A four-line letter, signed by the attending physician of Congress and released by Senator Mitch McConnell on Thursday, suggested that his recent spells of speechlessness were linked to “occasional lightheadedness” perhaps brought on by his recovery from a concussion last winter or “dehydration.”

But seven neurologists, relying on what they described as unusually revealing video of Mr. McConnell freezing up in public twice recently, said in interviews Thursday and Friday that the episodes captured in real time likely pointed to more serious medical problems afflicting the longtime Republican leader.

Some of the neurologists, while cautioning that they could not diagnose the minority leader from afar, said that the letter and other comments from Mr. McConnell’s office appeared to fall short of explaining why he abruptly stopped speaking during news conferences in late July and again on Wednesday.

“If I gave that tape to a medical student and that was his explanation, I’d fail him,” said Dr. Orrin Devinsky, a professor of neurology at the NYU Grossman School of Medicine, referring to the account given by the attending physician of Congress on Thursday. “Medically, these episodes need to be taken seriously.”

The neurologists said that the episodes justified close medical attention and could prompt treatment to keep them from recurring. While several possibilities were suggested, including mini-strokes, doctors said that the spells appeared most consistent with focal seizures, which are electrical surges in one region of the brain.

The senator’s aides have not revealed what type of follow-up care Mr. McConnell may be receiving. Doug Andres, a spokesman for the senator, said on Friday he had nothing to add beyond the letter by the Congressional physician, Dr. Brian P. Monahan, who did not respond to requests for comment.

More details about the medical history of Mr. McConnell, 81, including whether he has been having such episodes off camera, would also help rule out other possible explanations for the spells, neurologists said.

Whether caused by seizures or mini-strokes or something else, spells like Mr. McConnell’s would not preclude most patients from working or socializing normally, doctors said.

“Seizures have a stigma in our society, and that’s unfortunate because these are very brief electrical interruptions in behavior,” said Dr. Jeffrey Saver, a professor of neurology at U.C.L.A. “Between those rare episodes, which are usually well controlled with medicines, people function perfectly normally.”

Still, experts said that seizures carried some elevated risk of cognitive or behavioral problems and could affect older patients differently.

Rarely does the public get as complete a glimpse of a serious medical event in a public figure as it did twice in recent weeks with Mr. McConnell. For neurologists, videos like those showing Mr. McConnell from the moment he appeared to lose the ability to speak are far more than mere curiosities.

They can help form the basis of a diagnosis, as homemade videos of everyday patients occasionally do in standard neurology practices.

“They’re very helpful, because you’re not subject to the vagaries of someone’s description and you can capture the beginning of it, which is important especially for seizures,” said Dr. Anthony Kim, a professor of neurology at the University of California, San Francisco.

Details as small as the direction in which people’s eyes are pointed during such an episode offer potential clues about the cause, Dr. Kim said.

After watching Mr. McConnell’s symptoms play out — his abrupt stop in speech, his eyes fixed in the distance, his seeming recovery after about 30 seconds — Dr. Kim said that “the possibility at the top of my list would be a seizure.”

That Mr. McConnell’s second spell so closely mirrored the first pointed even more strongly to a seizure, neurologists said.

Mini-strokes, which result from a clot that reduces blood flow to the brain, can also cause brief periods of impaired speech. But they rarely produce the same constellation of symptoms each time they recur, given that clots are unlikely to travel at random to the same part of the brain twice.

Focal seizures, on the other hand, are often triggered by an irregularity in one specific part of the brain, creating what doctors refer to as stereotypic symptoms. They are known to stop patients dead in their tracks, seeming to cut them off from their surroundings.

Patients can often respond reflexively to questions during such an episode — as Mr. McConnell did on Wednesday, saying “yeah” when asked if he had heard a reporter’s question — even if they appear unable to voice their thoughts or engage with their environment.

Mr. McConnell suffered a concussion in March, a risk factor for seizures. The seizures can be caused by a bleed in the brain or a scar from a traumatic head injury. Previous strokes or other kinds of damage to brain tissue can also lead to seizures in older people, who as a group experience the onset of seizures almost as often as children do.

