How meningitis-causing bacteria invade the brain

Bacteria can slip into the brain by commandeering cells in the brain’s protective layers, a new study finds. The results hint at how a deadly infection called bacterial meningitis takes hold.

In mice infected with meningitis-causing bacteria, the microbes exploit previously unknown communication between pain-sensing nerve cells and immune cells to slip by the brain’s defenses, researchers report March 1 in Nature. The results also hint at a new way to possibly delay the invasion — using migraine medicines to interrupt those cell-to-cell conversations.
Bacterial meningitis is an infection of the protective layers, or meninges, of the brain that affects 2.5 million people globally per year. It can cause severe headaches and sometimes lasting neurological injury or death.

“Unexpectedly, pain fibers are actually hijacked by the bacteria as they’re trying to invade the brain,” says Isaac Chiu, an immunologist at Harvard Medical School in Boston. Normally, one might expect pain to be a warning system for us to shut down the bacteria in some way, he says. “We found the opposite…. This [pain] signal is being used by the bacteria for an advantage.”

It’s known that pain-sensing neurons and immune cells coexist in the meninges, particularly in the outermost layer called the dura mater (SN: 11/11/20). So to see what role the pain and immune cells play in bacterial meningitis, Chiu’s team infected mice with two of the bacteria known to cause the infection in humans: Streptococcus pneumoniae and S. agalactiae. The researchers then observed where that bacteria ended up in mice genetically tweaked to lack pain-sensing nerve cells and compared those resting spots to those in mice with the nerve cells intact.

Mice without pain-sensing neurons had fewer bacteria in the meninges and brain than those with the nerve cells, the team found. This contradicts the idea that pain in meningitis serves as a warning signal to the body’s immune system, mobilizing it for action.

Further tests showed that the bacteria triggered a chain of immune-suppressing events, starting with the microbes secreting toxins in the dura mater.

The toxins hitched onto the pain neurons, which in turn released a molecule called CGRP. This molecule is already known to bind to a receptor on immune cells, where it helps control the dura mater’s immune responses. Injecting infected mice with more CGRP lowered the number of dural immune cells and helped the infection along, the researchers found.

The team also looked more closely at the receptor that CGRP binds to. In infected mice bred without the receptor, fewer bacteria made it into the brain. But in ones with the receptor, immune cells that would otherwise engulf bacteria and recruit reinforcements were disabled.
The findings suggest that either preventing the release of CGRP or preventing it from binding to immune cells might help delay infection.

In humans, neuroscientists know that CGRP is a driver of headaches — it’s already a target of migraine medications (SN: 6/5/18). So the researchers gave five mice the migraine medication olcegepant, which blocks CGRP’s effects, and infected them with S. pneumoniae. After infection, the medicated mice had less bacteria in the meninges and brain, took longer to show symptoms, didn’t lose as much weight and survived longer than mice that were not given the medication.

The finding suggests olcegepant slowed the infection. Even though it only bought mice a few extra hours, that’s crucial in meningitis, which can develop just as quickly. Were olcegepant to work the same way in humans, it might give doctors more time to treat meningitis. But the effect is probably not as dramatic in people, cautions Michael Wilson, a neurologist at the University of California, San Francisco who wasn’t involved with the work.

Scientists still need to determine whether pain-sensing nerve cells and immune cells have the same rapport in human dura mater, and whether migraine drugs could help treat bacterial meningitis in people.

Neurologist Avindra Nath has doubts. Clinicians think the immune response and inflammation damage the brain during meningitis, says Nath, who heads the team investigating nervous system infections at the National Institute of Neurological Disorders and Stroke in Bethesda, Md. So treatment involves drugs that suppress the immune response, rather than enhance it as migraine medications might.

Chiu acknowledges this but notes there might be room for both approaches. If dural mater immune cells could head the infection off at the pass, it may keep some bacteria from penetrating the defenses, minimizing brain inflammation.

This study might not ultimately change how clinicians treat patients, Wilson says. But it still reveals something new about one of the first lines of defense for the brain.

The Milky Way may be spawning many more stars than astronomers had thought

The Milky Way is churning out far more stars than previously thought, according to a new estimate of its star formation rate.

