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When your cat leaves a mangled mouse on your pillow, he wants you to know that he’s a conqueror. In fact, he is part of a race of conquerors, the successful descendants of a winding journey in which cats made use of humans to conquer the world. Now researchers have used genetics to create the most extensive map ever made of cats’ path to worldwide domination, published this week in the journal Nature.
Modern domestic cats all descend from a single type of wild cat: Felis silvestris lybica. From archaeological studies, researchers believe that F. s. lybica’s reign begins in the Near East, in a region stretching from modern-day Turkey down to Lebanon. Around 10,000 years ago, farmers began storing grain, which attracted pesky mice. Cats, it turned out, could help out with that.
But F. s. lybica also ruled in Ancient Egypt, where they left their traces in cultural artifacts from cat mummies to statues and paintings. Researchers wanted to know: How did these two separate cat-doms lead to today's global feline success?
That wasn’t a question that could be answered with modern cat genetics alone. Around the world, the gene pools of modern cats are surprisingly similar, thanks to millennia of tagging along with human travelers and interbreeding wherever they went. “The modern domestic cats in Australia are the same as in Europe and as in America,” says Eva-Maria Geigl, paleogeneticist at the Institut Jacques Monod, CNRS and University Paris Diderot, and an author on the study.
So for this latest study, the team turned to the genetics of ancient cats around the globe to untangle their collective rise to power. By sifting through 9,000 years of genetic data, the researchers found that there were two separate waves of human-cat coexistence, with cats befriending both farmers and Vikings in their quest to spread around the globe. It also seems that over the course of this relationship, domestication happened fairly late in the game—if at all.
To collect enough samples, the researchers reached out to other scientists around the world for feline bones or teeth, whose toughness and stability make them most likely to harbor useable DNA. They ultimately analyzed over 200 ancient cat skeletons that spanned roughly 9,000 years. They also collected samples from modern cats for comparison. For each of these samples they looked at mitochondrial DNA, genetic material found in every cell that is passed on from mother to child, making it a useful tracer of evolution.
Combining the genetic information with the archeological and human historical records, the researchers teased out the basic pathways for kitty success. After cats befriended Near East farmers, and the farmers recognized their use, they began to crop up along the path of the farming movement. One striking example is a 9,500 year-old cat that was buried in a human grave on the island of Cyprus, where cats are not native. Some 6,000 years ago, after Neolithic farming practices began spreading, it seems that these people-friendly felines foraged northward and westward with humans into Bulgaria and Romania.
Thousands of years after cats in the Near East caught on, a second wave of cats began cohabitating with humans in Egypt. As we know from archeological evidence, cats began living with Ancient Egyptians from at least the 4th century B.C. But DNA shows that during Roman times, these Egyptian felines also began expanding through the Mediterranean, mixing with the Near East cats, and then heading up through the Baltics. Around the fifth and 13th centuries, they ventured through Europe and into Southwest Asia.
It seems cats had hit upon a winning strategy: Stick with humans. When the Viking era began, the expansion of Egyptian felines exploded, likely due to the popularity of ship cats that traveled along the trade routes keeping pests in check. “Rodents on ships not only eat and spoil the food, they also destroy the ropes, so rodents could be a disaster for sailors,” says Thierry Grange, a molecular biologist the Institut Jacques Monod, CNRS and University Paris Diderot and an author on the study. “Cats prevent these types of disasters.”
The researchers even found evidence of these human-loving cats at the Viking port of Ralswiek on the Baltic Sea, says Geigl, and the Iranian port of Siraf, confirming that the faithful mousers commonly joined sailing crews. And the cats’ venture didn’t end there: For thousands of years, these furry globetrotters have followed humans wherever they went, conquering every continent except for Antarctica.
This genetic tour de force was made possible not only thanks to the cheapness and efficiency of modern DNA sequencing, but also new methods in obtaining ancient DNA. The new research “adds to an array of studies coming out now with increasing success of obtaining ancient DNA,” says Melinda Zeder, curator of Old World archaeology at Smithsonian’s National Museum of Natural History. “(It) is clarifying the picture of initial domestication of animals ... and their dispersal … It’s a real technical accomplishment."
Yet while the new study may clarify how and when cats traveled with humans, it also raises new questions. Namely: Were these cats actually domesticated? And if so, when?
