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Fresh off his trip to the southern border earlier this week, former President Donald J. Trump on Saturday baselessly suggested that President Biden had “smuggled” violent anti-American forces across the border.

At a rally in Greensboro, N.C., Mr. Trump — who has been charged with conspiracy to defraud the United States as part of the 91 felony counts he currently faces in four separate criminal trials — broadly and without evidence asserted that Mr. Biden’s border policy amounted to a “conspiracy to overthrow the United States of America.”

Mr. Trump has previously suggested without evidence that Democrats were encouraging migrants to cross the border illegally in order to register them to vote. On Saturday, he told the crowd in Greensboro that he believed Mr. Biden was “giving aid and comfort” to America’s foreign enemies.

He went on to frame this year’s election as a question of “whether the foreign armies Joe Biden has smuggled across our border will be allowed to stay or whether they will be told to get the hell out of here and go back home.”

Mr. Trump has frequently blamed the surge of migrants at the border on Mr. Biden and Democrats, who he claims are too lenient on those who cross illegally. But there is no evidence to support the claim that Mr. Biden has trafficked migrants across the border.

Nor is there evidence to suggest that Democrats have been encouraging the surge of migrants at the border in order to register them illegally to vote, one of many claims that Mr. Trump has made as he has promoted widespread and frequently debunked assertions of voter fraud in the 2020 election.

Mr. Trump repeated those claims of fraud at the rally in Greensboro.

He said that Republicans needed to ensure a turnout in November that was so large as to be “too big to rig,” reviving his claim that Democrats could win this year only if they cheated. And his campaign distributed among the crowd signs reading “Too Big to Rig.”

But Mr. Trump also appeared to connect his predictions of voter interference to the migrant crisis, accusing Mr. Biden and his allies of trying to “collapse the American system, nullify the will of the actual American voters and establish a new base of power that gives them control for generations.”

The border crisis has worsened during the Biden administration. Republicans have accused Mr. Biden of being negligent on the issue, arguing that his promises to roll back Mr. Trump’s stringent border policies have led to the continuing influx.

Democrats have pointed to a surge in migration around the world. And Mr. Biden has blamed Republicans for trying to block bipartisan efforts to address the issue, including a bill in Congress that would have significantly cracked down on crossings. Republicans, at Mr. Trump’s urging, thwarted the bill.

Throughout his campaign, Mr. Trump has ramped up warnings about threats he portrays as damaging to traditional American values. The former president typically relies on the language of war to describe the border crisis, which he frames as an invasion.

On Thursday, Mr. Trump visited Eagle Pass, Texas, where Gov. Greg Abbott has dispatched the Texas National Guard to police the border. After receiving a briefing on the state’s efforts, Mr. Trump said that the migrants “look like warriors to me,” adding that “something’s going on — it’s bad.”

During his speech on Saturday, Mr. Trump again painted those crossing the border with a broad brush, saying that they were coming from “mental institutions and insane aslyums” or that they were former prisoners who had been sent across the border by leaders of other countries.

And he again rattled off a number of crimes allegedly committed by migrants, stoking fear and portraying Democrats as ignoring crime and disorder. But border officials, including some who worked for Mr. Trump, have said that most migrants who cross the border are members of vulnerable families fleeing poverty and violence.

Mr. Trump is planning an extreme expansion of his anti-immigration policies if he wins the election in 2025, including what he calls the “largest domestic deportation operation” in U.S. history.

A new Trump administration would build enormous camps in the United States to hold undocumented immigrants. And Mr. Trump would reimpose a Covid-era policy that would refuse asylum claims on the basis that migrants carry diseases like tuberculosis.

Former President Donald J. Trump capped off a clean sweep of Republican delegates in Michigan on Saturday during a raucous convention, which further exposed a deep fissure in the state party that threatens to fester in one of the most important battleground states.

Mr. Trump, the Republican front-runner, amassed at least 90 percent of the vote in all but one of the state’s 13 congressional districts against former Gov. Nikki Haley of South Carolina, who was ambassador to the United Nations under Mr. Trump.

A simple majority was needed in each district to win its share of delegates at the caucus-style event, giving Mr. Trump 39, to go along with the 12 that he won in Michigan’s primary, which was held on Tuesday. Ms. Haley emerged from that contest with four delegates.

Mr. Trump’s dominance earlier in the week left little doubt about the outcome of the convention on Saturday at the Amway Grand Plaza in Grand Rapids, Mich.

But a protracted fight over the state party’s rightful leader spilled over into the proceedings, where an estimated 200 Republican stalwarts from about 20 of Michigan’s 83 counties were denied credentials. Two groups boycotted the event and held breakaway conventions, one more than 100 miles to the north in Houghton Lake, Mich., and another more than 50 miles southeast in Battle Creek, Mich.

Many of those denied credentials were people aligned with Kristina Karamo, whom party leaders in January voted to remove as the state party’s chairwoman. They replaced her with Pete Hoekstra, a former U.S. representative who was Mr. Trump’s ambassador to the Netherlands.

Not all of Ms. Karamo’s backers were shut out on Saturday: A group from Saginaw County jeered and made thumbs-down gestures when Mr. Hoekstra spoke. He acknowledged the friction.

“It can be a little bit abusive,” Mr. Hoekstra told delegates during one of the caucuses, which took place in a ballroom replete with a chandelier and a portrait of President Gerald R. Ford, an old-guard Republican who was raised in Grand Rapids.

Mr. Hoekstra, speaking to reporters, denied that the credentialing snub was an act of retribution, saying that those turned away had not properly registered for the convention.

Ernest Dugan, a Saginaw County delegate and supporter of Ms. Karamo, said that he was disgusted by actions of party leaders, who have criticized her for money problems in the party and governance issues.

“The whole thing stinks to high you-know-where,” Mr. Dugan said.

As a Black Republican, he said he was troubled by the message that the party was sending with its ouster of Ms. Karamo, who is Black.

“A person of color wants to be in your group,” he said, adding, “Then you kick her to the curb?”

Until Friday, it had appeared that a rival convention, planned months ago by Ms. Karamo, might compete with the one organized by Mr. Hoekstra in Grand Rapids and recognized by the Republican National Committee. But after a series of court defeats disputing her removal as party chairwoman, Ms. Karamo scuttled her plans to hold the event in Detroit.

“We need to be united around Hoekstra,” said Jay A. Fedewa, chairman of the Genesee County Republican Party. “It’s disheartening that they don’t want to do that.”

Mr. Trump, whose victory in Michigan in the 2016 election propelled him to the presidency and who later lost the state to Joseph R. Biden Jr. in 2020, has recognized Mr. Hoekstra as chairman during the power struggle.