Gamma rays from aluminum-26, a radioactive isotope that arises primarily from massive stars, reveal that the Milky Way converts four to eight solar masses of interstellar gas and dust into new stars each year, researchers report in work submitted to arXiv.org on January 24. That range is two to four times the conventional estimate and corresponds to an annual birthrate in our galaxy of about 10 to 20 stars, because most stars are less massive than the sun.
At this rate, every million years — a blink of the eye in astronomical terms — our galaxy spawns 10 million to 20 million new stars. That’s enough to fill roughly 10,000 star clusters like the beautiful Pleiades cluster in the constellation Taurus. In contrast, many galaxies, including most of the ones that orbit the Milky Way, make no new stars at all.

“The star formation rate is very important to understand for galaxy evolution,” says Thomas Siegert, an astrophysicist at the University of Würzburg in Germany. The more stars a galaxy makes, the faster it enriches itself with oxygen, iron and the other elements that stars create. Those elements then alter star-making gas clouds and can change the relative number of large and small stars that the gas clouds form.

Siegert and his colleagues studied the observed intensity and spatial distribution of emission from aluminum-26 in our galaxy. A massive star creates this isotope during both life and death. During its life, the star blows the aluminum into space via a strong wind. If the star explodes when it dies, the resulting supernova forges more. The isotope, with a half-life of 700,000 years, decays and gives off gamma rays.

Like X-rays, and unlike visible light, gamma rays penetrate the dust that cloaks the youngest stars. “We’re looking through the entire galaxy,” Siegert says. “We’re not X-raying it; here we’re gamma-raying it.”

The more stars our galaxy spawns, the more gamma rays emerge. The best match with the observations, the researchers find, is a star formation rate of four to eight solar masses a year. That is much higher than the standard estimate for the Milky Way of about two solar masses a year.

The revised rate is very realistic, says Pavel Kroupa, an astronomer at the University of Bonn in Germany who was not involved in the work. “I’m very impressed by the detailed modeling of how they account for the star formation process,” he says. “It’s a very beautiful work. I can see some ways of improving it, but this is really a major step in the absolutely correct direction.”

Siegert cautions that it is difficult to tell how far the gamma rays have traveled before reaching us. In particular, if some of the observed emission arises nearby — within just a few hundred light-years of us — then the galaxy has less aluminum-26 than the researchers have calculated, which means the star formation rate is on the lower side of the new estimate. Still, he says it’s unlikely to be as low as the standard two solar masses per year.
In any event, the Milky Way is the most vigorous star creator in a collection of more than 100 nearby galaxies called the Local Group. The largest Local Group galaxy, Andromeda, converts only a fraction of a solar mass of gas and dust into new stars a year. Among Local Group galaxies, the Milky Way ranks second in size, but its high star formation rate means that we definitely try a lot harder.

Psychedelics may improve mental health by getting inside nerve cells

Psychedelics go beneath the cell surface to unleash their potentially therapeutic effects.

These drugs are showing promise in clinical trials as treatments for mental health disorders (SN: 12/3/21). Now, scientists might know why. These substances can get inside nerve cells in the cortex — the brain region important for consciousness — and tell the neurons to grow, researchers report in the Feb. 17 Science.

Several mental health conditions, including depression and post-traumatic stress disorder, are tied to chronic stress, which degrades neurons in the cortex over time. Scientists have long thought that repairing the cells could provide therapeutic benefits, like lowered anxiety and improved mood.
Psychedelics — including psilocin, which comes from magic mushrooms, and LSD — do that repairing by promoting the growth of nerve cell branches that receive information, called dendrites (SN: 11/17/20). The behavior might explain the drugs’ positive outcomes in research. But how they trigger cell growth was a mystery.

It was already known that, in cortical neurons, psychedelics activate a certain protein that receives signals and gives instructions to cells. This protein, called the 5-HT2A receptor, is also stimulated by serotonin, a chemical made by the body and implicated in mood. But a study in 2018 determined that serotonin doesn’t make these neurons grow. That finding “was really leaving us scratching our heads,” says chemical neuroscientist David Olson, director of the Institute for Psychedelics and Neurotherapeutics at the University of California, Davis.

