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The technique holds the power to repair or enhance any human gene. “It raises the most fundamental of issues about how we are going to view our humanity in the future and whether we are going to take the dramatic step of modifying our own germline and in a sense take control of our genetic destiny, which raises enormous peril for humanity,”

The paper’s authors, however, are concerned about countries that have less regulation in science. They urge that “scientists should avoid even attempting, in lax jurisdictions, germline genome modification for clinical application in humans” until the full implications “are discussed among scientific and governmental organizations.”

Though such a moratorium would not be legally enforceable and might seem unlikely to exert global influence, there is a precedent. In 1975, scientists worldwide were asked to refrain from using a method for manipulating genes, the recombinant DNA technique, until rules had been established.

Though highly efficient, the technique occasionally cuts the genome at unintended sites. The issue of how much mistargeting could be tolerated in a clinical setting is one that Dr. Doudna’s group wants to see thoroughly explored before any human genome is edited.

“We worry about people making changes without the knowledge of what those changes mean in terms of the overall genome,” Dr. Baltimore said. “I personally think we are just not smart enough — and won’t be for a very long time — to feel comfortable about the consequences of changing heredity, even in a single individual.”

Many ethicists have accepted the idea of gene therapy, changes that die with the patient, but draw a clear line at altering the germline, since these will extend to future generations. The British Parliament in February approved the transfer of mitochondria, small DNA-containing organelles, to human eggs whose own mitochondria are defective. But that technique is less far-reaching because no genes are edited.

There are two broad schools of thought on modifying the human germline, said R. Alta Charo, a bioethicist at the University of Wisconsin and a member of the Doudna group. One is pragmatic and seeks to balance benefit and risk. The other “sets up inherent limits on how much humankind should alter nature,” she said. Some Christian doctrines oppose the idea of playing God, whereas in Judaism and Islam there is the notion “that humankind is supposed to improve the world.” She described herself as more of a pragmatist, saying, “I would try to regulate such things rather than shut a new technology down at its beginning.

The Doudna group calls for public discussion, but is also working to develop some more formal process, such as an international meeting convened by the National Academy of Sciences, to establish guidelines for human use of the genome-editing technique.“We need some principled agreement that we want to enhance humans in this way or we don’t,” Dr. Jaenisch said. “You have to have this discussion because people are gearing up to do this.”

While conspiracy theories might falsely claim the virus was concocted in a lab, the virus’s genome makes clear that it arose in bats.

Sophisticated computer programs can then figure out how all of those mutations arose as viruses descended from a common ancestor. If they get enough data, they can make rough estimates about how long ago those ancestors lived. That’s because mutations arise at a roughly regular pace, like a molecular clock.

Tracking viral mutations demands sequencing all the genetic material in a virus — its genome. Once researchers have gathered the genomes from a number of virus samples, they can compare their mutations.

In January, a team of Chinese and Australian researchers published the first genome of the new virus. Since then, researchers around the world have sequenced over 3,000 more. Some are genetically identical to each other, while others carry distinctive mutations.

The most closely related coronavirus is in a Chinese horseshoe bat, the researchers found. But the new virus has gained some unique mutations since splitting off from that bat virus decades ago.

Dr. Boni said that ancestral virus probably gave rise to a number of strains that infected horseshoe bats, and perhaps sometimes other animals.

It’s entirely possible, Dr. Boni said, in the past 10 or 20 years, a hybrid virus arose in some horseshoe bat that was well-suited to infect humans, too. Later, that virus somehow managed to cross the species barrier.

already, the genomes of the virus are revealing previously hidden outlines of its history over the past few months.

While the coronavirus mutations are useful for telling lineages apart, they don’t have any apparent effect on how the virus works.

The deepest branches of the tree all belong to lineages from China. The Nextstrain team has also used the mutation rate to determine that the virus probably first moved into humans from an animal host in late 2019.

In January, as the scope of the catastrophe in China became clear, a few countries started an aggressive testing program. They were able to track the arrival of the virus on their territory and track its spread through their populations.