Debra Ell, a party leader from Saginaw County, compartmentalized her loyalty to Ms. Karamo and the former president.

“Almost everybody, honestly — we love Trump, by the way — but everybody that Trump has endorsed in Michigan has not won,” said Ms. Ell, who wore a pin with Ms. Karamo’s picture on it. “So bless his heart. We love him, but stay out of our politics.”

At the convention, where one delegate went into cardiac arrest, Mr. Trump outperformed his showing in the primary on Tuesday. Mr. Hoekstra attributed the former president’s sweep to the fact that the process was limited to Republicans. Primaries in Michigan are open to all voters, regardless of party affiliation.

“These folks are focused on winning in November,” he said. “Right? Not fighting other Republicans.”

Still, a woman holding a “Hoekstra Is an Impostor” sign lingered nearby.

At the breakaway gathering in Houghton Lake, about 300 Republicans who boycotted the Grand Rapids convention held their own vote to award delegates, a move Mr. Hoekstra and the R.N.C. have said won’t count. All votes went to Mr. Trump.

Daire Rendon, a former state representative who faces felony charges related to a voting-machine breach after the 2020 election — one intended to overturn Mr. Trump’s defeat in Michigan — presided over the event. She wore a blue Trump cap with a Q pin — for the QAnon conspiracy movement — on it.

“This is not going to impact the national election,” Ms. Rendon said. “But what it does is impact the party here in our state because what we’ve done is we’ve gone back to the party of the old white guys when we had a new grass-roots party being led by Kristina Karamo, who was the younger, dynamic version of a rebirth of the Republican Party, embracing a set of values that the Republican Party has always said it stood for.”

The breakaway group then held what amounted to a straw poll, asking supporters of Ms. Karamo to stand. They did the same for Mr. Hoekstra; no one stood for him.

“We have a unanimous vote,” Ms. Rendon said.

The Smokehouse Creek fire is the largest on record in Texas. Since it started on Monday, the blaze has burned over a million acres of land in the state’s Panhandle, much of which is cattle country.

The fire, which spread around the town of Canadian, is still largely uncontrolled. It has devastated cattle ranches, destroyed homes and killed at least two people. It is now one of the most destructive fires in U.S. history, and conditions are expected to worsen over the weekend.

Here’s a look at the damages from the fire so far.

Friday, March 1

The aftermath of the fire in Canadian.

Cattle killed by the fire in Shellytown.

Thursday, Feb. 29

Fire burning across open field near Sanford.

Cows walking through land burned by the fire at the B&C Cattle ranch in the town of Miami.

Firefighters working in Miami.

Firefighters working through rubble of burned homes in Stinnett.

Austin Breeding examining the burned carcass of a pronghorn in Miami.

Wednesday, Feb. 28

Grasslands burning in Roberts County.

Sheriff Brent Clapp, right, and his sons Rider, left, and Rangle picked through the ashes of their home in Canadian.

A destroyed home in Canadian.

Food was passed out to community members on Wednesday night in Canadian.

The fire has so far burned an area over five times the size of New York City.

Plants burned by the fire in Canadian.

Tuesday, Feb. 27

A driver filming while driving down a road surrounded by wildfire in Hutchinson County.

In this photo provided by the Flower Mound Fire Department, firefighters are at the scene of a fire.

Wildfires burning in Moor County.

When it comes down to it, a lot of Democrats wish President Biden were not running this fall. Only 28 percent of Democrats in a new survey by The New York Times and Siena College expressed enthusiasm about his candidacy and 38 percent said flatly that Mr. Biden should not be their nominee.

But even as many Democrats both in Washington and around the country quietly pine for someone else to take on former President Donald J. Trump, who leads nationwide in the poll by 5 percentage points, no one who matters seems willing to tell that to Mr. Biden himself. Or if they are, he does not appear to be listening.

Surrounded by a loyal and devoted inner circle, Mr. Biden has given no indication that he would consider stepping aside to let someone else lead the party. Indeed, he and the people close to him bristle at the notion. For all the hand-wringing, the president’s advisers note, no serious challenge has emerged and Mr. Biden has dominated the early Democratic primaries even more decisively than Mr. Trump has won his own party’s nominating contests.

The Biden team views the very question as absurd. The president in their view has an impressive record of accomplishment to run on. There is no obvious alternative. It is far too late in the cycle to bow out without considerable disruption. If he were ever to have opted against a second term, it would have been a year ago when there would have been time for a successor to emerge. And other than someone with Biden in their name, it is hard to imagine who would have enough influence to even broach the idea with him, much less sway him.

“There is no council of elders and I’m not sure if there was that an incumbent president, no matter who it was, would listen to them,” said David Plouffe, the architect of President Barack Obama’s campaigns and one of the strategists who helped him pick Mr. Biden as his vice-presidential running mate in 2008. “He thinks, ‘Hey, I won and I beat the guy who’s going to run against me and I can do it again.’”

Members of Mr. Biden’s team insist they feel little sense of concern. The president’s closest aides push back in exasperation against those questioning his decision to run again and dismiss polls as meaningless this far before the vote. They argue that doubters constantly underestimate Mr. Biden and that Democrats have won or outperformed expectations in 2018, 2020, 2022, 2023 and even a special House election this year.

“Actual voter behavior tells us a lot more than any poll does and it tells a very clear story: Joe Biden and Democrats continue to outperform while Donald Trump and the party he leads are weak, cash-strapped, and deeply divided,” Michael Tyler, the Biden campaign’s communications director, said on Saturday. “Our campaign is ignoring the noise and running a strong campaign to win — just like we did in 2020.”

Outside the White House, though, many Democrats wish that the no-panic White House would exhibit some urgency. Mr. Biden’s weakness in polls, especially those showing him trailing in all of the half-dozen swing states necessary to assemble an Electoral College majority, have generated widespread anxiety within the party. Some privately say that Georgia and Arizona may be out of reach, requiring Mr. Biden to sweep Michigan, Wisconsin and Pennsylvania.

The discontent is not necessarily a judgment on the merits of Mr. Biden’s presidency. Many Democrats say he has done a good job on many fronts — winding down the pandemic, rebuilding the economy, managing wars in Europe and the Middle East and enacting landmark legislation on infrastructure, climate change, health care, industrial policy, veterans’ care and other issues.

But his support has been undercut by concern about his age, his support for Israel’s war on Hamas, the record influx of migrants at the southwest border and the lingering effects of inflation even though it has come back down. More than 100,000 Democrats in Michigan, or 13 percent of the total, just cast protest votes for “uncommitted” to voice their dissatisfaction, most notably over Gaza.