To figure out why these two types of chemicals affect neurons differently, Olson and colleagues tweaked some substances to change how well they activated the receptor. But those better equipped to turn it on didn’t make neurons grow. Instead, the team noticed that “greasy” substances, like LSD, that easily pass through cells’ fatty outer layers resulted in neurons branching out.

Polar chemicals such as serotonin, which have unevenly distributed electrical charges and therefore can’t get into cells, didn’t induce growth. Further experiments showed that most cortical neurons’ 5-HT2A receptors are located inside the cell, not at the surface where scientists have mainly studied them.

But once serotonin gained access to the cortical neurons’ interior — via artificially added gateways in the cell surface — it too led to growth. It also induced antidepressant-like effects in mice. A day after receiving a surge in serotonin, animals whose brain cells contained unnatural entry points didn’t give up as quickly as normal mice when forced to swim. In this test, the longer the mice tread water, the more effective an antidepressant is predicted to be, showing that inside access to 5-HT2A receptors is key for possible therapeutic effects.

“It seems to overturn a lot about what we think should be true about how these drugs work,” says neuroscientist Alex Kwan of Cornell University, who was not involved in the study. “Everybody, including myself, thought that [psychedelics] act on receptors that are on the cell surface.”
That’s where most receptors that function like 5-HT2A are found, says biochemist Javier González-Maeso of the Virginia Commonwealth University in Richmond, who was also not involved in the work.

Because serotonin can’t reach 5-HT2A receptors inside typical cortical neurons, Olson proposes that the receptors might respond to a different chemical made by the body. “If it’s there, it must have some kind of role,” he says. DMT, for example, is a naturally occurring psychedelic made by plants and animals, including humans, and can reach a cell’s interior.

Kwan disagrees. “It’s interesting that psychedelics can act on them, but I don’t know if the brain necessarily needs to use them when performing its normal function.” Instead, he suggests that the internal receptors might be a reserve pool, ready to replace those that get degraded on the cell surface.

Either way, understanding the cellular mechanisms behind psychedelics’ potential therapeutic effects could help scientists develop safer and more effective treatments for mental health disorders.

“Ultimately, I hope this leads to better medicines,” Olson says.

Hominids used stone tool kits to butcher animals earlier than once thought

Nearly 3 million years ago, hominids employed stone tool kits to butcher hippos and pound plants along what’s now the shores of Kenya’s Lake Victoria, researchers say.

Evidence of those food preparation activities pushes back hominids’ use of these tool kits, known as Oldowan implements, by roughly 300,000 years, say paleoanthropologist Thomas Plummer of Queen’s College, City University of New York and colleagues. That makes these finds possibly the oldest known stone tools.

Several dating techniques place discoveries at the Kenyan site, known as Nyayanga, at between around 2.6 million and 3 million years old. Based on where artifacts lay in dated sediment layers, these finds are probably close to about 2.9 million years old, the scientists report in the Feb. 10 Science.
Until now, the oldest Oldowan tools dated to roughly 2.6 million years ago at an Ethiopian site more than 1,200 kilometers north of Nyayanga (SN: 6/3/19). Excavations at another site in Kenya, called Lomekwi 3, have yielded large, irregularly shaped rocks dating to about 3.3 million years ago (SN: 5/20/15). But claims that these finds, which include some sharp edges, represent the oldest known stone tools are controversial.

Similarities of the Nyayanga artifacts to those found at sites dating to as late as around 1.7 million years ago “reinforce the long trajectory of Oldowan technology in the early stages of human evolution,” says archaeologist Manuel Domínguez-Rodrigo of Rice University in Houston and the University of Alcalá in Madrid. He did not participate in the new study.