Some seizures are provoked by triggers like abnormal blood sugar levels. But if someone has had two seizures that cannot be explained in that way, neurologists said that would typically be enough for a diagnosis of epilepsy, a common neurological disorder affecting more than three million Americans that can arise at any age. They would generally prescribe anti-seizure medication.

“Two seizures you definitely would want to treat,” said Dr. Sami Khella, the chief of neurology at Penn Presbyterian Medical Center. “You don’t want them to happen — they’re not good for you.”

Many patients function completely normally and show normal brain wave activity between seizures, allowing them to remain active and working even as some may end up forgoing activities like driving.

But they do cause patients to miss periods of time during episodes. A seizure at an inopportune moment, like when crossing the street, can be dangerous. And focal seizures involving one region of the brain can generalize, causing episodes characterized by jerking movements or epileptic spasms.

Beyond that, one or two seizures can beget more, a cycle that neurologists try to interrupt with treatment. “The more the brain seizes, the more it learns to seize,” Dr. Khella said.

Other complications can follow. A phenomenon known as sudden, unexpected death in epilepsy kills an estimated one in 1,000 people with epilepsy each year.

“If you do get epilepsy as an elderly individual, there are concerns about things like memory, about cognitive function, because your resilience at 80 is going to be far less than when you’re 20 or 30,” said Dr. Devinsky, who directs NYU Langone’s Comprehensive Epilepsy Center.

Neurologists said they could not rule out other possible explanations for Mr. McConnell’s episodes.

Dr. Gavin Britz, a neurosurgeon at Houston Methodist, said he would want to exclude Parkinson’s disease, which can also cause freezing episodes.

But neurologists agreed that suggestions that Mr. McConnell was merely lightheaded, while possible, were difficult to square with the video. Dehydration could exacerbate other conditions, they said, but such patients would be unlikely to stay upright or recover so quickly without fluids, as Mr. McConnell appeared to do.

“We don’t have 100 percent information, so we’re kind of in the dark,” Dr. Devinsky said. “But we do have this very powerful clinical information, which is quite honestly how I have to diagnose seizures and epilepsy all the time, often without the video.”

The nation’s most thinly staffed nursing homes would be required to hire more workers under new rules proposed on Friday by the Biden administration, the greatest change to federal nursing home regulations in three decades.

The proposed standard was prompted by the industry’s troubled performance earlier in the coronavirus pandemic, when 200,000 nursing home residents died. But the proposal falls far short of what both the industry and patient advocates believe is needed to improve care for most of the 1.2 million Americans in nursing homes.

The proposal, by the Centers for Medicare and Medicaid Services, would require all facilities to increase staff up to certain minimum levels, but it included no money for nursing homes to pay for the new hires.

C.M.S. estimated that three-quarters of the nation’s 15,000 homes would need to add staff members. But the increases at many of those facilities would be minor, as the average nursing home already employs nurses and aides at, or very close to, the proposed levels.

“The standards are a lot lower than what a lot of experts, including myself, have called for over the years,” said David Grabowski, a professor of health care policy at Harvard Medical School. “There are some real positives in here, but I wish the administration had gone further.”

The government said it would exempt nursing homes from punishment if they could prove that there was a local worker shortage and that the facilities had made sincere efforts to recruit employees.

“Fundamentally, this standard is wholly inadequate to meet the needs of nursing home residents,” said Richard Mollot, the executive director of the Long Term Care Community Coalition, an advocacy group based in New York.

Executives in the nursing home industry said that without extra money from Medicare or Medicaid — the two federal insurers that pay for most nursing home care — the requirement would be financially unattainable.

“It’s meaningless to mandate staffing levels that cannot be met,” Katie Smith Sloan, the president and chief executive of LeadingAge, an association that includes nonprofit nursing homes, said in a statement. “There are simply no people to hire — especially nurses. The proposed rule requires that nursing homes hire additional staff. But where are they coming from?”

The new staffing standard would require homes to have daily average nurse staffing levels amounting to at least 0.55 hours per resident. That translates to one registered nurse for every 44 residents. But that is below what the average nursing home already provides, which is 0.66 hours per resident, a 1:36 ratio, federal records show.

At least one registered nurse would have to be on duty at all times under the proposed plan — one of the biggest changes for the facilities, as they currently must have nurses for only eight consecutive hours each day.

