Today in Science: How many people have lived on Earth?

A newsletter for unflinching, ever-curious science-lovers. ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­

October 3, 2025—We've crossed another planetary boundary, some galaxies are moving faster than the speed of light, and mathematicians calculate the total number of humans that have ever lived. —Andrea Gawrylewski, Chief Newsletter Editor TODAY'S NEWS Dmytro Varavin/iStock/Getty Images How many people have ever lived on Earth? Mathematicians have used different techniques to estimate. | 6 min read If you liked the "unknot" puzzles yesterday, here are more knotty mysteries that have emerged in the burgeoning field of knot theory. | 5 min read Jane Goodall challenged what it meant to be a scientist in three big ways. | 3 min read  Adidas officially unveiled the 2026 FIFA World Cup's new Trionda ball. Here's the mathematics behind the Trionda ball's design. | 5 min read More News TOP STORIES Boundaries Breached In a new report, researchers examined nine geophysical limits that make up a sort of planetary life-support system; staying within these limits, they say, is the best hope for maintaining the climatic conditions humans and other organisms on Earth have adapted to. As of 2025, humans have pushed Earth past another of these planetary boundaries: Levels of ocean acidification have exceeded a critical threshold, becoming the seventh out of nine boundaries crossed.   How it works: Carbon dioxide concentrations reached a record global high of 422.7 parts per million (ppm) last year. Much of that carbon dioxide gets absorbed by the ocean, increasing its acidity, which can have profound impacts on marine ecosystems. At low enough pH levels, corals and shells can begin to dissolve. These effects could destabilize entire ecosystems and devastate many commercially valuable species, such as oysters.    What the experts say: "The movement we're seeing is absolutely headed in the wrong direction. The ocean is becoming more acidic, oxygen levels are dropping, and marine heatwaves are increasing. This is ramping up pressure on a system vital to stabilize conditions on planet Earth," Levke Caesar, co-lead of PIK's Planetary Boundaries Science Lab, said in the new evaluation's press statement. Amanda Montañez; Source: "Planetary Health Check 2025: A Scientific Assessment of the State of the Planet," Planetary Boundaries Science, Potsdam Institute for Climate Impact Research (data). The most distant galaxies in this deep field image from the James Webb Space Telescope appear as small, faint dots. ESA/Webb, NASA & CSA, G. Östlin, P. G. Perez-Gonzalez, J. Melinder, the JADES Collaboration, M. Zamani (ESA/Webb), CC BY 4.0 INT Faster-Than-Light Galaxies Nothing can move quicker through space than the speed of light. The distinciton is important. The universe—space itself— is constantly expanding, but the rate of expansion grows the farther away from us you measure. A galaxy one megaparsec from us (about 3.26 million light-years) will be receding at 70 km/sec. A galaxy two megaparsecs away will be moving twice as fast, or at 140 km/sec, and so on. So at a certain point, a galaxy will be moving away from us at the speed of light. Calculations show that this distance, which is called the Hubble sphere, is about 14 billion light years away. Anything farther away would be moving faster than light from our perspective.   How it works: Though these galaxies are moving quicker from us than the speed of light, they are not moving through space faster than light. They are moving with it, writes Phil Plait in his weekly column. He gives an analogy: "Imagine a boat on the ocean that can move across the water at 20 km/hour. If the boat is headed away from you, that's how fast you'll see it moving. But now imagine the boat's in a current moving at 30 km/hour away from you. You'd now measure the boat moving at 50 km/hour, even though the speed of the boat relative to the water is only 20. To be clear, this is only an analogy and shouldn't be taken too far. But it helps to picture how this works."   What this means: The cosmos was born approximately 13.8 billion years ago. Hundreds of millions of years later, galaxies formed. Light from distant galaxies has taken about 12 billion years to reach us, but over that time, the universe has expanded. Technically, the light has traveled much farther than 12-billion-light-years to reach us. By the time it reaches us, the galaxy is more like 23-billion light-years away. —Andrea Tamayo, Newsletter Writer

