found that abundant sulfate and scant oxygen created conditions that kept down levels of methane — a potent greenhouse gas — around 1.8 billion to 800 million years ago (SN: 11/14/15, p. 18). So something other than methane kept Earth from becoming a snowball during this dim phase in the sun’s life. Researchers report on this new wrinkle in the so-called faint young sun paradox (SN: 5/4/13, p. 30) the week of September 26 in the Proceedings of the National Academy of Sciences.

Limited oxygen increases the production of microbe-made methane in the oceans. With low oxygen early in Earth’s history, many scientists suspected that methane was abundant enough to keep temperatures toasty. Oxygen may have been too sparse, though. Recent work suggests that oxygen concentrations at the time were as low as a thousandth their present-day levels (SN: 11/28/14, p. 14).

Stephanie Olson of the University of California, Riverside and colleagues propose that such low oxygen concentrations thinned the ozone layer that blocks methane-destroying ultraviolet rays. They also estimate that high concentrations of sulfate in seawater at the time helped sustain methane-eating microbes. Together, these processes severely limited methane to levels similar to those seen today — far too low to keep Earth defrosted.