The proposed rule also calls for 2.45 nurse aide hours per resident per day, meaning a ratio of about one aide for every 10 residents. While the federal government sets no specific staffing requirements for nurse aides, the average home already provides 2.22 nurse aide hours a day, a ratio of about 1:11.

“The federal minimum staffing standards proposed by C.M.S. are robust yet achievable,” the agency said in a statement. “The proposal also makes clear that the numerical staffing levels are a floor — not a ceiling — for safe staffing.”

Registered nurses are at the top of the chain of command at nursing homes, overseeing assessments of residents and handling complex clinical tasks. Nurses delegate more straightforward clinical roles to licensed practical nurses.

Certified nurse assistants, often called nurse aides, are generally the most plentiful in a nursing home and help residents with basic needs like bathing, getting out of bed and eating.

On average, registered nurses make $37 an hour while licensed practical nurses earn $28 an hour, according to C.M.S. Aides often start at minimum wage or slightly above, earning $17 an hour on average.

“People have more choice,” said Tina Sandri, the chief executive of Forest Hills of DC, a nursing home in Washington, referring to nursing home staff. “They can go to hospitals and make more and do less than they do here in a nursing home.”

“We’ve lost staff to hospitals that had $20,000 signing bonuses,” she added, “and as a nonprofit, we can’t compete with that.”

Nursing home officials say they cannot afford to pay higher wages because state Medicaid programs reimburse them too little. Patient advocates, however, note that some for-profit homes are providing substantial returns to investors.

Medicare and Medicaid spent $95 billion on nursing home care and retirement community care in 2021, according to C.M.S. The agency estimated that the new standards would cost homes another $4 billion in three years, when all homes except those in rural areas would need to comply. Rural homes would have five years.

Ellen Quirk, a retired certified nurse assistant in Hayes, Va., recalled that sometimes she would care for all of the residents on a single floor in the nursing home, which could be 20 or more people, by herself. It’s challenging for an aide to care for more than five to seven people at a time, she said.

“If it’s more than that, then things aren’t done properly,” Ms. Quirk, 63, said. “Things are skipped over, like a bath or changing them every couple of hours or feeding them properly.”

“I’ve seen patients that roll over and fall out of bed,” she added. “Sometimes they get bed sores because beds are saturated in urine for hours and hours.”

The nursing home industry has been pressing federal and state governments to pay for a bevy of enticements to long-term care workers, including educational subsidies for those who have worked in nursing homes, loan forgiveness and career opportunities for certified nursing assistants working toward their nursing degrees.

The administration said it would offer $75 million in scholarships and tuition as part of the new proposal. The administration is accepting comments for the next 60 days before it finalizes the new standard.

Jordan Rau is a senior correspondent at KFF Health News in Washington, D.C.

No place on the planet has escaped the influence of Homo sapiens, from the rainforests cleared for farms to microplastic-laced deep oceans to climate-altered jet streams. Last November, the world population reached 8 billion.

But as omnipresent as humans may be today, a team of scientists now claims that our species came very close to never appearing at all.

Researchers in China have found evidence suggesting that 930,000 years ago, the ancestors of modern humans suffered a massive population crash. They point to a drastic change to the climate that occurred around that time as the cause.

Our ancestors remained at low numbers — fewer than 1,280 breeding individuals — during a period known as a bottleneck. It lasted for over 100,000 years before the population rebounded.

“About 98.7 percent of human ancestors were lost at the beginning of the bottleneck, thus threatening our ancestors with extinction,” the scientists wrote. Their study was published on Thursday in the journal Science.

If the research holds up, it will have provocative implications. It raises the possibility that a climate-driven bottleneck helped split early humans into two evolutionary lineages — one that eventually gave rise to Neanderthals, the other to modern humans.

But outside experts said they were skeptical of the novel statistical methods that the researchers used for the study. “It is a bit like inferring the size of a stone that falls into the middle of the large lake from only the ripples that arrive at the shore some minutes later,” said Stephan Schiffels, a population geneticist at Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.

For decades now, scientists have reconstructed the history of our species by analyzing the genes of living people. The studies all take advantage of the same basic facts of our biology: every baby is born with dozens of new genetic mutations, and some of those mutations can be handed down over thousands or even millions of years.