But the United States fumbled in making its first diagnostic kits and initially limited testing only to people who had come from China and displayed symptoms of Covid-19.“It was a disaster that we didn’t do testing,”

As new cases arose in other parts of the country, other researchers set up their own pipelines. The first positive test result in New York came on March 1, and after a couple of weeks, patients surged into the city’s hospitals.

Dr. Heguy and her colleagues found some New York viruses that shared unique mutations not found elsewhere. “That’s when you know you’ve had a silent transmission for a while,”

And researchers at Mount Sinai started sequencing the genomes of patients coming through their hospital. They found that the earliest cases identified in New York were not linked to later ones.“Two weeks later, we start seeing viruses related to each other,”

Dr. Gonzalez-Reiche and her colleagues found that these viruses were practically identical to viruses found around Europe.

hey write that the viruses reveal “a period of untracked global transmission between late January to mid-February.”

Dr. van Bakel and his colleagues found one New York virus that was identical to one of the Washington viruses found by Dr. Bedford and his colleagues. In a separate study, researchers at Yale found another Washington-related virus. Combined, the two studies hint that the coronavirus has been moving from coast to coast for several weeks.

Dr. Boni and his colleagues found that the genome of the new virus contains a number of mutations in common with strains of coronaviruses that infect bats.

Some viruses evolve so quickly that they require vaccines that can produce several different antibodies. That’s not the case for Covid-19. Like other coronaviruses, it has a relatively slow mutation rate compared to some viruses, like influenza.

The lowest amount of African admixture occurs in the Druse, a religious group of the Middle East that prohibited slavery and has been closed to converts since A.D. 1043.

Another mixing event is the injection of European-type DNA into the Kalash, a people of Pakistan, at some time between 990 and 210 B.C. This could reflect the invasion of India by Alexander the Great in 326 B.C. The Kalash claim to be descended from Alexander’s soldiers

Though all humans have the same set of genes, their genomes are studded with mutations, which are differences in the sequence of DNA units in the genome. These mutations occur in patterns because whole sets of mutations are passed down from parent to child and hence will be common in a particular population

The dating system is based on measuring the length of chromosome segments of a particular ancestry that occur in a population. When people of two different populations intermarry, their children’s genomes carry large chunks of DNA of one parent’s ancestry interspersed with large chunks from the other’s.

from the average size of the chunks in a person’s genome, the geneticists can calculate the number of generations since the mixing event.

One of the most widespread events his group has detected is the injection of Mongol ancestry into populations within the Mongol empire, such as the Hazara of Afghanistan and the Uighur Turks of Central Asia.

the European colonization of America is recorded in the genomes of the Maya and Pima Indians. And Cambodian genomes mark the fall of the Khmer empire in the form of ancestral DNA from the invading Tai people.

They find among Northern Italians an insertion of Middle Eastern DNA that occurred between 776 B.C. and A.D. 550, and may represent the Etruscans, a mysterious people said by the ancient Greek historian Herodotus to have emigrated from Lydia in Turkey.

his method cannot yet detect genetic mixing between very similar populations, as was the case with the English and their invaders from Scandinavia and Northern Germany.

“In some sense we don’t want to talk to historians,” Dr. Falush said. “There’s a great virtue in being objective: You put the data in and get the history out. We do think this is a way of reconstructing history by just using DNA.”

The study reveals, for the first time, the genetic sequence of the virus that may have infected Mr. Trump and dozens of others, researchers said.

Viruses constantly mutate, picking up tiny, accidental alterations to their genetic material as they reproduce. Few mutations alter how a virus functions. But by comparing patterns of mutations across many genetic sequences, scientists can construct family trees of a virus, illuminating how it spreads.

The genomes believed by these researchers to be connected to the White House outbreak do not identify a recent geographic source, in part because they are unusual.

The results show that even weeks after it was identified, the White House outbreak would be better understood by sequencing samples of more people who were infected.

In a study released on Thursday, the C.D.C. cited genetic sequencing and intensive contact tracing that documented an super-spreading event at a high school retreat in Wisconsin.

But the Trump administration is not known to have conducted its own genetic analysis of people infected in the outbreak. The White House declined to respond to questions on genetic sequencing of Mr. Trump and the cluster of aides and officials who tested positive or became ill.

