When the messenger Pheidippides ran from the battlefield of Marathon to Athens to announce the Greek victory over the Persians in the 5th century B.C., he did it without shoes. His time was not officially tracked.

Millenniums later, at the London Marathon on Sunday, Sabastian Sawe and Yomif Kejelcha became the first to break the two-hour barrier in an official marathon, and Tigist Assefa set a women’s world record. All three did it in featherweight footwear.

The shoe, the Adidas Adizero Adios Pro Evo 3, weighs 97 grams, or about 3.4 ounces, depending on the shoe size. It’s the lightest running sneaker approved for competition. It sold out on Monday.

The race to near-weightlessness has been a driving force of innovation in running sneakers in the 25 centuries since shoeless Pheidippides’s run.

Heavier shoes are slower, a 2016 study showed, although that analysis was only for three-kilometer time trials. The study’s authors hid lead pellets in some Nike racing shoes and didn’t tell the subjects.

“When we added 100 grams per shoe, they ran about 1 percent slower, and when we added 300 grams, they ran about 3 percent slower,” said Rodger Kram, an emeritus professor of integrative physiology at the University of Colorado, Boulder. He called the 1 percent per 100 grams “a rule of toe.”

But it took many steps before running shoes evolved to the blistering pace they now allow.

‘The Platonic ideal’

Shoes once relied on leather, wood or metal reinforcements that were heavy and stiff. The rise of the rubber sole added flexibility and waterproofing. Better durability and grip were bonuses.

People could also run or walk in rubber soles without being heard. That’s why they’re called sneakers.

The first flat-soled rubber and canvas shoe was developed in 1868, almost 30 years after Charles Goodyear discovered the process of curing rubber called vulcanization. Converse and Keds made them popular in the 1920s.

“Converse All Stars are the Platonic ideal of the sneaker,” said Nicholas Smith, the author of “Kicks: The Great American Story of Sneakers.”

He noted they have not changed much.

“Canvas on top, rubber at the bottom.”

‘Chariots of Fire’

Competitive runners soon faced a trade-off: a heavier shoe with better traction or a lighter flat sole.

The heavier option, which transferred maximum force from the runner’s foot to the ground for acceleration, came from J.W. Foster of Bolton, England, who had the insight that led to the first metal spikes in shoes in the 1890s. Top British runners, including those competing in the 1924 Paris Olympics, wore these new spikes to notch their fastest times. Their story is told in “Chariots of Fire,” the Oscar-winning 1981 film.

It was at another Olympics, in Berlin, that Jesse Owens wore six-spiked shoes created by Adidas to become the most successful runner of the 1936 Games. His shoes, made of specially tanned calf leather and cowhide, weighed 201 grams.

A broken waffle iron

Running tracks made of urethane, a rubber compound, began to emerge in the 1960s. When one was installed in 1969 at the University of Oregon, the track coach, Bill Bowerman, found that runners’ spikes dug in too deep while flat shoes offered too little traction.

Having breakfast one morning in 1971, he noticed that the grid pattern on the waffle iron his wife was using might just be what his runners needed to get a grip on the track.

He tried pouring liquid urethane into the waffle iron, but only managed to seal it shut. He kept trying other waffle irons until he had the mold he wanted, according to Nike archivists.

But Bowerman wanted more than grippier sneakers.

“He was also devoted to making the lightest running shoe possible,” said Elizabeth Semmelhack, the author of “Out of the Box: The Rise of Sneaker Culture.”

Bowerman’s shoe, which Nike called the Waffle Trainer, “stands out because it had an extremely thin rubber sole but with a high tread, and then the upper of the shoe was made out of nylon.”

The light weight also helped bring running to the masses.

“You can bet there wouldn’t be so many people running today if they had to carry all the extra baggage we had back then,” Bowerman, a Nike co-founder, said in 1979.

The waffle-pattern nubs on the soles compressed under weight and helped bring spring to a runner’s step. But they only hinted at how sneakers could be cushioned. For many runners, it was not enough.

Then Nike introduced ethylene-vinyl acetate, better known as EVA foam, on the heel of its Cortez shoe in 1972. EVA offered a thicker, air-infused layer of separation from the road and absorbed more of the shock. The age of adding thick slabs of rubber for cushioning was over.

That led to a new quest that continues today: How much cushioning can you build into a shoe?

Lighter than foam

Sneaker makers next turned to a gas and a semisolid to help spread the energy of a foot’s impact with the ground.

In 1979, the Nike Air Tailwind began the airbag era, in which pressurized gas is stored in a flexible urethane bag within the sole.