Skeletal remains of at least three hippos unearthed near a total of 56 stone artifacts at Nyayanga display butchery marks, the investigators say. Wear patterns on another 30 stone tools from Nyayanga indicate that these items were used to cut, scrape and pound animal tissue and a variety of plants. And antelope fossils found at Nyayanga display damage from hominids removing meat with sharp stones and crushing bones with large stones to remove marrow.
These discoveries are among 330 Oldowan artifacts and 1,776 animal bones unearthed at Nyayanga from 2015 through 2017. Oldowan finds included three parts of an ancient tool kit — rounded hammerstones, angular or oval cores and sharp-edged flakes. Toolmakers struck a core held in one hand with a hammerstone held in the other hand, splitting off flakes that could be used to cut or scrape.

Whoever wielded stone tools at the Kenya site close to 3 million years ago “had access to a well-balanced diet for hunter-gatherers,” says coauthor Rick Potts, a paleoanthropologist at the Smithsonian Institution in Washington, D.C.
The evolutionary identity of ancient Nyayanga toolmakers remains a mystery. Plummer’s group unearthed two large, peg-shaped molars belonging to Paranthropus, a big-jawed, small-brained hominid line that inhabited eastern and southern parts of Africa until around 1 million years ago. The Nyayanga teeth are the oldest known Paranthropus fossils.

But there is no way to confirm that Paranthropus made and used the newly recovered stone tools. Individuals who died at Nyayanga and left behind their fossilized teeth were not necessarily part of groups that periodically butchered hippos there, Domínguez-Rodrigo says.

Members of the Homo genus appeared in East Africa as early as around 2.8 million years ago and could have made Oldowan tools at Nyayanga, says archaeologist Sileshi Semaw of the National Research Center for Human Evolution in Burgos, Spain (SN: 3/4/15). But Paranthropus can’t be discounted as a toolmaker. A large male Paranthropus skull discovered in 1959, dubbed Nutcracker Man, lay near Oldowan artifacts dated to 1.89 million years ago, says Semaw, who was not part of Plummer’s group (SN: 3/3/20).

Previous discoveries indicated that Oldowan toolmakers eventually occupied much of Africa, Asia and Europe, either via the spread of toolmaking groups or through independent inventions.

Discoveries at Nyayanga fit a current consensus that stone-tool making must have begun shortly after hominids evolved substantially smaller canine teeth around 5 million years ago, says archaeologist John Shea of Stony Brook University in New York, who was not involved in the new study. Stone tools did the work formerly performed by big canines, including slicing prey carcasses, mashing edible plants and helping individuals communicate anger or dominance over others, Shea suspects.

If that tool-crafting timeline is correct, then even Australopithecus afarensis, known for Lucy’s famous partial skeleton, might have made and used stone tools by around 3.4 million years ago (SN: 8/11/10).

Any way you slice it, Oldowan finds at Nyayanga now provide the earliest hard evidence of stone tools.

NASA’s exoplanet count surges past 5,000

It’s official: The number of planets known beyond our solar system has just passed 5,000.

The exoplanet census surpassed this milestone with a recent batch of 60 confirmed exoplanets. These additional worlds were found in data from NASA’s now-defunct K2 mission, the “second life” of the prolific Kepler space telescope, and confirmed with new observations, researchers report March 4 at arXiv.org.

As of March 21, these finds put NASA’s official tally of exoplanets at 5,005.

It’s been 30 years since scientists discovered the first planets orbiting another star — an unlikely pair of small worlds huddled around a pulsar (SN: 1/11/92). Today, exoplanets are so common that astronomers expect most stars host at least one (SN: 1/11/12), says astronomer Aurora Kesseli of Caltech.
“One of the most exciting things that I think has happened in the last 30 years is that we’ve really started to be able to fill out the diversity of exoplanets,” Kesseli says

Some look like Jupiter, some look — perhaps — like Earth and some look like nothing familiar. The 5,005 confirmed exoplanets include nearly 1,500 giant gassy planets, roughly 200 that are small and rocky and almost 1,600 “super-Earths,” which are larger than our solar system’s rocky planets and smaller than Neptune (SN: 8/11/15).
Astronomers can’t say much about those worlds beyond diameters, masses and densities. But several projects, like the James Webb Space Telescope, are working on that, Kesseli says (SN: 1/24/22). “Not only are we going to find tons and tons more exoplanets, but we’re also going to start to be able to actually characterize the planets,” she says.