These queries are more challenging than they may first appear. What constitutes domestication, like what constitutes a species, is still a matter of fierce scientific debate. Many researchers, Zeder included, define it in terms of a relationship: “For me, domestication is a two-way relationship in which the animal ... is actually benefiting from its relationship from humans,” she says. But that kind of relationship isn’t something that is easy to pinpoint using DNA alone.
Another marker of domestication that researchers often use is distinct changes in the animal's physical looks, like the floppy ears in dogs—a trait that humans likely didn't specifically select for, but seems to be associated with desirable qualities like a less aggressive personality, and can be identified in the genome. Yet modern house cats, besides being slightly smaller and stubbier, don’t look much different from their wildcat cousins, says Giegl. “It's basically still the same shape,” he says. “It has still the same behavior. It has still the same food habits.”
Genetics can’t tell the entire story of domestication, but it can offer clues. In this case, researchers traced a genetic marker for the splotchy tabby fur color. A similar increase in color variation crops up in other animals when selective breeding began, and could be linked with a range of desirable behavioral traits, explains Zeder. It’s also possible that ancient humans could be selecting for these marks, since it may have helped them spot their animals in a crowd. Either way, identifying when this coloring started in cats could help them pin down when selective breeding (rather than just cohabitation) began.
Researchers found the tabby marker in roughly 80 percent of the modern cats tested. However, it didn’t appear in the ancient kitties until around 1300 A.D. This means that efforts to breed cats to look or act a specific way likely didn’t happen until very late in the game. Some scientists even suggest that modern house cats still aren’t fully domesticated—something that will come as little surprise to cat owners.
While the genetic picture is growing clearer, much is still fuzzy when it comes to our cat conquerors, says Wim Van Neer, bioarchaeologist at the University of Leuven who came up with the idea for the study after finding several cats buried in a human cemetery in Egypt dating back 6,000 years ago, the oldest human-cat relations found in the region so far.
Van Neer still wants to know: Where did the first cats—those worshipped in ancient Egyptian—come from? To answer this, researchers need to find still-older Egyptian cats with intact ancient DNA, not an easy proposition in the hot and humid tombs. In the future, researchers could also use isotopes, variations of an element that weigh different amounts, to learn more about kitty diet, as well as study ancient cat jaws to learn more about how their delicate physique has changed through the ages.
What’s certain is that, while cats have changed little as they followed humans around the world, both have grown and benefited from the relationship. The rest, of course, is hiss-tory.
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Scientists have discovered a surprisingly powerful aid in the never-ending quest for a better night's sleep—the smartphone.
Staring at the device won't help you sleep, but phones did enable researchers to gather a mountain of real-world sleep data from thousands of volunteers around the world. The study explores the daily tug-of-war between our bodies' natural rhythms and those of our social calendars.
Two years ago, mathematicians Daniel Forger and Olivia Walch of the University of Michigan designed a free phone app, called ENTRAIN, that helps travelers overcome jet lag by creating optimized personal lighting schedules. The app is driven by a mathematical model that works effectively only when users accurately input such information as their location, sleep hours and daily exposure to light. The scientists, seeing potential in such data, asked users to anonymously volunteer the information collected by the app. Some 10,000 people from 100 countries did just that.
“It's pretty amazing that for almost no cost we ended up with, I think, one of the richest and most interesting datasets on human sleep ever collected,” Forger says. “The unsung heroes in this are all the people who agreed to send us their data.”
What they shared revealed some notable patterns, Forger and Walch, along with UM colleague Amy Cochran, report today in Science Advances. Some nations, for example, are home to night owls while others have citizens who enjoy more beauty sleep. Residents of Singapore and Japan clocked in at the low end, averaging only 7 hours 24 minutes of sleep per night. The Netherlands, in contrast, topped out at 8 hours and 12 minutes of sleep on average each night.
Women most everywhere seem to schedule about half an hour more sleep per night than men. “That's huge,” Forger says. “Half an hour actually makes a huge difference in terms of your overall performance.” Middle-aged men get the least sleep, on average, and often sleep less than the recommended 7 to 8 hours.
As people get older, though, their sleeping schedules look much more alike. “When we looked at the sleep habits of different age groups in our population, we noticed that the distributions of bedtime and wake time were getting narrower as age increased,” Walch notes. This may be real-world support for the results of past studies, she adds, that found that older people had narrower windows of time in which to fall asleep and stay asleep.
Paul Kelley, who researches sleep and circadian neuroscience at the University of Oxford, said it was encouraging to see technology and mathematical models applied to sleep science. “Inventive new methods and new outcomes may offer additional ways to understand our biological timing systems,” he notes, while cautioning that such research remains a work in progress.