Asics pioneered gel cushioning technology in 1986 with its silicone-based shock absorption. Nike countered in 1987 with the Air Max 1, which had a “window” in the sole designed to show off the air pocket. “At the time that was the cutting edge,” Mr. Smith said.

Though air is lighter than foam, it had to be kept in a rubber vessel, which added weight. So did adding silicone gel packets to the heel and forefoot.

While air bubbles felt more springy, and gel more dampening, they both were able to absorb shocks longer than the standard foam.

An icon of the 1990s tackled a different problem: how laces become loosened during a run. Reebok, building upon its wildly successful Pump basketball sneakers, which could be inflated with the press of a button on the tongue, introduced the Instapump Fury, a colorful, open-paneled, split-soled running sneaker.

“The Instapump used an air bladder that could fit your particular foot, the nuances of your own foot, very, very closely,” Ms. Semmelhack said. “Then you didn’t have to adjust any lacing throughout your run or at any time. So it was very innovative.”

Rise of the super shoes

But as new technologies made foam lighter, shoemakers soon couldn’t get enough of it.

One brand, Hoka, wedged so much cushy foam into its soles, beginning in 2009, that the shoes looked swollen. Runners could hardly “feel the ground anymore,” Smith said. The company’s designs helped push amateur runners to chunkier shoes.

But it was the thick-soled Nike Vaporflys that captured the most attention. They came with a carbon-fiber plate in the midsole that was very light and gave stability to all the squishy foam. The plate stores and releases energy with each stride, and is meant to spring runners forward.

The Vaporflys also use polyether block amide, or PEBA, a bouncier, lighter foam. The twin technologies led to the nickname “super shoes.”

“There was concern that this additional innovation in the running shoe was the equivalent of doping,” Semmelhack said. The shoes cushioned the feet of all three medalists in the men’s marathon at the 2016 Rio de Janeiro Olympics. The New York Times found in 2019 that the shoes gave a significant advantage, but they were never banned.

Adidas used its lightest foam in the sole of the Adizero Adios Pro Evo 3, which were worn by Sunday’s record-breaking marathoners. It also said that the shoe has carbon fiber rods to mimic the human foot’s bone structure.

In other words, closer to the bare feet in Pheidippides’s run.

When President Trump named a new leadership team at the Centers for Disease Control and Prevention two weeks ago, public attention focused on Dr. Erica Schwartz, his nominee to be the agency’s director. Her public support of vaccines was interpreted by some as a sign that Health Secretary Robert F. Kennedy Jr.’s quest to limit childhood immunizations might be coming to an end.

But another senior official Mr. Trump named to the team shares many of Mr. Kennedy’s views, suggesting the potential for continuing tension at the public health agency.

Mr. Trump appointed Dr. Sara Brenner, a deputy commissioner at the Food and Drug Administration and a self-described “MAHA mom,” to be Mr. Kennedy’s senior counselor for public health, a post that, unlike the C.D.C. director, does not require Senate confirmation. A look at Dr. Brenner’s background suggests she is aligned with Mr. Kennedy on some of his signature issues, including skepticism about vaccines and a strong belief in the importance of fitness.

The public health counselor serves as the liaison between the health secretary and the C.D.C. (and on occasion the White House). As such, Dr. Brenner, who starts the job in the next couple of weeks, will be Mr. Kennedy’s eyes and ears at an agency he has been warring with through most of his tenure as the nation’s top health official.

She will be based in Washington in the office of the secretary, and will most likely meet with the top officials at the C.D.C. at least once every day. She will also be Mr. Kennedy’s liaison to the National Institutes of Health, an agency that has come under fire for making large cuts to medical research grants.

Dr. Nirav Shah, who was the C.D.C.’s principal deputy director from March 2023 through February 2025, said Dr. Brenner could wield a powerful influence on the nation’s public health policies through her role. (Dr. Shah, a Democrat, is currently running for governor in Maine.)

He cited the work of previous officials who held that role and acted as “a conduit and a sounding board for agencies to help them get where they wanted to go.”

“If Sara follows that lead,” he added, “then there’s a possibility that it will be constructive. But if she rather asserts herself as a stand-in for the secretary or the director, then what we will see is more political interference.”

The Department of Health and Human Services would not make Dr. Brenner available for an interview.

“Dr. Brenner was selected for this role because of her experience as a physician and her work on federal public health policy,” Andrew Nixon, a spokesman for the department, said in an emailed statement.

“She brings a strong understanding of both clinical care and research and will work with Secretary Kennedy to optimize coordination with the C.D.C. and N.I.H.,” he added.

At a Make America Healthy Again event last May, Dr. Brenner said she had been pregnant during the pandemic and chose not to receive the Covid vaccine because she was concerned about the vaccine’s “biodistribution patterns.”