And the search is far from over. NASA’s newest exoplanet hunter, the TESS mission, has confirmed more than 200 planets, with thousands more yet to verify, Kesseli says (SN: 12/2/21). Ongoing searches from ground-based telescopes keep adding to the count as well.

“There’s tons of exoplanets out there,” Kesseli says, “and even more waiting to be discovered.”

Levitating plastic beads mimic the physics of spinning asteroids

Some asteroids can barely hold it together.

Rather than solid lumps of rock, ‘rubble pile’ asteroids are loose collections of material, which can split apart as they rotate (SN: 3/16/20). To understand the inner workings of such asteroids, one team of scientists turned to levitating plastic beads. The beads clump together, forming collections that can spin and break up, physicist Melody Lim of the University of Chicago reported March 15 at a meeting of the American Physical Society in Chicago.

It’s an elegant dance that mimics the physics of asteroid formation, which happens too slowly to observe in real-life space rocks. “These ‘tabletop asteroids’ compress phenomena that take place over kilometers [and] over hundreds of thousands of years to just centimeters and seconds in the lab,” Lim said. The results are also reported in a paper accepted in Physical Review X.
Lim and colleagues used sound waves to levitate the plastic beads, which arranged themselves into two-dimensional clumps. Acoustic forces attract the beads to one another, mimicking the gravitational attraction between bits of debris in space. Separate clumps then coalesced similarly to how asteroids are thought to glom onto one another to grow.
When the experimenters gave the structures a spin using the sound waves, the clumps changed shape above a certain speed, becoming elongated. That could help scientists understand why ‘rubble pile’ asteroids, can have odd structures, such as the ‘spinning tops’ formed by asteroids Bennu and Ryugu (SN: 12/18/18).

Eventually, the fast-spinning clumps broke apart. This observation could help explain why asteroids are typically seen to spin up to a certain rate, but not beyond: Speed demons get split up.

Social media crackdowns during the war in Ukraine make the internet less global

Since Russia’s invasion of Ukraine in late February, people around the world have watched the war play out in jarring detail — at least, in countries with open access to social media platforms such as Twitter, Facebook, TikTok and the messaging app Telegram.

“The way that social media has brought the war into the living rooms of people is quite astounding,” says Joan Donovan, the research director of the Shorenstein Center on Media, Politics and Public Policy at Harvard University. Fighting and explosions play out nearly in real time, and video messages from embattled Ukrainian president Volodymyr Zelenskyy have stirred support across the West.

But that’s not all. Social media is actually changing the way wars are fought today, says political scientist Thomas Zeitzoff of American University in Washington, D.C., who is an expert on political violence.
The platforms have become important places to recruit fighters, organize action, spread news and propaganda and — for social scientists — to gather data on conflicts as they unfold.

As social platforms have become more powerful, governments and politicians have stepped up efforts to use them — or ban them, as in Russia’s recent blocking of Facebook, Twitter and Instagram. And in a first, the White House held a special briefing on the Ukraine war with TikTok stars such as 18-year-old Ellie Zeiler, who has more than 10 million followers. The administration hopes to shape the messages of young influencers who are already important sources of news and information for their audiences.

The Ukraine war is shining a spotlight on social media’s role as a political tool, says Donovan, whose Technology and Social Change Project team has been following the spread of disinformation in the conflict. “This is a huge moment in internet history where we’re starting to see the power of these tech companies play out against the power of the state.” And that, she says, “is actually going to change the internet forever.”

Science News interviewed Donovan and Zeitzoff about social media’s influence on the conflict and vice versa. The following conversations have been edited for length and clarity.

SN: When did social media start to play a role in conflicts?

Zeitzoff: Some people would say the Zapatista uprising in Mexico, way back in the 1990s, because the Zapatistas used the internet [to spread their political message]. But I think the failed Green Revolution in Iran in 2007 and 2008 was one of the first, and especially the Arab Spring in the early 2010s. There was this idea that social media would be a “liberation technology” that allows people to hold truth to power.