Many people don't get enough shuteye. A recent CDC study found that one in three U.S. adults doesn't get the recommended seven minimum hours on a regular basis. And people in other nations around the world are similarly exhausted. That creates problems far more serious than grumpy breakfast conversations and coffee cravings. Sleep deprivation can boost one's chances of obesity, diabetes, high blood pressure, heart disease, stroke, stress and other ailments. And fatigue makes people perform all kinds of mental and physical tasks poorly, which is why sleep scientists keep suggesting that school days should start later.
A primary cause of all this missed sleep is the daily tug-of-war between our bodies' natural inclinations to rest and a host of competing factors created by human society.
Natural sleep patterns are guided by circadian rhythms that are set and reset by the natural cycle of day and night, adjusted by input from our eyes. Forger and Walch had used existing data from other studies to create their mathematical model that simulates these natural circadian rhythms.
This model also enabled them to explore the patterns that appeared during analysis of the ENTRAIN sleep data. For example, they found that people who spend time outdoors in natural light tend to go to bed earlier, and get more sleep, than those who spend most of their day in artificial light. But those data don't reveal if the light itself is causing more sleep, Walch says. For example, these people may report sleeping more because they have physical jobs, which keep them outdoors and tire them out. The model provided a way to test the impacts of outdoor light alone, and its results suggest that natural light does make people sleep more regardless of what they do while outside.
The results also led Forger to suggest an interesting hypothesis about how the battle between social influence and circadian rhythms plays out each day: “We noticed that when people wake up was not a good predictor of whether people in a certain country would sleep more or less, but when they go to bed really was,” he says. “So the reason why people are getting less sleep in certain countries is that they are going to bed later, rather than waking up earlier than people in other countries.”
That finding suggests to Forger that bedtime may be pushed back by social influences, such as working late or going out with others, but that wake time remains strongly guided by biological factors.
“I'd assumed wake time would really be a function of societal effects like alarm clocks,” he says. “But our data support the hypothesis that our biological clocks are governing when we wake up. For instance, we found that people in countries that have a later sunrise sleep in more.” The timing of sunset, meanwhile, may affect the total amount of sleep a person gets.
That hypothesis, however, is at odds with the results of other studies. “All our data and that of other people speak against this, and 85 percent of alarm clock users also demonstrate the opposite,” says Till Roenneberg, a professor at the University of Munich’s Institute of Medical Psychology. Roenneberg's work, in fact, suggests that your alarm clock may be hazardous to your health.
“There are, in my view, no easy answers to scheduling our 24/7 existence, [but] it is painfully clear we are currently damaging the lives of most people at the moment, and more immediate actions are required,” Kelley says. “The fundamental point is that there is wide variation in our individual [biological] timings over 24 hours. [It's] not a one size fits all phenomena.”
On Tuesday, NASA astronaut Scott Kelly and Russian cosmonaut Mikhail Kornienko touched down in Kazakhstan after spending a whopping 340 days aboard the International Space Station (ISS).
As part of NASA’s "Year in Space" project, Kelly and his Earth-bound identical twin brother, retired astronaut Mark Kelly, provided samples of blood, saliva and urine and underwent a barrage of physical and psychological tests designed to study the effects of long-duration spaceflight on the human body.
Studies of identical and fraternal twins have long been used to untangle the influences of genes and the environment on particular traits. Identical twins share all of their genes, while fraternal twins only share 50 percent. If a trait is more common among identical twins than fraternal twins, it suggests genetic factors are partly responsible.
"Twins studies are the only real way of doing natural experiments in humans," says Tim Spector, a professor of genetic epidemiology at Kings College, London. "By studying twins, you can learn a great deal about what makes us tick, what makes us different, and particularly the roles of nature versus nature that you just can't get any other way.”
Spector is director of the TwinsUK Registry, which includes data from 12,000 twins and is used to study the genetic and environmental causes of age-related complex traits and diseases. He estimates that twins research is currently being conducted in more than 100 countries, and that most of those projects draw upon information contained in large databases such as the TwinsUK Registry.