Scientists not involved in the research who reviewed the results agreed with the conclusion that the two samples sharing rare mutations strongly suggested they are part of the same outbreak.

“These genomes are probably going to be identical or nearly identical to the genome that infected the president,” said Michael Worobey, head of the department of ecology and evolutionary biology at the University of Arizona.

For months, the White House minimized the threat of the virus and eschewed basic safety precautions at official events, like wearing a mask or keeping people six feet apart.

At least 11 people who attended a Rose Garden celebration on Sept. 26 for Judge Barrett, which included an indoor event without masks, became infected with the coronavirus, including Mr. Trump.

The work is convincing, and it is the best way to piece together the progression of such an outbreak, said David Engelthaler, head of the infectious disease branch of the Translational Genomics Research Institute in Arizona, where he and colleagues have sequenced thousands of genomes to track the spread of the coronavirus, including devastating outbreaks at Native American reservations in the state.

The Ust’-Ishim man’s genome suggests he belonged to a group of people who lived after the African exodus, but before the split between Europeans and Asians.

By comparing the Ust’-Ishim man’s long stretches of Neanderthal DNA with shorter stretches in living humans, Dr. Paabo and his colleagues estimated the rate at which they had fragmented. They used that information to determine how long ago Neanderthals and humans interbred.

Humans and Neanderthals interbred 50,000 to 60,000 years ago, according to the new data.

The findings raised questions about research suggesting that humans in India and the Near East dated back as far as 100,000 years ago. Some scientists believe that humans expanded out of Africa in a series of waves.

the new study offered compelling evidence that living non-Africans descended from a group of people who moved out of Africa about 60,000 years ago.Any humans that expanded out of Africa before then probably died out, Mr. Stringer said.

One of the oldest genomes studied came from a thigh bone discovered in Goyet Cave in Belgium and given the unwieldy name GoyetQ116-1. Radiocarbon dating pegs the Goyet individual at some 35,000 years old,

Around 1,000 years after the Goyet individual was found, a new culture swept through Europe: the Gravettians. Analysis of genetic material from the time shows that art and artifacts weren't the only things changing. The Gravettians' DNA was significantly different from their Aurignacian predecessors, suggesting that they were a completely separate lineage.

Goyet guy's descendants retreated to the Iberian Peninsula (modern day Spain and Portugal) and waited for their time to come again.

It did, some 15,000 years later. Probably spurred by climate changes as glaciers began to recede, this dormant lineage expanded back into the rest of Europe, bearing a new culture known as Magdalenian.

Not long after that, their genomes started to look like those of people from the Middle East and the Caucasus, suggesting that new arrivals from the southeast were mingling with — and in some cases supplanting — the existing population.

This was a surprise, because researchers used to think that transition happened much later, when Turkish farmers introduced agriculture to Europe some 8,500 years ago.

The genetic analysis allowed researchers to trace the inexorable decline of Neanderthal DNA, which was two to three times more prominent in early human genomes than it is in modern-day ones. This supports theories that early humans interbred with Neanderthals, but that their DNA was toxic to us and gradually weeded out by natural selection over the course of millennia.

The problem echoes the country’s catastrophic stumbles early in the pandemic, when a lack of testing allowed the virus to spread widely. Currently, only a tiny fraction of all positive coronavirus tests in the United States are forwarded for further sequencing.

t if scientists don’t know what strains are moving through the population, the mutations that matter may pop up undetected.

For months, scientists have been sounding alarms and trying to ramp up genetic sequencing of test samples, but the effort has been plagued by a lack of funding, political will and federal coordination

Centers for Disease Control and Prevention Director Rochelle Walensky said Friday that the government is increasing the level of sequencing nationwide.“We have scaled up surveillance dramatically just in the last 10 days, in fact. But our plans for scaling up surveillance are even more than what we’ve done so far,”

Ultimately, the country needs real-time data — similar to the dashboards now used to track daily cases, hospitalizations and deaths — to track variants and their prevalence across the country

“None of that exists right now. We’re incredibly behind compared to other countries,”

The U.S. effort is so underdeveloped that it’s impossible to say exactly how many virus cases are sequenced daily.

The CDC has warned that the variant found in the United Kingdom — which British scientists said could be up to 70 percent more transmissible — could become dominant in the United States by March.