But as the Arab Spring gave way to the Arab Winter [and its resurgence of authoritarianism], people started challenging that notion. Yes, it makes it easy to get a bunch of people out on the street [to protest], but it also makes it easier for governments to track these folks.
SN: How do you see social media being used in the Ukrainian conflict, and what’s different now?
Donovan: Some of the platforms that are more well-known, like Facebook and Twitter, are not as consequential as newer platforms like Telegram and TikTok. For instance, Ukrainian groups on Facebook started to build other channels for communication right before the Russian invasion because they felt that Facebook might get compromised. So Telegram has been a very important space for getting information and sharing news.

Telegram has also become a hot zone for propaganda and misinformation, where newer tactics are emerging such as fake debunked videos. These are videos that look like they’re news debunks showing that Ukraine is participating in media manipulation efforts, but they’re actually manufactured by Russia to make Ukraine look bad.

Zeitzoff: I think social media has probably afforded the Ukrainians an easier ability to communicate to their diaspora communities, whether in Canada, the United States or across Europe. It’s also increasingly affording unprecedented battlefield views.

But I think the bigger thing is to think about what these new suites of technology allow, like Volodymyr Zelenskyy holding live videos that basically allow him to show proof of life, and also put pressure on European leaders.

SN: Despite Russia’s big investments in disinformation, is Ukraine winning the social media war?

Zeitzoff: Up to the beginning of the conflict, many Ukrainians were skeptical of Zelenskyy’s ability to lead. But you look back at his presidential campaign, and he was doing Facebook videos where he would talk into the camera, in a very sort of intimate style of campaigning. So he knew how to use social media beforehand. And I think that has allowed Ukraine to communicate to Western audiences, basically, ‘give me money, give me weapons,’ and that has helped. There is an alternative scenario where perhaps if Russia’s military were slightly better organized and had a better social media campaign, it would become very difficult for Ukraine to hold.

And I would say that Russia’s propaganda has been sloppier. It’s not as good of a story. Ukraine already has the underdog sympathy, and they’ve been very good at capitalizing on it. They show their battlefield successes and highlight atrocities committed by Russians.

And the other thing is that social media has helped to organize foreign fighters and folks who have volunteered to go to Ukraine.

SN: Social media is also an enormous source of misinformation and disinformation. How is that playing out?

Donovan: We’re seeing recontextualized media [on TikTok and elsewhere], which is the reuse of content in a new context. And it usually also misrepresents the time and place of the content.

For instance, we’ve seen repurposed video game footage as if it was the war in Ukraine. While we [in the United States] don’t need real-time information to understand what’s happening in Ukraine, we do need access to the truth. Recontextualized media gets in the way of our right to truth.

And we want to make sure the information getting to people in Ukraine is as true and correct and vetted as possible, because they’re going to make a life-or-death decision that day about going out in search of food or trying to flee a certain area. So those people do need real-time accurate information.

There’s one other story about the way in which hope and morale can be decimated by disinformation. Among Ukrainians, there’s a lot of talk about when or if the United States or NATO will send planes. And there were these videos going around suggesting that the United States had already sent planes, and showing paratroopers jumping out. People were sharing these until they got to a reputable news source and heard the news that NATO was still not sending planes. So it can be something as innocent as a video that provides a massive amount of hope to people who share it, and then it’s all snatched away.

SN: What aren’t we seeing on social media?

Donovan: There’s a missing piece, which is that many social media algorithms are set to remove things that are torturous or gory. And so the very violent and vicious aftermath of war is something that the platforms are suppressing, just by virtue of their design.

So in order to get a complete picture of what has happened in Ukraine, people are going to have to see those videos [from other news sources] and be a global witness to the atrocity.

SN: Where is this all heading?

Zeitzoff: I think the biggest thing that’s changing is this decoupling of social media networks across great powers. So you have the Great Firewall [that censors the internet] in China, and I think Russia will be doing something very similar. And how does that influence the free flow of information?

Donovan: We try to understand how information warfare plays out as kind of a chess match between different actors. And what’s been incredible about the situation in Russia is you have this immense titan, the tech industry, pushing back on Russia by removing state media from their platforms. And then Russia counters by removing Facebook and Instagram in Russia.

