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In 1977, a University of Oxford statistician named Richard Peto pointed out a simple yet puzzling biological fact: We humans should have a lot more cancer than mice, but we don't. Dr. Peto's argument was beguilingly simple. Every time a cell divides, there's a small chance it will gain a mutation that speeds its growth. Cells that accumulate several of these mutations may become cancerous. The bigger an animal is, the more cells it has, and the longer an animal lives, the more times its cells divide. We humans undergo about 10,000 times as many cell divisions as mice - and thus should be far more likely to get cancer.

The Ebola epidemic that tore through West Africa in 2014 claimed 11,310 lives, far more than any previous outbreak. A combination of factors contributed to its savagery, among them a mobile population, crumbling public health systems, official neglect and hazardous burial practices. But new research suggests another impetus: The virus may have evolved a new weapon against its human hosts. In studies published on Thursday in the journal Cell, two teams of scientists report that a genetic mutation may have made Ebola more deadly by improving the virus's ability to enter human cells.

For the first time, biologists have succeeded in growing human stem cells in pig embryos, shifting from science fiction to the realm of the possible the idea of developing human organs in animals for later transplant.

But those measures did little to curb the smog the first time. Toxic air cloaked Beijing until strong winds blew the pollutants south. The main source of foul air in the capital comes from coal-burning factories in surrounding provinces, not from activity in the city itself, and cities in those provinces did not take the same drastic emergency measures that Beijing did. Many parts of northern China, including the provinces of Hebei, Henan and Shanxi, regularly have air quality that is worse than that of Beijing.

Could it be that Alzheimer's disease stems from the toxic remnants of the brain's attempt to fight off infection? Provocative new research by a team of investigators at Harvard leads to this startling hypothesis, which could explain the origins of plaque, the mysterious hard little balls that pockmark the brains of people with Alzheimer's.

A new study, published on Wednesday in Molecular Biology and Evolution, identifies gene variants in Inuit who live in Greenland, which may help them adapt to the cold by promoting heat-generating body fat. These variants possibly originated in the Denisovans, a group of archaic humans who, along with Neanderthals, diverged from modern humans about half a million years ago.

Children from an Amish community in Indiana had an even lower prevalence of allergies than European farmers, making them among the least allergic subgroup ever measured in the developed world.

Microsoft scientists have demonstrated that by analyzing large samples of search engine queries they may in some cases be able to identify internet users who are suffering from pancreatic cancer, even before they have received a diagnosis of the disease.

The threat of antibiotic resistance has become so dire that the United Nations General Assembly is holding a meeting to discuss it this month in New York City. Although WHO has been sounding the alarm on antibiotic resistance for years, this month's high-level U.N. meeting represents only the fourth time in the international body's history that its General Assembly-a global deliberative body that primarily grapples with issues like war and economics-has held a meeting to tackle a health topic.

High on Bolivia's semiarid Andean plains at more than 12,000 feet and subject to climatic whims, the shallow saline lake has dried up before only to rebound to a size twice the area of Los Angeles. But recovery may no longer be possible, scientists say. "This is a picture of the future of climate change," said Dirk Hoffman, a German glaciologist who studies how rising temperatures from the burning of fossil fuels has accelerated glacial melting in Bolivia.

Scientists have genetically engineered monkeys so that they exhibit behaviors similar to autism, with a goal of testing potential therapies on the animals in hopes that their resemblance to humans will yield more answers about the disorder. The scientists found that the monkeys showed "very similar behaviors related to human autism patients, including repetitive behaviors, increased anxiety and, most importantly, defects in social interactions," said Zilong Qiu, a leader of the research at the Institute of Neuroscience at the Chinese Academy of Sciences in Shanghai. The team is now imaging the brains of the monkeys, he said, "trying to identify the deficiency in the brain circuits that is responsible for the autism-like behavior."

Federal regulators on Thursday approved a genetically engineered salmon as fit for consumption, making it the first genetically altered animal to be cleared for American supermarkets and dinner tables. The approval by the Food and Drug Administration caps a long struggle for AquaBounty Technologies, a small company that first approached the F.D.A. about approval in the 1990s. The agency made its initial determination that the fish would be safe to eat and for the environment more than five years ago.

Perhaps the biggest misconception is that as long as you lose weight, it doesn't matter what you eat. But it does. Yet being thin and being healthy are not at all the same thing. Being overweight is not necessarily linked with disease or premature death. What you eat affects which diseases you may develop, regardless of whether you're thin or fat.

A team of scientists unveiled a new tree of life on Monday, a diagram outlining the evolution of all living things. The researchers found that bacteria make up most of life's branches. And they found that much of that diversity has been waiting in plain sight to be discovered, dwelling in river mud and meadow soils.

The uproar over the new ease and precision with which scientists can manipulate the DNA of living things has centered largely on the complicated prospect of editing human embryos. But with the federal government's approval last week of a fast-growing salmon as the first genetically altered animal Americans can eat, a menagerie of gene-edited animals is already being raised on farms and in laboratories around the world - some designed for food, some to fight disease, some, perhaps, as pets.

In a striking advance that helps open the door to organ transplants from animals, researchers have created gene-edited piglets cleansed of viruses that might cause disease in humans.

Microsoft scientists have demonstrated that by analyzing large samples of search engine queries they may in some cases be able to identify internet users who are suffering from pancreatic cancer, even before they have received a diagnosis of the disease.

I'd come that afternoon to witness what has become a rarity: the non-forensic medical autopsy. "Autopsy" means to see for oneself, and that is exactly what doctors once did. Fifty years ago we performed autopsies on roughly half of patients who died in hospitals. Today, the number of autopsies has dwindled to less than 10 percent, with next to none in nonacademic hospitals.

But then on Dec. 22, 1938, Marjorie Courtenay-Latimer came across a strange blue fin poking out of a pile of fish that struck her as extraordinary. With its fleshy, lobed fins and its tough armored scales, the specimen looked very unlike fish we see in our oceans today. That is because the coelacanth has managed to survive in roughly its current form for hundreds of millions of years. In the course of researching this film, we learned all kinds of amazing facts about the coelacanth. For instance, unlike most fish, they give birth to live young. They produce eggs the size of grapefruits, which then hatch internally. From video footage taken by the explorer Hans Fricke - which we used as inspiration for our sets and puppets - we know coelacanths are prone to some odd behavior. They've been spotted doing headstands underwater.

The relative importance of nature and nurture has been debated for centuries, and has had strong - and sometimes misguided - influences on public policy.