Mr. Biden, 81, is just a little older than Mr. Trump, 77, and both have exhibited moments of confusion and memory lapses. After his annual physical this past week, Mr. Biden’s doctor pronounced him “fit for duty.” But polls show that more of the public is unsettled by Mr. Biden’s advancing years than Mr. Trump’s.

“Would I rather that Joe Biden were 65? Sure, that would be great,” said Elaine Kamarck, director of the Center for Effective Public Management at the Brookings Institution and a member of the Democratic National Committee. “But he’s not. And that’s why I think we’re in the silly season where everybody is casting around for some alternate scenario.”

The alternate scenarios remain far-fetched. The long-shot challenger, Representative Dean Phillips of Minnesota, has gotten no traction and with Super Tuesday coming up this week it is almost certainly too late for a more heavyweight candidate to jump into the contest even if any were willing to take on the president, which none seem prepared to do.

Plenty of dinner-table conversations in Washington these days focus on what would happen if Mr. Biden changed his mind at the last minute the way President Lyndon B. Johnson did in 1968 or experienced a health situation that prompted him to drop out. If that happened before the Democratic National Convention in August, it would set the stage for the first open competition at a convention in decades. After the convention, any vacancy at the top of the ticket would be filled by the Democratic National Committee.

All the talk, though, is just that. Mr. Biden is helped by the fact that no one from the next generation of Democrats waiting in the wings, like Vice President Kamala Harris or Governors Gavin Newsom of California or Gretchen Whitmore of Michigan, has a proven national following or track record of success in primaries.

“You could name five or six alternatives to Biden but they haven’t been through the system,” said Ms. Kamarck, one of the country’s leading experts on the nomination process who has just published the fourth edition of her quadrennial guide, “Primary Politics: Everything You Need to Know about How America Nominates Its Presidential Candidates.”

“We don’t know enough about them to hand them a nomination,” she continued. “It’s crazy. The whole thing is so nutty. There is no alternative.”

Ms. Kamarck said that more and more, Democrats have come to accept that. “Democrats are increasingly getting very, very vocal in their defense of Biden,” she said. “The guy’s a good guy. He’s not senile. He’s made good choices. The economy’s the best economy in the world. I mean, shut up. Let’s get behind this guy.”

The notion that someone outside his family could talk Mr. Biden into stepping aside has always been a fanciful one. There are few Democrats with the kind of gravitas that might mean something to Mr. Biden. He still feels sore that Mr. Obama gently pushed him not to run in 2016, deferring to Hillary Clinton, who went on to lose to Mr. Trump in the fall. Mr. Biden is old enough to have no mentors left and few peers from his Senate days. And Jill Biden and other family members strongly support this final run.

“There were only two people who could prevent Joe Biden from being the nominee — Joe Biden if he decides not to run or someone serious who would challenge him,” said Mr. Plouffe. And no matter how appealing a younger Democrat might seem in theory, he added, nothing is certain until someone actually runs and wins. “The political graveyard is full of people who look good on paper,” he said.

Mr. Plouffe agreed that “the concerns about his age are more pronounced than people thought” a year ago. “The only thing you can do is normalize it and ultimately take the fight to Trump.” He said he was pleased to see Mr. Biden get out more, go on late-night television and utilize Tik Tok. The more voters see him, Mr. Plouffe reasoned, the less any particular miscue might matter.

An important moment for the president to assert himself will come on Thursday night when he delivers his State of the Union address to what historically should be his largest television audience of the year. He will talk about his record and what he wants to do for the next four years. But as important as any policy pronouncement will be how he presents himself.

The president’s advisers express confidence that when the moment of decision arrives, most voters will again prefer Mr. Biden, whatever his faults, to Mr. Trump, a twice-impeached defeated former president who faces 91 felony counts, has been found liable in civil trials for sweeping business fraud and sexual assault and talks of being a “dictator” for a day.

“Where most Democrats are,” said Mr. Plouffe, “is, ‘OK, this is going to be really hard, a high degree of difficulty, but ultimately there’s probably enough of the country who doesn’t want to sign up for a second Trump term that we can make this work.’”

Depending on whom you ask, the University of Idaho’s plan to take over the University of Phoenix, a for-profit online school, is either a sweet deal or a potential disaster.

C. Scott Green, the president of University of Idaho, said he viewed the agreement with a price tag of $550 million as a hedge against what is known as the “demographic cliff,” an expected drop in the number of college-age students.

But critics of the university’s plan, like U.S. senators including Elizabeth Warren, nonprofits and a union, have questioned why the state’s top public university would team up with the University of Phoenix, known historically for its low graduation rates and misleading claims, so much so that it was recently ridiculed on “Saturday Night Live.”

The University of Idaho is just the latest publicly funded state school to consider partnering with a for-profit company as a way to develop online enrollment. Arrangements at Arizona State, Purdue and, most recently, the University of Arizona have delivered varying results as higher education faces an existential crisis.

“There are going to be lots of universities that don’t survive,” Mr. Green, an alumnus of the University of Idaho and of Harvard Business School, said in an interview.

Mr. Green, who inherited a deficit when he became president in 2019, set out to run the university as a business. He cut spending, laid off employees and merged programs. He has also worked to entice students to the campus in Moscow, a city in a remote area of the state called the Palouse, which is distinctive for its vast rolling hills covered in wheat. He even published a book on navigating the university through crisis.

College enrollment across the country is expected to peak by next year and then fall precipitously as a result of lower birthrates after the economic downturn, according to research by Nathan D. Grawe, a professor at Carleton College.

Undergraduate enrollment at Idaho has inched up recently, to around 7,400 last fall, an increase of 3.4 percent since 2022. But the future is cloudy, especially for a state with one of the country’s lowest rates of students enrolling in college immediately after high school.

Mr. Green says the University of Phoenix can deliver enrollment and revenue. But it comes with its own complicated legacy.

Founded in 1976, the University of Phoenix grew rapidly, and by 2010, it enrolled more than 450,000 students, mostly online. It aggressively promoted its brand, even acquiring naming rights to an N.F.L. stadium.

Because its enrollment skews toward lower-income students and veterans, its operations have been fueled by billions of dollars in federally backed loans and grants. But along with its growth came allegations of deceptive representation. Thousands of students said they had enrolled and amassed debt, but never gotten degrees.

In 2019, the University of Phoenix reached a $191 million federal settlement following allegations that, from 2012 to 2016, it promoted nonexistent deals with companies such as Microsoft and Twitter that would help students get jobs. The Federal Trade Commission said it would reimburse 147,000 students as a result of those claims.

Alphi Black, an Army veteran from Los Angeles, is trying to have her student loans forgiven after having enrolled at the University of Phoenix following what she said were misleading sales pitches. After earning her degree in 2018, she came to view it as a handicap.