She co-wrote a memo saying there was “no clear evidence” that the benefits of Covid vaccines for children under 18 outweighed the risk of harm. She also intervened in the F.D.A.’s review of Novavax’s Covid vaccine, asking for more data on the shot at the 11th hour — a highly unusual step for the agency’s deputy commissioner.

Sarah Despres, who served as the public health counselor during the Biden administration, would not comment on Dr. Brenner specifically. But she said a person in that role would receive enough notice of new initiatives and scientific reports, including in the agency’s flagship journal, for her or the secretary’s office to block them.

“If you’re interested in interfering, you will have a heads-up,” Ms. Despres said.

Dr. Brenner, who holds degrees in medicine and public health, is currently a principal deputy commissioner at the F.D.A. In interviews, four former senior officials who worked closely with her described her as ambitious and eager to please her bosses, even if that meant going against the interests of the F.D.A.’s rank-and-file employees. (They asked to remain anonymous because of fear of retaliation from the administration.)

Two former colleagues recalled Dr. Brenner saying that people should not reflexively believe in vaccines but should insist on facts as they do for other medical products — echoing comments made by Mr. Kennedy and others who question vaccines. Dr. Brenner seemed unaware of the large body of evidence on vaccine safety that already exists, the colleagues said.

Asked about Dr. Brenner’s views on vaccines, Mr. Nixon said, “Anonymous characterizations don’t reflect her record.”

Aaron Siri, a lawyer who for years joined with Mr. Kennedy to bring lawsuits over vaccine safety, said he believed Dr. Brenner would “at least critically consider concerns” from people with opposing views.

“Wherever she comes out at the end, at least you feel like the data and evidence were given a more objective overall review,” he said.

Dr. Brenner joined the F.D.A. in 2019 as a midlevel scientist in the agency’s Center for Devices and Radiological Health. Shortly after Mr. Trump’s inauguration, in January 2025, she was catapulted to the position of acting commissioner, a job she held until April when his nominee for the job, Dr. Martin Makary, became the agency’s leader.

Before her ascent, some top officials were unfamiliar with her. Dr. Janet Woodcock, who worked at the agency for decades and was its principal deputy commissioner from 2022 to 2024, said she had “never even heard her name.”

Dr. Robert Califf, who led the F.D.A. during parts of the Obama and Biden administrations, said Dr. Brenner “was part of some briefings and seemed very professional,” but he did not know her well enough to comment on her work.

Dr. Brenner may have come to the attention of Mr. Trump’s team when she worked at the White House Office of Science and Technology Policy during his first term. After being named acting commissioner, she fully embraced the MAHA movement, signing her introductory emails to the F.D.A. staff with the slogan.

She was the agency’s leader while widespread layoffs were decimating the ranks of staff scientists. In her communications with staff members, she amplified the messages of the administration and did not address the distress of the employees, the former colleagues said. At the same time, she made it clear that she did not want to be in the office five days a week and sometimes showed up to meetings in workout clothes, the colleagues said.

In a video posted by the F.D.A. to social media as a “spin on the Pete and Bobby challenge” — referring to fitness videos made by Defense Secretary Pete Hegseth and Mr. Kennedy — she knocks out 50 burpees, 50 forearm to full planks and 100 bicycle situps in seven minutes and one second.

Sounding much like Mr. Kennedy, she says, “I’ve been encouraging people my whole career to understand that taking care of their body is one of the most important things that you can do on a daily basis.”

Mr. Nixon, the health department spokesman, said that “Dr. Brenner exemplifies what it means to be a MAHA mom through her commitment to improving children’s health and well-being through nutrition and fitness.”

The other three officials Mr. Trump named to lead the C.D.C. have all been enthusiastically received by public health experts.

Dr. Schwartz, the White House’s nominee to be the agency’s director, has degrees in medicine and public health and has praised vaccines, prompting cautious optimism that she will steady the C.D.C. It is unclear when she might be confirmed by Senate. The White House has not yet filed papers to move her nomination forward.

The agency has been without an official director since Mr. Kennedy fired Susan Monarez after a dispute over vaccine policy 29 days after she began the job. The agency has also experienced widespread layoffs, resignations and a shooting since Mr. Trump returned to the White House.

In hearings before Congress over the past few days, Mr. Kennedy has contradicted himself, telling one group of lawmakers that he would not commit to carrying out Dr. Schwartz’s policies on vaccines, but later recanting that stance in front of another.

But Dr. Schwartz is also a longstanding member of the armed forces, used to following orders in that context, some public health experts noted.