By comparing genetic variations in DNA, scientists can trace people’s ancestry to ancient populations that lived in different parts of the world, moved around and interbred. They can even infer the size of those populations at different times in history.

These studies have gotten more sophisticated as DNA sequencing technology has grown more powerful. Today, scientists can compare the entire genomes of people from different populations.

Every human genome contains over 3 billion genetic letters of DNA, each of which has been passed down for thousands or millions of years — creating a vast record of our history. To read that history, researchers now use increasingly powerful computers that can carry out the vast numbers of calculations required for more realistic models of human evolution.

Haipeng Li, an evolutionary genomics researcher at Chinese Academy of Sciences in Shanghai, and his colleagues spent over a decade creating their own method for reconstructing evolution.

The researchers named the method FitCoal (short for Fast Infinitesimal Time Coalescent). FitCoal lets scientists cut up history into fine slices of time, allowing them to create a model of a million years of evolution divided into periods of months.

“It is a tool we created to figure out the history of different groups of living things, from humans to plants,” Dr. Li said.

At first he and his colleagues focused on animals like fruit flies. But once enough genetic data from our own species had been sequenced, they turned to the history of humans, comparing the genomes of 3,154 people from 50 populations around the world.

The researchers explored various models in order to find one that best explains today’s genetic diversity among humans. They ended up with a scenario that included a near-extinction event among our ancestors 930,000 years ago.

“We realized we had discovered something big about human history,” said Wangjie Hu, a computational biologist at the Icahn School of Medicine at Mount Sinai in New York and an author of the study.

Before the bottleneck, the scientists concluded, the population of our ancestors included about 98,000 breeding individuals. It then shrank to fewer than 1,280 and stayed that small for 117,000 years. Then the population rebounded.

Dr. Hu and his colleagues argue in their paper that this bottleneck is consistent with the fossil record of our human ancestors.

Our branch of the evolutionary tree split from that of other apes about seven million years ago in Africa. Our ancestors had evolved to be tall and big-brained in Africa by about a million years ago. Afterward, some of those early humans spread out to Europe and Asia, evolving into Neanderthals and their cousins, the Denisovans.

Our own lineage continued to evolve into modern humans in Africa.

After decades of fossil hunting, the record of ancient human relatives remains relatively scarce in Africa in the period between 950,000 and 650,000 years ago. The new study offers a potential explanation: there just weren’t enough people to leave behind many remains, Dr. Hu said.

Brenna Henn, a geneticist at the University of California, Davis, who was not involved in the new study, said that a bottleneck was “one plausible interpretation.” But today’s genetic diversity might have been produced by a different evolutionary history, she added.

For example, humans might have diverged into separate populations then come together again. “It would be more powerful to test alternative models,” Dr. Henn said.

Dr. Hu and his colleagues propose that a global climate shift produced the population crash 930,000 years ago. They point to geological evidence that the planet became colder and drier right around the time of their proposed bottleneck. Those conditions may have made it harder for our human ancestors to find food.

But Nick Ashton, an archaeologist at the British Museum, noted that a number of remains of ancient human relatives dating to the time of the bottleneck have been found outside Africa.

If a worldwide disaster caused the human population in Africa to collapse, he said, then it should have made human relatives rarer elsewhere in the world.

“The number of sites in Africa and Eurasia that date to this period suggests that it only affected a limited population, who may have been ancestors of modern humans,” he said.

Dr. Li and his colleagues also drew attention to the fact that modern humans appear to have split from Neanderthals and Denisovans after their proposed population crash. They speculate that the two events are related.

The researchers noted that most apes have 24 pairs of chromosomes. Humans have only 23, thanks to the fusion of two sets. After the crash, the scientists suggest, a fused set of chromosomes may have arisen and spread through the tiny population.

“All humans with 24 pairs of chromosomes became extinct, while only the small isolated population with 23 pairs of chromosomes fortunately survived and passed down from generation to generation,” said Ziqian Hao, a bioinformatics researcher at Shandong First Medical University and an author of the study.

But Dr. Schiffels isn’t buying the story of the bottleneck quite yet: “The finding is very surprising indeed, and I think the more surprising the claim, the better the evidence should be.”