Prospective employers “kind of laughed,” she said. “They said, ‘It’s not a real school.’”

Other University of Phoenix graduates, though, say their degrees have been valuable. In December, more than 200 of them wrote to Miguel Cardona, the education secretary, in support of Idaho’s acquisition.

“We are often dismayed at the level of focus and vitriol directed at our alma mater. It seems certain officials believe we should have pursued our degrees at a different institution,” the letter to Mr. Cardona said.

Jake Searle, a former Army pilot who lives in Kuna, Idaho, was one of the graduates who signed the letter. A working father who found it difficult to attend a traditional campus, Mr. Searle, now 41, obtained two University of Phoenix degrees, including an M.B.A. in 2019.

“The University of Phoenix was the first out of the gate,” said Mr. Searle, who now works in petroleum marketing. “They were the ones that designed and developed the online platform that I would argue every other program has adopted.”

The University of Phoenix has transformed itself, according to Andrea Smiley, a spokeswoman for the school. It has closed low-performing programs and has seen higher graduation rates since 2016, when it was acquired for $1.1 billion by a group of investors, including funds associated with Apollo Global Management. Apollo Global is led by the billionaire Marc Rowan, who directed the recent donor revolt at the University of Pennsylvania that resulted in the resignation of its president, M. Elizabeth Magill.

“The University of Phoenix is proud of who we are today and the value we offer our students and alumni,” Ms. Smiley said in an email, citing “improving student outcomes, positive external reviews by our accreditor, the satisfaction of our students with our career-focused education, and our fiscal health.”

Emphasizing the value of its enrollment, which the university says it has intentionally shrunk to a more manageable 85,000 students, and its net income of about $75 million, the University of Phoenix has been shopping itself around.

It has not been a smooth process. Last year, the University of Arkansas’s board of trustees rejected a proposal, despite the chancellor’s push for a $500 million agreement.

“Why would you lie down with a dog? You’re going to get fleas,” said C.C. Gibson III, an Arkansas lawyer and former member of the university’s board, citing Phoenix’s reputational problems.

In Idaho, the plan has roiled state politics. While Gov. Brad Little has endorsed it, Raúl Labrador, the state’s attorney general, is suing to block it. Mr. Labrador is questioning the secrecy surrounding the Idaho State Board of Education vote last year that approved the complex arrangement, in which the University of Phoenix would technically be acquired by a newly created nonprofit organization.

Members of the Idaho Legislature are questioning the deal, bolstered by a legal opinion from a lawyer with the state government who says the board lacked authority to approve it. The controversy was fanned when Idaho Education News disclosed that the University of Idaho had paid the law firm Hogan Lovells, where Mr. Green was formerly the chief operating officer, more than $7 million for advice on the deal.

“From everything I can see, and from what I know about corporate acquisitions and restructurings, this deal carries substantial risk,” said Rod Lewis, a former general counsel for a major technology company who also once headed the board that oversees the state’s public universities.

In a recent opinion piece describing his reservations, Mr. Lewis asked whether the state could be on the hook for a $685 million bond issue that is being planned to finance the deal.

There is also the sense that the University of Idaho may be late to the party. Arizona State University and Purdue already sponsor major online programs, said Byron Jones, the former chief financial officer for the University of Phoenix.

“The online market itself is kind of flattening out because of the saturation rates,” Mr. Jones said.

At the University of Arizona, a budget crisis has raised questions about its acquisition of the for-profit Ashford University in 2020. Robert Shireman, a former deputy under secretary at the U.S. Education Department, points to the program, currently operating at a loss, as a cautionary sign that public universities face “innumerable hazards and complications” when teaming up with for-profit schools.

Still, the enrollment cliff is not going away.

Even though Idaho isn’t among the states expected to be hit the hardest, Mr. Green said that other universities were already trying to poach his prospective students. At a recent recruitment event at a high school in Idaho Falls, universities from as far away as Tennessee showed up, he said.

“Our competitors are already here,” Mr. Green said. “I mean, it was unbelievable. So, you know, people are going to come for our students, because they’re going to be desperate.”

Nancy Wallace, who toiled tirelessly to clean up the only freshwater river that flows in New York City, the Bronx River, and to reclaim it for recreation and as a natural habitat, died on Feb. 15 at her home in Marblehead, Mass. She was 93.

Her death was confirmed by her daughter Lane Wallace.

Living in White Plains, N.Y., in the 1980s, Mrs. Wallace galvanized a broad campaign to rescue the river, at the time an inaccessible 23-mile watercourse that was home to more flotsam, like the carcasses of junked cars and rusted refrigerators, than fauna.

The river is mostly tidal and brackish from East Tremont Avenue in the Bronx down to where Hunts Point meets the saltwater East River, but it is considered generally fresh as it flows south from its source near the Kensico Dam in Westchester County.

While “naturally fresh” is a phrase not typically associated with the Bronx, New York’s only borough on the American mainland is, in fact, home to Pelham Bay Park, the city’s largest, as well as the New York Botanical Garden, the Bronx Zoo and the Hunts Point Produce Market — and a mostly freshwater river runs through it all.

A career educator and civic leader, Mrs. Wallace joined the board of the Bronx River Restoration in 1982. The following year, when the executive director left, she agreed to fill in temporarily. She held the job for 22 years, until she retired in 2006 — one year before biologists from the Wildlife Conservation Society in the Bronx reported a sighting of a beaver in the river for the first time in two centuries.

Mrs. Wallace almost single-handedly raised money from local governments, philanthropies and individuals for the restoration; won the support of officials in the Bronx and Westchester County; persuaded local businesses to donate construction equipment and supplies; and recruited Boy Scout troops and the City Volunteer Corps, among other groups, to help with the cleanup and restoration.

“For the long term, the basic thing we have to do is change people’s attitude toward the river,” she told The New York Times in 1988. “After all, it only became a problem because of what people did to it.”

The Bronx River Restoration and the umbrella Bronx River Alliance sought to turn an eyesore — for anyone who could find it along the Bronx River Parkway, the nation’s first — into a spot for hiking and canoeing. Conservationists reclaimed the riverbank to create improbable oases, like Starlight Park, hugging the Sheridan Expressway near East 173rd Street. The city’s parks department now calls the park “a vital link along the Bronx River Greenway.”

Ann Seaver Coolidge Upton was born on Sept. 2, 1930, in Marblehead. Her mother, Anna (Pennypacker) Upton, headed the welfare department in Marblehead, and her father, Edward, was a lawyer. Ann was called Nancy, in keeping with a family tradition of giving children an informal name.

She earned a bachelor’s degree in English from Smith College in Massachusetts in 1951 and briefly interrupted master’s degree studies to marry Bruce A. Wallace, a mechanical engineer, whom she had met while working with inner-city teenagers in Paterson, N.J.