While it may be a while before we see results from the astronaut twins, researchers are hopeful that the opportunity will yield some unique insights into human health. Here are some examples of what we've learned from past twins studies—both famous and infamous:
The Birth of Eugenics
Victorian scientist Francis Galton, a half-cousin of Charles Darwin, was one of the first people to recognize the value of twins for studying the heritability of traits. In an 1875 paper titled "The History of Twins," Galton used twins to estimate the relative effects of nature versus nature (a term that Galton himself coined). But his firm belief that human intelligence is largely a matter of nature led him to a darker path: He became a vocal proponent of eugenics (another term that he coined) and the idea that "a highly gifted race of men" could be produced through selective breeding.
Genes and I.Q.
In 2003, Eric Turkheimer, a psychology professor at the University of Virginia, took a fresh look at the research on the heritability of I.Q., which relied heavily on twin studies. Turkheimer noticed that most of the studies that found I.Q. is largely due to genetics involved twins from middle-class backgrounds, and he wondered what the pattern was among poorer people. When he looked at twins from poor families, he found that the I.Q.s of identical twins varied just as much as the I.Q.s of fraternal twins. In other words, the impact of growing up poor can overwhelm a child's natural intellectual gifts.
Genetic Basis for Everyday Diseases
Working with data and biological samples in the TwinsUK Registry, Spector and his colleagues have shown in more than 600 published papers that many common diseases such as osteoarthritis, cataracts and even back pain have a clear genetic basis to them. "When I started in this field, it was thought that only 'sexy' diseases [such as cancer] were genetic," Spector says. "Our findings changed that perception."
Heritable Eating Disorders
One of the newer twin registries to come online, the Michigan State University Twin Registry (MSUTR) was founded in 2001 to study genetic and environmental influences on a wide range of psychiatric and medical disorders. One of the most surprising findings to come out of the group's research is that many eating disorders such as anorexia have a genetic component to them.
"People thought for the longest time that it was due entirely to culture, the media and social factors,” says MSUTR co-director Kelly Klump. "Because of twins studies, we now know that genes account for the same amount of variability in eating disorders as they do in schizophrenia and bipolar disorder. We would have never known that without twins studies."
The Genetics of Obesity
A classic twin study conducted by geneticist Claude Bouchard in 1990 looked at the importance of genes for body-fat storage. Bouchard, now at Louisiana State University, housed a dozen lean young male twins in a dormitory and overfed them by 1,000 calories a day for three months. Although every participant was heavier by the end of the experiment, the amount of weight and fat gained varied considerably, from 9 pounds to 29 pounds. Weight gain within pairs of twins was much more similar than weight gain between different twin pairs, and the twins in each pair tended to gain weight in the same places, whether it be in the abdomen, buttocks or thighs.
Untangling the "Gay Gene"
Numerous twin studies have attempted to elucidate the importance of genes in sexual orientation. In 2008, researchers led by Niklas Langström, a psychiatrist at the Karolinska Institute in Stockholm, drew upon the treasure trove of twin data contained in the Swedish Twin Registry, the largest in the world, to investigate genetic and environmental influences that determine whether or not a person is gay. The scientists found that genetics accounted for only 35 percent of the differences between identical and fraternal gay men and even less—roughly 18 percent—in gay women.
The study, one of the most comprehensive to date, indicates that a complex interplay of genetics and environmental factors work together to shape people’s sexual orientations. But like other twins studies on this controversial subject, Langström’s study was criticized for possible recruitment bias, since only 12 percent of the males in the Swedish registry were included in the study.
Twins Reared Apart
In 1979, Thomas Bouchard conducted what is perhaps the most fascinating twin study yet. Then director of the Minnesota Center for Twin and Family Research, Bouchard looked at identical and fraternal twins separated in infancy and reared apart. He found that identical twins who had different upbringings often had remarkably similar personalities, interests and attitudes. In one of the most famous examples, Bouchard came across twins who had been separated from birth and reunited at the age of 39.
"The twins," Bouchard later wrote, "were found to have married women named Linda, divorced, and married the second time to women named Betty. One named his son James Allan, the other named his son James Alan, and both named their pet dogs Toy."
But MSUTR's Klump is quick to point out that Bouchard's findings are not proof of genetic determinism. "What they show is that we we enter the world not as random beings or blank slates,” Klump says. “As we walk through life, we have a lot of free choice, but some portion of that free choice is probably based on things that we're really good at and things that we like to do. Bouchard's study tells us that there is a dynamic interplay between what we like, what we want and the environments that we choose."
Bernard Karfiol, New York: American Artists Group, 1945.
Black-and-white study print (8x10).
Orig. negative: 8x10, Safety, BW.