This is the first time that we’ve seen these companies take action based on the request of other governments. In particular, Nick Clegg [the president of global affairs at Meta, the parent company of Facebook, Instagram and the messaging service WhatsApp] said that they were complying with Ukrainian asks. That means that they are taking some responsibility for the content that is being aired on their platforms. Whatever outcome happens over the next month, I don’t think the internet is going to be as global as it once was.

Here’s the best timeline yet for the Milky Way’s big events

A new analysis of nearly a quarter million stars puts firm ages on the most momentous pages from our galaxy’s life story.

Far grander than most of its neighbors, the Milky Way arose long ago, as lesser galaxies smashed together. Its thick disk — a pancake-shaped population of old stars — originated remarkably soon after the Big Bang and well before most of the stellar halo that envelops the galaxy’s disk, astronomers report March 23 in Nature.

“We are now able to provide a very clear timeline of what happened in the earliest time of our Milky Way,” says astronomer Maosheng Xiang.
He and Hans-Walter Rix, both at the Max Planck Institute for Astronomy in Heidelberg, Germany, studied almost 250,000 subgiants — stars that are growing larger and cooler after using up the hydrogen fuel at their centers. The temperatures and luminosities of these stars reveal their ages, letting the researchers track how different epochs in galactic history spawned stars with different chemical compositions and orbits around the Milky Way’s center.

“There’s just an incredible amount of information here,” says Rosemary Wyse, an astrophysicist at Johns Hopkins University who was not involved with the study. “We really want to understand how our galaxy came to be the way it is,” she says. “When were the chemical elements of which we are made created?”

Xiang and Rix discovered that the Milky Way’s thick disk got its start about 13 billion years ago. That’s just 800 million years after the universe’s birth. The thick disk, which measures 6,000 light-years from top to bottom in the sun’s vicinity, kept forming stars for a long time, until about 8 billion years ago.

During this period, the thick disk’s iron content shot up 30-fold as exploding stars enriched its star-forming gas, the team found. At the dawn of the thick disk era, a newborn star had only a tenth as much iron, relative to hydrogen, as the sun; by the end, 5 billion years later, a thick disk star was three times richer in iron than the sun.

Xiang and Rix also found a tight relation between a thick disk star’s age and iron content. This means gas was thoroughly mixed throughout the thick disk: As time went on, newborn stars inherited steadily higher amounts of iron, no matter whether the stars formed close to or far from the galactic center.

But that’s not all that was happening. As other researchers reported in 2018, another galaxy once hit our own, giving the Milky Way most of the stars in its halo, which engulfs the disk (SN: 11/1/18). Halo stars have little iron.

The new work revises the date of this great galactic encounter: “We found that the merger happened 11 billion years ago,” Xiang says, a billion years earlier than thought. As the intruder’s gas crashed into the Milky Way’s gas, it triggered the creation of so many new stars that our galaxy’s star formation rate reached a record high 11 billion years ago.

The merger also splashed some thick disk stars up into the halo, which Xiang and Rix identified from the stars’ higher iron abundances. These “splash” stars, the researchers found, are at least 11 billion years old, confirming the date of the merger.

The thick disk ran out of gas 8 billion years ago and stopped making stars. Fresh gas around the Milky Way then settled into a thinner disk, which has given birth to stars ever since — including the 4.6-billion-year-old sun and most of its stellar neighbors. The thin disk is about 2,000 light-years thick in our part of the galaxy.

“The Milky Way has been quite quiet for the last 8 billion years,” Xiang says, experiencing no further encounters with big galaxies. That makes it different from most of its peers.

If the thick disk really existed 13 billion years ago, Xiang says, then the new James Webb Space Telescope (SN: 1/24/22) may discern similar disks in galaxies 13 billion light-years from Earth — portraits of the Milky Way as a young galaxy.

Spinosaurus’ dense bones fuel debate over whether some dinosaurs could swim

A fierce group of predatory dinosaurs may have done much of their hunting in the water.

An analysis of the bone density of several sharp-toothed spinosaurs suggests that several members of this dino group were predominantly aquatic, researchers report March 23 in Nature.