She changed her name legally to Ann U. Wallace in 1977, when she ran successfully for the Common Council in White Plains, though she continued to be known as Nancy Wallace.

After teaching and traveling once her husband had completed his service in the U.S. Army, she settled with him in White Plains in 1959. She worked with the local parent-teacher association on a plan to desegregate the local public school system and, as a member of the Common Council, shepherded that city’s groundbreaking anti-discriminatory Fair Housing Law.

In 1982, she was importuned to join the board of Bronx River Restoration and readily accepted.

“I’d always been interested in environmental issues and causes — even my children were taught to fold their paper lunch bags very carefully and bring them home so we could reuse them,” she said in 2005.

In addition to her daughter Lane, Mrs. Wallace is survived by her husband; another daughter, Gail Wallace; a son, David; her sister, Lane Upton Serota; and three grandchildren. She moved back to her hometown with her husband in 2012.

The river restoration project had been underway for nearly a decade before Mrs. Wallace’s arrival on the board. It started as a partnership between Anthony Bouza, a Bronx police commander who was seeking to distract teenagers from delinquency, and Ruth Anderberg, who had harbored the idea of a restoration for a decade and who ultimately quit her job as a secretary at Fordham University to help initiate it.

Among Mrs. Wallace’s staunchest supporters were José E. Serrano, a state assemblyman from the Bronx at the time who later became a U.S. representative; Bea Castiglia-Catullo, a fellow member of the river restoration board; and the New York City parks commissioner, Henry J. Stern, who predicted in 1988 that the river would someday be “what it should be — a place for modern-day Huckleberry Finns.”

In helping to rescue the river, Mrs. Wallace combined political acumen, a knack for consensus building and skills in community organizing to seek common ground among suburban Westchester towns and the less affluent neighborhoods that abutted the river from the Bronx border at 242nd Street to the East River eight miles to the south.

“We’re not hoping to drink the water or anything,” she told The Times in 1988. “But we’re hoping to get it clean enough so the fish will want to come back.”

They did — American eels and the endangered alewife herring among them.

President Biden is struggling to overcome doubts about his leadership inside his own party and broad dissatisfaction over the nation’s direction, leaving him trailing behind Donald J. Trump just as their general-election contest is about to begin, a new poll by The New York Times and Siena College has found.

With eight months left until the November election, Mr. Biden’s 43 percent support lags behind Mr. Trump’s 48 percent in the national survey of registered voters.

Only one in four voters think the country is moving in the right direction. More than twice as many voters believe Mr. Biden’s policies have personally hurt them as believe his policies have helped them. A majority of voters think the economy is in poor condition. And the share of voters who strongly disapprove of Mr. Biden’s handling of his job has reached 47 percent, higher than in Times/Siena polls at any point in his presidency.

The poll offers an array of warning signs for the president about weaknesses within the Democratic coalition, including among women, Black and Latino voters. So far, it is Mr. Trump who has better unified his party, even amid an ongoing primary contest.

Mr. Biden has marched through the early nominating states with only nominal opposition. But the poll showed that Democrats remain deeply divided about the prospect of Mr. Biden, the 81-year-old chief executive, leading the party again. About as many Democratic primary voters said Mr. Biden should not be the nominee in 2024 as said he should be — with opposition strongest among voters younger than 45 years old.

Mr. Trump’s ability to consolidate the Republican base better than Mr. Biden has unified the base of his own party shows up starkly in the current thinking of 2020 voters. Mr. Trump is winning 97 percent of those who say they voted for him four years ago, and virtually none of his past supporters said they are casting a ballot for Mr. Biden. In contrast, Mr. Biden is winning only 83 percent of his 2020 voters, with 10 percent saying they now back Mr. Trump.

“It’s going to be a very tough decision — I’m seriously thinking about not voting,” said Mamta Misra, 57, a Democrat and an economics professor in Lafayette, La., who voted for Mr. Biden in 2020. “Trump voters are going to come out no matter what. For Democrats, it’s going to be bad. I don’t know why they’re not thinking of someone else.”

Mr. Trump’s five-point lead in the survey, which was conducted in late February, is slightly larger than in the last Times/Siena national poll of registered voters in December. Among the likely electorate, Mr. Trump currently leads by four percentage points.

In last year’s survey, Mr. Trump led by two points among registered voters and Mr. Biden led by two points among the projected likely electorate.

One of the more ominous findings for Mr. Biden in the new poll is that the historical edge Democrats have held with working-class voters of color who did not attend college continues to erode.

Mr. Biden won 72 percent of those voters in 2020, according to exit polling, providing him with a nearly 50-point edge over Mr. Trump. Today, the Times/Siena poll showed Mr. Biden only narrowly leading among nonwhite voters who did not graduate from college: 47 percent to 41 percent.

An excitement gap between the two parties shows up repeatedly in the survey: Only 23 percent of Democratic primary voters said they were enthusiastic about Mr. Biden — half the share of Republicans who said they were about Mr. Trump. Significantly more Democrats said they were either dissatisfied or angry at Mr. Biden being the leader of the party (32 percent) than Republicans who said the same about Mr. Trump (18 percent).

Both Mr. Trump and Mr. Biden are unpopular. Mr. Trump had a weak 44 percent favorable rating; Mr. Biden fared even worse, at 38 percent. Among the 19 percent of voters who said they disapproved of both likely nominees — an unusually large cohort in 2024 that pollsters and political strategists sometimes call “double haters” — Mr. Biden actually led Mr. Trump, 45 percent to 33 percent.

The candidate who had won such “double haters” was victorious in the elections in both 2016 and 2020.

For now, though, unhappiness with the state of the country is plainly a drag on Mr. Biden’s prospects. Two-thirds of the country feels the nation is headed in the wrong direction — and Mr. Trump is winning 63 percent of those voters.

The share of voters who believe the nation is on the right track remains a dismal and diminutive minority at 24 percent. Yet even that figure is a marked improvement from the peak inflationary days in the summer of 2022, when only 13 percent of voters felt the nation was headed in the proper direction.

“If we get Trump for another four years, we get a little better on economics,” said Oscar Rivera, a 39-year-old independent voter who owns a roofing business in Rochester, N.Y.

Mr. Trump’s policies were generally viewed far more favorably by voters than Mr. Biden’s. A full 40 percent of voters said Mr. Trump’s policies had helped them personally, compared to only 18 percent who said the same of Mr. Biden’s.

Only 12 percent of independent voters like Mr. Rivera said Mr. Biden’s policies had personally helped them, compared to 43 percent who said his policies had hurt them.

