Around the turn of the century, 3.8 million people banded together in a real-time search for aliens — with screensavers. It was a big moment in a century-long concerted search for extraterrestrial intelligence. So far, alien life hasn't been found. But for people like astronomer James Davenport, that doesn't mean the hunt is worthless — or should be given up.

No, according to James, the search is only getting more exciting as new technology opens up a whole new landscape of possibilities. So today on the show: The evolving hunt for alien life.

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Forests are the lungs of our planet. Not only do they absorb carbon dioxide and create oxygen, they also regulate temperature, absorb rainwater to help prevent flooding, and provide critical habitat for the majority of the world's land-based plant and animal species.

But due to land conversion for agriculture and road construction, timber harvesting, and increasing wildfires, the trees that make up our forests are increasingly endangered. That's why we need reforestation: replanting new forests to replace the old ones.

For the past century, the United States Forest Service has helped to replant new forests and manage existing ones. But as scientists face the ongoing threat of climate change, they're developing new strategies... including one that tries to literally predict the future.

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On Wednesday, the Food and Drug Administration announced it is banning the dye called Red No. 3, a food dye additive in many processed foods, like sodas, sweets and snacks. Recently, it and other dyes were linked to behavior issues in children. But high levels of Red No. 3 were linked to cancer in rats decades ago. So why is the ban happening now?

Senior editor and science desk correspondent Maria Godoy answers our questions about Red No. 3 and other dyes that may replace it. Plus, how should parents think about feeding their kids products that may contain Red No. 3 before the ban takes affect.

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Imagine the tree of life. The tip of every branch represents one species, and if you follow any two branches back through time, you'll hit an intersection. If you keep going back in time, you'll eventually find the common ancestor for all of life. That ancestor is called LUCA, the last universal common ancestor, and there is no fossil record to tell us what it looked like.

Luckily, we have Jonathan Lambert. He's a science correspondent for NPR and today he's talking all things LUCA: What we think this single-celled organism may have looked like, when it lived and why a recent study suggests it could be older and more complex than scientists thought.

Have other questions about ancient biology? Email us at [email protected] — we'd love to hear from you!

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In a previous version of this episode, we said that the research team used carbon-dated fossils to calibrate a molecular clock aimed at estimating the age of LUCA. In fact, the researchers used radio isotopic-dated fossils for that purpose.

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When you picture a dinosaur, what does it look like? For Jingmai O'Connor, paleobiologist and associate curator of reptiles at the Field Museum of Chicago, the dinosaurs she studies look a lot more like birds.

"If you looked at an artist's reconstruction of something like Velociraptor or Microraptor ... you would see that it pretty much looks the same as a bird," Jingmai says. "In terms of the plumage, the soft tissues covering the body, it would have looked very, very birdlike."

In this episode, Short Wave delves into the dinosaur-avian connection. Which dinosaurs had feathers? Were they using them to fly? And once and for all – what are those ancient dinosaurs' relationship to birds today?

Have other dinosaur questions you want us to unravel? Email us at [email protected] — we'd love to hear from you!

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The COVID-19 mRNA vaccine generates enough of an antibody response to protect against severe disease for six months. But other vaccines offer years-long — even lifelong — immunity, such as the measles or yellow fever vaccines. Is there a way for scientists to tell how long a person's immunity will last? A team at Stanford Medicine might have found a way to do just that — with the help of some of the cells found in our bone marrow.

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Air quality in the Los Angeles region has plummeted due to smoke from the ongoing wildfires. With all that smoke comes possible risks to human health. So what actually is smoke and why is it so harmful? Jessica Gilman, an atmospheric chemist with the National Oceanic and Atmospheric Administration, explains what smoke is made of, how it behaves in the atmosphere and smoke's role in climate change. Plus, tips for how to lessen your exposure.

Check out the CDC's recommendations for avoiding smoke inhalation here. Read more of NPR's coverage of the fires.

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Over 29,000 acres in the greater Los Angeles area are on fire right now. The fires emerged after the Santa Ana winds swept into the Los Angeles area Tuesday. The largest is the Palisades fire, which is quickly burning through the Pacific Palisades and Malibu communities. It's one of four ongoing critical fires — only one of which is partially contained. Ahead of the windy week, a Red Flag Warning was issued for an increased fire risk due to the strong winds, low humidity and higher temperature. Today, we dig into the Santa Ana winds: What they are and how they combined with other factors to create conditions for the most destructive fire in Los Angeles' history.

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Fluoridating the public water supply has been common practice for nearly 80 years in the U.S. It's an acclaimed public health intervention that helps prevent cavities. For just as long, some have raised concerns about the practice that can veer from evidence-based to unsubstantiated conspiracy. An analysis by government researchers, published Monday in JAMA Pediatrics, is adding to the debate. The research found that exposing babies and kids to high levels of fluoride might be associated with neurodevelopmental harm. Frankly, it's a lot to digest — so we invited health correspondent Pien Huang onto the show to wade through the debate.

Questions, story ideas or want us to dig into another public health debate? Email us at [email protected] — we'd love to hear from you!

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Some weapons used by the U.S. military are so powerful they can pose a threat to the people who fire them. Today, we meet two Marines, William Wilcox and Michael Lozano, who spent years firing missiles and rockets, then developed the same rare brain condition: arteriovenous malformation, or AVM. The condition sends high pressure blood from a tangle of abnormal blood vessels directly into fragile veins, which can leak or burst. Most AVMs are caused by genetic changes that affect the growth of blood vessels, so the connection between weapon blasts and AVM isn't always immediately clear. But NPR's brain correspondent Jon Hamilton reports that recent research suggests that blast waves can alter genes in the brain — and that the evidence is even stronger for less extreme blood vessel changes.

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There are hundreds of atomic clocks in orbit right now, perched on satellites all over Earth. We depend on them for GPS location, Internet timing, stock trading ... and space navigation?

Today on the show, hosts Emily Kwong and Regina G. Barber learn how to build a better clock. In order to do that, they ask: How do atomic clocks really work, anyway? What makes a clock precise? And how could that process be improved for even greater accuracy?

For more about Holly's Optical Atomic Strontium Ion Clock, check out the OASIC project on NASA's website.
For more about the Longitude Problem, check out Dava Sobel's book, Longitude.

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