Mr. Trump named two others, Sean Slovenski, a seasoned health care executive, and Dr. Jennifer Shuford, a highly respected epidemiologist and physician, as deputies to Dr. Schwartz. But more quietly, the administration has also appointed Stephen Sayle, a former tobacco executive, as deputy director for legislative affairs. And several other political appointees with known antipathy to vaccines, including Stuart Burns, remain at the agency.

Mr. Kennedy and his appointees have already skirted the normal procedures to try to alter the childhood vaccine schedule, although a federal judge has blocked those efforts. And Dr. Jay Bhattacharya, the C.D.C.’s acting head, recently canceled publication of a report showing the effectiveness of Covid shots last winter.

It is unclear where Dr. Brenner stands on other issues and whether she would go along with Mr. Kennedy’s agenda without question.

“It’s important for people who work in the Office of the Secretary to have their own red lines, and to be willing to push back if a request from their boss comes that is inappropriate,” Ms. Despres said.

A very large sea lion is drawing onlookers to a pier popular among both other sea lions and tourists in San Francisco, wowing visitors and catapulting him to online fame.

“POV: You pull up to the gym and see that one guy who’s just built different,” reads the caption on one video of the sea lion. Commenters across several Reddit threads dedicated to the mammal call him an “AbsoluteUnit,” “Harbor Master” and “Sir Chungus Maximus.”

But the name that has stuck is Chonkers — because, well, he is chonky.

Video posted on social media last month showed the sea lion leaping from the bay onto a pontoon full of smaller sea lions, who immediately scattered to make way. Chonkers, about two to three times as large, weighs about 1,500 to 2,000 pounds, according to experts. He has a light-tan coat and a boxy, bearlike head.

He is not a freak of nature. He is a Steller sea lion.

His species, unlike the California sea lions most typically found around San Francisco, rarely shows up at Pier 39 — a popular tourist spot near one end of Fisherman’s Wharf. There, thousands of the less sizable animals laze atop wooden floats installed for their rest.

Steller sea lions typically migrate farther offshore, said Laura Gill, the public programs manager at the Marine Mammal Center in Sausalito, which has been tracking Chonkers since March 13. She and other experts believe Chonkers most likely swam about 30 miles from a rookery in the Farallon Islands, making his way under the Golden Gate Bridge and into the San Francisco Bay.

The bay’s waters are more shallow than the open sea, Ms. Gill said, where sea lions can avoid predators and more easily track prey like anchovies and rockfish. The floating platforms, she added, are a comfortable alternative to the harsh ocean, where, in order to rest, a sea lion may have to haul itself onto jagged rocks.

“They can just rest on those platforms for hours, days at a time,” she added, “and not have to worry about the elements.” Though life is great at the dock, the journey there is long, which may deter other Steller sea lions, Ms. Gill said.

Steller sea lions are similar in appearance to California sea lions, but they grow much larger. Males can be up to 11 feet long and weigh up to about 2,500 pounds.

They usually forage at night, feasting on more than a hundred species of fish and cephalopods. But individual sea lions may adopt their own hunting strategies, according to the National Oceanic and Atmospheric Administration.

“He’s kind of out of the ordinary,” Ms. Gill said of Chonkers. “He is probably just very food-motivated.”

On the Farallon Islands, Steller sea lions and their smaller counterparts, the California sea lion, compete intensely for food, Ms. Gill said.

“Chonkers is kind of just working smarter and not harder,” she added, “trying to find the easy, safe place where there’s less competition, less predators.” At Pier 39, Ms. Gill said, Chonkers is “king of the dock.”

Sheila Chandor, who has been harbor master at Pier 39 since 1985, said Chonkers had been visiting the pier for about 15 years, but had previously stayed just a day or two at a time.

“It was quite a spectacle to see this wonderful mammal,” Ms. Chandor recalled of the first time she saw him. This time, Ms. Chandor added, Chonkers had stayed far longer and appeared to have been joined by a juvenile Steller sea lion for a few days.

“I sort of hope we don’t end up with more,” Ms. Chandor said, noting that the wooden floats were built for California sea lions, which weigh up to about 700 pounds. (As it is, there is hardly enough space for all the animals, which regularly pile upon and push one another off the floats.)

“When he’s sitting on the dock, he’s so big compared to the others,” Ms. Chandor said. “He makes them all look like little kittens.”

“He’s such a big guy,” she marveled. “He’s truly huge.”

Ms. Chandor described international tourists, locals and other visitors coalescing at the pier on Wednesday morning in the hopes of witnessing the great beast.

“They didn’t know each other, but they were all saying, ‘Are you here to see Chonkers?’” she said.