Mr. Rivera, who is Puerto Rican, said he doesn’t like the way Mr. Trump talks about immigration and the southern border, but is planning to vote for him anyway. “Biden? I don’t know,” Mr. Rivera said. “It looks like we’re weak, America’s weak. We need someone stronger.”

Overall, Mr. Biden and Mr. Trump were dead even among prized independent voters, drawing 42 percent each.

But over and over, the Times/Siena poll revealed how Mr. Trump has cut into more traditional Democratic constituencies while holding his ground among Republican groups. The gender gap, for instance, is no longer benefiting Democrats. Women, who strongly favored Mr. Biden four years ago, are now equally split, while men gave Mr. Trump a nine-point edge. The poll showed Mr. Trump edging out Mr. Biden among Latinos, and Mr. Biden’s share of the Black vote is shrinking, too.

There are, of course, unpredictable X factors in a race where the Republican front-runner is facing four indictments, 91 felony counts and a criminal trial set to begin at the end of March in New York State Supreme Court.

The poll showed that 53 percent of voters currently believe Mr. Trump has committed serious federal crimes, down from 58 percent in December. But viewed another way, Mr. Trump’s current lead over Mr. Biden is built with a significant number of voters who believe he is a criminal.

The country, meanwhile, remains divided on some of the thorniest domestic and international issues.

By a narrow margin, more voters favor making it more difficult for migrants at the southern border to seek asylum (49 percent to 43 percent). Mr. Trump and Mr. Biden made dueling appearances at the border this week; illegal border crossings set record highs at the end of 2023.

As the Israel-Hamas conflict rages in its fifth month, 40 percent of voters said they sympathized more with Israel compared to 24 percent who said they sympathized more with the Palestinians. Mr. Trump was winning 70 percent of those who backed Israel primarily; Mr. Biden was winning 68 percent of those who sided with the Palestinians, even as he has faced demonstrations and a protest vote over his pro-Israel stance.

Philip Kalarickal, a 51-year-old anesthesiologist in Decatur, Ga., is a Democrat dismayed by Mr. Biden’s handling of the humanitarian fallout from the conflict in Gaza.

“Joe Biden should be doing more to ensure that the Israeli government goes about this in a way that provides safety for them but without the civilian toll,” Dr. Kalarickal said, adding that he would reluctantly back Mr. Biden this fall, given that he lives in a swing state.

“I understand that my vote or lack of vote carries a consequence, and I look at the alternative and that’s worse than the current thing,” Dr. Kalarickal said. “But I do want to register my displeasure. The way I vote doesn’t mean I like it.”

The Biden campaign hopes that more and more voters like Mr. Kalarickal snap back into their usual partisan patterns in the coming months. The return of such reluctant Democrats is one reason the Biden campaign has been optimistic that polling will narrow, and eventually flip, as the choice between Mr. Trump and Mr. Biden becomes clearer.

Nikki Haley, Mr. Trump’s Republican rival, who has made the case that he will lose in November, leads Mr. Biden by double the margin of the former president: a hypothetical 45 percent to 35 percent. But she has struggled to gain traction in the primary and the poll portends landslide losses on Super Tuesday next week, with 77 percent of Republican primary voters picking Mr. Trump over her.

Alyce McFadden and Ruth Igielnik contributed reporting.

The New York Times/Siena College poll of 980 registered voters nationwide was conducted on cellular and landline telephones, using live interviewers, from Feb. 25 to 28, 2024. The margin of sampling error for the presidential ballot choice question is plus or minus 3.5 percentage points among registered voters. Cross-tabs and methodology are available here.

Two scientists who worked at Canada’s top microbiology lab passed on secret scientific information to China, and one of them was a “realistic and credible threat to Canada’s economic security,” documents from the national intelligence agency and a security investigation show.

The hundreds of pages of reports about the two researchers, Xiangguo Qiu and Keding Cheng, who were married and born in China, were released to the House of Commons late Wednesday after a national security review by a special parliamentary committee and a panel of three retired senior judges.

Canadian officials, who have warned that the country’s academic and research institutions are a target of Chinese intelligence campaigns, have tightened rules around collaborating with foreign universities. Canadian universities can now be disqualified from federal funding if they enter into partnerships with any of 100 institutions in China, Russia and Iran.

The release of the documents was the subject of a prolonged debate in Parliament that began before the last federal election, in September 2021. Opposition parties asked to see the records at least four times and found the Liberal government to be in contempt of Parliament in 2021. The government filed a lawsuit in an attempt to keep the records hidden, but dropped it when the vote was called.

The release comes as the country is holding a special inquiry led by a judge to look into allegations that China and other foreign nations have interfered in Canadian elections and political parties. Some of Prime Minister Justin Trudeau’s political opponents have charged that his government has failed to respond adequately to Chinese meddling in Canadian affairs.

But Mark Holland, the federal health minister in Canada, told reporters late Wednesday that at “no time did national secrets or information that threatened the security of Canada leave the lab.”

The couple were escorted out of their labs at the National Microbiology Laboratory in Winnipeg, Manitoba, during the summer of 2019 and later stripped of their security clearances. They were fired in January 2021.

The same year, the government released heavily redacted records about their dismissal, setting off a battle with opposition parties that were demanding more detail about the security breach.

The large cache of newly released documents, which have significantly fewer redactions, offer more details about the scientists’ unauthorized cooperation and information exchanges with Chinese institutions. The documents also revealed that Dr. Qiu had not disclosed formal agreements with Chinese agencies in which a Chinese institution agreed to pay substantial amounts of research money. It also agreed to pay her an annual salary of 210,000 Canadian dollars (about $155,000).

The couple could not be located, and they did not appear to have any obvious local representatives. Some Canadian news outlets have reported, based on undisclosed sources, that they moved to China after being dismissed. The Royal Canadian Mounted Police opened a criminal investigation in 2021, but its status is unclear and no charges have been laid.

The documents released on Wednesday do not include any general response from the couple. But they show that during questioning by investigators, Dr. Qiu repeatedly said that she was not aware that she had broken any security rules, blamed the health agency for not fully explaining procedures and frequently tried to mislead investigators until confronted by contradictory evidence.

In a letter to Dr. Qiu, the public health agency said that she “did not express remorse or regret. You failed to accept responsibility for your actions and deflected blame onto P.H.A.C.” It added that she did not show “any signs of corrective behavior, rehabilitation or desire for resolution of the situation.”

The Canadian Security Intelligence Service also found that Dr. Qiu repeatedly misrepresented her ties to researchers and organizations in China, relationships it characterized as “close and clandestine.”

In one secret report, the intelligence agency said that when she was asked about her exchanges with scientists and organizations in China, she “continued to make blanket denials, feign ignorance or tell outright lies.”