Explore the universe and feel the awe of science. Subscribe to Scientific American with a special discount for Today in Science readers.   PLAY NOW See how well you read Scientific American this week by taking today's quiz! Also, try to solve Spellements and our killer version of Sudoku. This week, Bruce F. and Amir C. both found the word modally in the Spellements puzzle. It's an adverb relating to modality. Excellent word. More Games MOST POPULAR STORIES OF THE WEEK Marsh Will-o'-the-Wisps Sparked by Strange Chemistry | 3 min read   Smallmouth Bass Evolve to Evade Electric Culling in Adirondack Lake | 2 min read People Are More Likely to Cheat When They Use AI | 4 min read Thanks for reading Today in Science this week. We're barreling past troubling markers of the planet's health, and the stakes can feel overwhelming. But it gives me real comfort to be part of this community of science-minded readers—curious, informed, and unwilling to look away. If change is going to come, I suspect it will begin with people like you. Wishing you a restful weekend. You can always email me: newsletters@sciam.com. See you Monday.   —Andrea Gawrylewski, Chief Newsletter Editor With contributions by Andrea Tamayo

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Week in Science: Jane Goodall's scientific legacy

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October 3—This week, people are more comfortable with cheating when they delegate tasks to AI, Saturn's moon has life-friendly chemistry, and the beautiful legacy of primatologist Jane Goodall. All that and more below! —Andrea Gawrylewski,  Chief Newsletter Editor

Top Stories

'Ghost Fire' in Marshes Sparked by Strange Chemistry A phenomenon called microlightning may explain ghostly blue marsh lights Rachel Nuwer

Sponsor Content Provided by Archaeological Paths An Evening with President Lech Walesa Meet the man who defended freedom and became one of the world's greatest leaders. Lech Walesa, the living legend who pushed the world toward democracy, is coming to the U.S. and Canada on his first-ever Grand Lecture Tour. Register now!

3 Ways Jane Goodall Challenged What It Means to Be a Scientist Here are three big ways that Jane Goodall transformed science Rachel Fieldhouse, Mohana Basu, Nature magazine

People Are More Likely to Cheat When They Use AI Participants in a new study were more likely to cheat when delegating to AI—especially if they could encourage machines to break rules without explicitly asking for it Rachel Nuwer

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NASA's First-Ever Alien-Hunting Space Telescope Could Enlighten Our New Dark Age The Habitable Worlds Observatory is poised to tell us whether Earth-like planets are common—if it can get off the ground Nadia Drake, Lee Billings

How Many People Have Ever Lived on Earth? Is it really possible that half of all people who have ever been age 65 or older are still alive today? We explore the amazing mathematics of demography to find out Manon Bischoff

Mildly Menacing Mating Calls Lead to Discovery of New Gecko Species Scientists found new gecko species hidden in plain sight in pristine deserts of southern Africa, thanks to their loud, barking mating calls Ashley Balzer Vigil

Saturn's Moon Enceladus Has Complex, Life-Friendly Chemistry A fresh analysis of old data has found rich organic chemistry within the hidden ocean of Saturn's moon Enceladus Jacek Krywko

Rock Art Discovery Reveals Unknown Arabian Nomads from 12,000 Years Ago Camels in ancient Arabia may have led hunter-gatherers through deserts once thought uninhabitable Martin J. Kernan

How Scientists Finally Found a Treatment That Slows Huntington's Disease After years of heartbreak, researchers have found an experimental treatment that can slow the progression of Huntington's disease, according to early results from a small clinical trial Allison Parshall

Why It's Dangerous to Avoid Tylenol While Pregnant Untreated fevers during pregnancy can cause more harm than taking acetaminophen will Rachel Fieldhouse, Nature magazine

Meet the Microbes That Munch Mountains of Mining Waste Biomining uses engineered microbes to harvest critical minerals Vanessa Bates Ramirez

Lab-Made Mini Organs Could Transform Female Reproductive Medicine Artificial tissues that mimic the placenta, endometrium, ovary and vagina could point to treatments for common conditions such as preeclampsia and endometriosis Cassandra Willyard, Nature magazine

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