That finding is the latest salvo in an ongoing challenge to the prevailing view that all dinosaurs were land-based animals that left the realms of water and air to marine reptiles such as Mosasaurus and flying reptiles such as Pteranodon. But, other researchers say, it still doesn’t prove that Spinosaurus and its kin actually swam.
Back in 2014, Nizar Ibrahim, a vertebrate paleontologist now at the University of Portsmouth in England, and colleagues pieced together the fossil of a 15-meter-long Spinosaurus from what’s now Morocco. The dinosaur’s odd collection of features — a massive sail-like structure on its back, short and muscular legs, nostrils set well back from its snout and needlelike teeth seemingly designed for snagging fish — suggested to the researchers that the predator might have been a swimmer (SN: 9/11/14). In particular, it had very dense leg bones, a feature of some aquatic creatures like manatees that need the bones for ballast to stay submerged.

In the new study, Ibrahim and his team returned to that question of bone density to assess whether it’s a reliable proxy for how much time a creature spends in the water. The team assembled “a massive dataset” of femur and dorsal rib bone densities from “an incredible menagerie of extinct and living animals, reaching out to museum curators all around the world,” Ibrahim says.

That menagerie includes spinosaurs like showy, sail-backed Spinosaurus as well as its equally sharp-toothed cousins Baryonyx and Suchomimus. It also includes other groups of dinosaurs, extinct marine reptiles, pterosaurs, birds, modern crocodiles and marine mammals.

The team then compared these bone analyses with the water-dwelling habits of the various creatures in the study. That work confirms that density is “an excellent indicator” for species in the early stages of a transition from land-dwelling to water-dwelling, the team reports. Those compact bones can aid such transitional creatures, which might not yet have features like fins or flippers to help them maneuver in the water more easily, in hunting underwater — what the team calls “subaqueous foraging.”

The analyses also show that not only did Spinosaurus have very dense bones, but Baryonyx did too. That suggests that both of these dinos were subaqueous foragers, the team says. That idea builds on previous work by Ibrahim and colleagues that proposed that Spinosaurus didn’t just spend much of its time in the water, but could actually swim in pursuit of prey, thanks to its odd, paddle-shaped tail (SN: 4/29/20).
The idea of a swimming Spinosaurus hasn’t been convincing to all. In 2021, a study in Palaeontologia Electronica examined Spinosaurus’ anatomy in detail and came to a different conclusion. The dinosaur was not a highly specialized aquatic predator, wrote David Hone, a zoologist and paleobiologist at Queen Mary University of London, and Thomas Holtz Jr., a vertebrate paleontologist at the University of Maryland in College Park. Instead, Spinosaurus may have just waded in the shallows, heronlike, to do its fishing.

The new study has not convinced those skeptics. Spinosaurus has “clearly got very dense bones. This is really good evidence that they’re hanging around in water — but we kind of knew that,” Hone says. “It’s not clear what they’re doing in the water. That’s the contentious part.”

Take hippos, which spend much of their time mostly submerged, Hone says. “Hippos have bone densities entirely comparable to Spinosaurus and Baryonyx, but they don’t eat in the water” and they don’t swim, he adds.

“Everyone has been in agreement that Spinosaurus was more aquatic than other big theropods” like Tyrannosaurus rex, Holtz says. That Baryonyx also had dense bones was a bit of an interesting surprise, he adds.

But dense bones or not, Holtz says, “it still doesn’t turn them into aquatic hunters.” He describes several anatomical features — Spinosaurus’ long slender neck, tilted head and arrangement of neck muscles that suggest a downward striking motion — that point more to a wading creature that hunted from above the water surface than one that chased its prey underwater.

Kiersten Formoso, a vertebrate paleobiologist at the University of Southern California in Los Angeles, says that the new comparison of bone densities among a wide variety of creatures is a valuable addition, one that she anticipates referring to in her own work studying the transition of ancient creatures from land to water. But she too is not convinced that it proves that Spinosaurus and Baryonyx could actually swim.

“I would never detach Spinosaurus from the water,” Formoso says. But, she adds, more work needs to be done on its biomechanics — how it might have moved — to understand how adroitly aquatic the dinosaur might have been.