An internal investigation report for the Public Health Agency of Canada, which includes the lab, shows that the couple fell under suspicion in 2018, when Dr. Qiu was named an inventor on a patent granted in China that appeared to use research developed by the agency for an Ebola vaccine.

That revelation, in turn, suggested that the couple had engaged in several violations of security rules at the laboratory, portions of which are designed for work on the world’s most lethal microbes, including ones that could be used for biological warfare.

Those breaches included attempts by graduate students of Dr. Qiu at the University of Manitoba, all of whom were Chinese nationals, to remove material from the lab and being allowed to wander through the facility unescorted.

In one episode, X-rays revealed that a parcel delivered to the lab for Dr. Cheng — and labeled “kitchen utensils” — contained vials of mouse proteins. The discovery underscored that Dr. Cheng had broken protocols, according to the documents.

An investigation by the intelligence agency found that Dr. Qiu had a formal agreement with Hebei Medical University to work on a “talent program,” something it described as a project “to boost China’s national technological capabilities.”

A report documenting the investigation added that it “may pose a serious threat to research institutions, including government research facilities, by incentivizing economic espionage.” That agreement promised about 1.2 million Canadian dollars (roughly $884,000) in research funding. The agency said the couple did not disclose, as required, that they maintained a bank account in China.

Dr. Qiu, the intelligence service said, also had a résumé she used only in China that showed she was a visiting professor at three Chinese health research institutes and a visiting researcher at a fourth one.

Exactly what information Dr. Qiu may have provided to China and how China may have used it is not clear either from the internal investigation or the intelligence agency reports.

The intelligence service said that many of the institutions she worked with researched “potentially lethal military applications.” When asked as part of an internal investigation about the potential military uses of her work, Dr. Qiu said that the idea had not occurred to her, the documents show.

The internal investigation found that a trip Dr. Qiu made to Beijing in 2018 was paid for by a Chinese biotechnology company.

Mr. Holland said that the lab’s management had demonstrated an “inadequate understanding of the threat of foreign interference.”

He added, “I believe that an earnest effort was made to adhere to those policies, but not with the rigor that was required.”

In a statement, Pierre Poilievre, the Conservative leader, said that the Chinese government and its agencies, “including the People’s Liberation Army, were allowed to infiltrate Canada’s top-level lab.” The statement added, using the abbreviation for the People’s Republic of China, “They were able to transfer sensitive intellectual property and dangerous pathogens to the P.R.C.”

Vjosa Isai contributed reporting from Toronto.

Let’s just say it: Joe Biden should be expected to win this election. He’s an incumbent president running for re-election with a reasonably healthy economy against an unpopular opponent accused of multiple federal crimes.

And yet President Biden is not winning, at least not now. Polls show him trailing in states worth well over 270 electoral votes, and this morning he lags Donald J. Trump in our newest New York Times/Siena College national poll by five percentage points among registered voters, 48 percent to 43 percent.

That’s the largest lead Mr. Trump has ever had in a Times/Siena national poll. In fact, it’s the largest lead Mr. Trump has held in a Times/Siena or Times/CBS poll since first running for president in 2015.

Why is President Biden losing? There are many possible reasons, including his age, the war in Gaza, the border and lingering concerns over inflation. But ultimately, they add up to something very simple: Mr. Biden is very unpopular. He’s so unpopular that he’s now even less popular than Mr. Trump, who remains every bit as unpopular as he was four years ago.

President Biden’s unpopularity has flipped the expected dynamic of this election. It has turned what looked like a seemingly predictable rematch into a race with no resemblance to the 2020 election, when Mr. Biden was a broadly appealing candidate who was acceptable to the ideologically diverse group of voters who disapproved of Mr. Trump.

Instead, many voters will apparently agonize between two candidates they dislike. It’s exactly what Democrats sought to avoid when they nominated Mr. Biden in 2020. It’s what Democrats largely avoided in the 2018 and 2022 midterm elections, when they mostly nominated acceptable candidates or ran incumbents against right-wing opponents. And it’s exactly what led to the election of Mr. Trump in 2016.

Overall, 19 percent of registered voters in the Times/Siena survey have an unfavorable view of both candidates — a group sometimes referred to as “double haters.” These voters say they backed Mr. Biden by a three-to-one margin among those who voted in 2020, but now he holds the support of less than half. Every vote counts, but these voters will undoubtedly be pivotal in deciding the November election.

The double haters might ultimately return to Mr. Biden’s side. There are still eight months left until November, and it’s not as if these voters like Mr. Trump. If they do come back to Mr. Biden, perhaps their return will have seemed inevitable in retrospect.

But from today’s vantage point, we can’t know what will happen. What we know is that the choice for these voters is much more difficult for them than it was four years ago, when they said they liked Mr. Biden. They don’t today. It creates the conditions for a volatile race, and it might just be enough to flip their preference for president as well.

You can read the full article on the poll here.

A few other items of note:

• In our last poll in December, Mr. Biden led by two points among likely voters, even though he trailed by two among the wider set of registered voters. But in this poll, Mr. Trump holds a four-point lead among likely voters. That’s still better for Mr. Biden than his five-point deficit among registered voters, and it continues a pattern of unusual Biden strength among the likeliest voters, but the difference is no longer enough to give Mr. Biden the lead.

  Mr. Biden’s strength remains relatively concentrated among the most regular voters, as he holds a 46-45 lead among those who have voted in a midterm or a primary. He trails by only two points among those “almost certain to vote.” But many other voters will turn out in a general election, and at least in this particular poll they’re enough to give Mr. Trump a modest lead.

• The poll found Mr. Trump leading Nikki Haley in the Republican primary, 77-20. That’s pretty good for Mr. Trump, of course, but it’s actually Ms. Haley’s best result in a month. And according to our poll, there’s a simple reason for her strength: Biden voters, who now make up 15 percent of those who say they will probably vote in the Republican primary. In fact, a near majority of Ms. Haley’s supporters (48-31) say they voted for Mr. Biden in the last election instead of Mr. Trump.

• Mr. Biden’s support among nonwhite voters keeps sinking. He held just a 49-39 lead among the group, even though nonwhite respondents who voted in the 2020 election said they backed Mr. Biden, 69-21.

• Despite the positive economic news over the last few months, 51 percent of voters still said the economy was “poor.” In a strange way, perhaps that’s good news for Mr. Biden: Maybe his standing will improve if or when voters begin to gain confidence that the economy has turned the corner.

• Even at this late stage, Democrats are still divided over whether Mr. Biden should be the nominee, with 46 percent saying he should be and 45 percent saying he shouldn’t. We didn’t ask whether Mr. Biden should drop out of the race. We considered it — in fact, we discussed it for days — but many respondents may not know the complications involved in a contested convention.

ORIGINS

DNA has shown that the extinct humans thrived around the world, from chilly Siberia to high-altitude Tibet — perhaps even in the Pacific islands.

Neanderthals may have vanished 40,000 years ago, but they are no strangers to us today. Their stocky skeletons dazzle in museums around the world. Their imagined personas star in television ads. When Kevin Bacon noted on Instagram that his morning habits are like those of a Neanderthal, he did not stop to explain that our ancient cousins interbred with modern humans expanding out of Africa.

But there’s no such familiarity with the Denisovans, a group of humans that split from the Neanderthal line and survived for hundreds of thousands of years before going extinct. That’s largely because we have so few of their bones. In a new review paper, anthropologists tally all of the fossils that have been clearly identified as Denisovan since the first discovery in 2010. The entire list consists of half a broken jaw, a finger bone, a skull fragment, three loose teeth and four other chips of bone.

“The bits of Denisovan we have, it’s almost nothing,” said Janet Kelso, a paleoanthropologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, who helped write the review.

Nevertheless, many scientists are growing increasingly fascinated by Denisovans. Like us, they were extraordinarily resilient, arguably more so than Neanderthals. “I find Denisovans way more interesting,” said Emilia Huerta-Sánchez, a geneticist at Brown University.

What the Denisovans lack in fossils they make up for in DNA. Geneticists have been able to extract bits of genetic material from teeth and bones dating back 200,000 years. They have found genetic clues in the dirt of cave floors. And billions of people on Earth carry Denisovan DNA, inherited from interbreeding.

The evidence offers a picture of remarkable humans who were able to thrive across thousands of miles and in diverse environments, from chilly Siberia to high-altitude Tibet to woodlands in Laos — perhaps even in the Pacific islands. Their versatility rivals our own.

“What we have found out about Denisovans is that, from a behavioral perspective, they were much more like modern humans,” said Laura Shackelford, a paleoanthropologist at the University of Illinois.

The Denisovans get their name from the Denisova Cave in Siberia, where their remains were first identified. Russian paleontologists had been pulling up bits of bone from the cave floor for years when Dr. Kelso and other researchers offered to search them for DNA.

A molar tooth somewhere between 122,700 and 194,400 years old contained Neanderthal-like genes. But the tooth’s DNA was distinct enough to suggest it had come from a separate branch of human evolution. A finger bone dating back 51,600 to 76,200 years belonged to the same lineage, demonstrating that it had existed for tens of thousands of years — if not more.

Since then, researchers have found more Denisovan fossils in the cave, and they have also gathered loose genetic material from the cave floor. The samples date from 200,000 to 50,000 years ago. A 90,000-year-old bone fragment belonged to a Denisovan-Neanderthal hybrid, showing that the two groups sometimes interbred.

Dr. Kelso and her colleagues quickly came to suspect that the Denisovans had not been limited to Siberia. The researchers found that some stretches of the ancient humans’ DNA closely matched genetic material carried around by people in East Asia, Native Americans, Aboriginal Australians and people in New Guinea and other islands in the area.

When modern humans expanded out of Africa some 60,000 years ago, the Denisovans must have been in their path in order to interbreed and introduce some of their genes into our lineage. But it wasn’t until 2019 that scientists found the first fossil trace of Denisovans beyond Siberia, in a high-altitude cave in Tibet.

Researchers there found part of a jaw dating back more than 160,000 years with Denisovan-like teeth. It also contained proteins with a molecular structure that might be expected from a Denisovan, based on their genes. The following year, the researchers reported that the cave floor contained Denisovan DNA.

In 2022, Dr. Shackelford and her colleagues made a discovery that could extend the Denisovan range to Southeast Asia, right in the path of modern humans in their early waves out of Africa. In a cave in Laos, they found a tooth about as old as the Denisovan jaw, and matching the tooth embedded in it.

The Laotian tooth did not yield any DNA, however, so the researchers have started sifting through sediments in nearby caves. “We have loads of DNA,” Dr. Shackelford said. “But we don’t know yet what all that DNA represents.”

Other researchers are surveying the Denisovan DNA inherited by living people. The pattern of mutations documented so far suggests that several genetically distinct groups of Denisovans interbred with our ancestors. What’s more, none of those Denisovan groups was closely related to the ones who occupied the Denisova cave.

Some of the most intriguing results have come from studies on people in New Guinea and the Philippines. They show signs of repeated instances of interbreeding with Denisovans that were distinct from what occurred on mainland Asia. Dr. Kelso and other experts on Denisovans suspect that when sea levels were low during the last ice age, Denisovans may have walked to New Guinea and the Philippines, where they lived for thousands of years before modern humans arrived.

Put together, these findings suggest that Denisovans thrived in vastly different environments. They endured the harsh winters of Siberia and the thin air of the Tibetan plateau. In Laos, Dr. Shackleford and her colleagues have found that Denisovans lived in open woodlands with herds of dwarf elephants and other mammals to hunt. And they may have lived in rainforests in New Guinea and the Philippines.

That flexibility stands in sharp contrast to Neanderthals, who adapted to the cold climate of Europe and western Asia but did not expand elsewhere.

The Denisovans’ versatility may have helped them last for a long time. People in New Guinea may have inherited some Denisovan DNA from interbreeding just 25,000 years ago.

Dr. Shackelford said findings like these raised the possibility that Denisovans and modern humans coexisted and interacted for tens of thousands of years — though whether they communicated is unclear. “That’s really going down the rabbit hole,” Dr. Shackelford said.

After the Denisovans disappeared, their genetic legacy lived on. Certain genes of Denisovans have become more common because they provide an evolutionary advantage in modern humans. In Tibet, Dr. Huerta-Sánchez and her colleagues have found a Denisovan gene that helps people survive at high altitudes. She has also found that Native Americans carry a Denisovan gene for a mucus protein, though its benefit remains a mystery.

In New Guinea, some Denisovan genes have been favored by people living in the lowlands, while others are favored in the highlands. The lowland genes appear to help fight infections. It’s possible that high rates of malaria and other diseases make those genes valuable.

But in the highlands, the Denisovan genes with the evolutionary advantage are active in the brain. Michael Dannemann, an evolutionary geneticist at the University of Tartu in Estonia who led the New Guinea study, speculated that at high altitudes in New Guinea, people might face periods of food shortages. “You might have to adapt body parts that use a lot of energy, and one that consumes a lot of energy in humans is the brain,” he said.

Dr. Shackelford predicted that the search for more Denisovan fossils would be hard, because the humid conditions in places like Laos don’t favor the survival of skeletons. “I’m begging for bones,” she said. “But I am going to be wanting bones for a long time.”