It also recently contracted with four private companies — Quest, Labcorp, Illumina and Helix — to conduct more sequencing. By mid-February, those contracts should hit full capacity, analyzing 6,000 samples per week, CDC officials said.

Illumina estimates that the country needs to sequence 5 percent of its coronavirus cases to detect a new variant when the variant represents about 0.1 percent to 1.0 percent of the country’s case

However, the United States so far has only sequenced about 0.32 percent of its total cases

the country ranks 38th out of 130 countries reporting whole-genome sequencing data.

The United States has sequenced 84,177 samples out of 25.7 million cases as of Friday, according to a Washington Post analysis. By comparison, the United Kingdom, in ninth place, has sequenced 214,000 genomes — almost 6 percent — of the country’s 3.7 million cases.

Unlike the United States, the U.K. invested in genetic sequencing early on in the pandemic, launching its genomics consortium in March with a $27 million investment and a multimillion-dollar boost late last year.

Even before the emergence of mutations such as the variants first discovered in South Africa and the United Kingdom, U.S. experts had been warning for months about the need for a national standard for genetic surveillance.

In May, the CDC launched a surveillance program for the coronavirus called SPHERES (SARS-CoV-2 Sequencing for Public Health Emergency Response, Epidemiology, and Surveillance). But, in practice, the program relied on a haphazard patchwork of academic labs contributing genetic sequencing on a volunteer basis.

A July report by the National Academies of Science said that “poor funding, coordination, and capacity” had led to a “patchy, typically passive, and reactive” U.S. sequencing effort.

the cost to collect molecular data from each blood sample was about $2,500 — which did not include the cost of analysis. But the price for tests similar to Dr. Snyder’s will also decline in the future

Dr. Church looks forward to the day when current research becomes a routine clinical procedure that combines inherited genomic information with analyses of RNA, proteins, metabolites and microbes in our bodies.

Dr. Snyder is a co-founder of a company, Personalis, in Palo Alto, Calif., that is developing software and other tools to interpret genomes after sequencing

While some diseases, like cystic fibrosis, are caused by a single genetic mutation, the most common ones are not. Instead, mutations to a number of different genes can each raise the risk of getting, say, heart disease or breast cancer. Discovering these mutations can shed light on these diseases and point to potential treatments. But many of them are rare, making it necessary to search large groups of people to find them.

The wealth of data created in Iceland may enable scientists to begin doing that

For example, they found eight people in Iceland who shared a mutation on a gene called MYL4. Medical records showed that they also have early onset atrial fibrillation, a type of irregular heartbeat.

they identified a gene called ABCA7 as a risk factor for Alzheimer’s disease.Previous studies had suggested a gene in the genetic neighborhood of ABCA7 was associated with the disease.But the Icelandic study pinpointed the gene itself — and even the specific mutation involved.

Since Dr. Stefansson and his colleagues submitted their initial results for publication, they have continued gathering DNA from Icelanders.The scientists now have full genomes from about 10,000 Icelanders and partial genetic information on 150,000 more.

Dr. Stefansson said that means that his firm could generate a report for genetic disease on every person in Iceland

Iceland is a particularly fertile country for doing genetics research. It was founded by a small number of settlers from Europe arriving about 1,100 years ago. Between 8,000 and 20,000 people came mainly from Scandinavia, Ireland and Scotland.

The country remained isolated for the next thousand years, and so living Icelanders have a relatively low level of genetic diversity. This makes it easier for scientists to detect genetic variants that raise the risk of disease, because there are fewer of them to examine.

celand also has impressive genealogical records. Through epic poems and historical documents, many Icelanders can trace their ancestry back to the nation’s earliest arrivals. Geneticists use national genealogy databases to look for diseases that are unusually common in relatives — a sign that they share a mutation.

the company is now investigating a gene, found by Decode, with a strong link to cardiovascular disease in Iceland. (He declined to name the gene.)

Working along an east-west transect through central England and Wales, the scientists discovered that the mix of Y-chromosomes characteristic of men in the English towns was very different from that of men in the Welsh towns: Wales was the primary Celtic holdout in Western Britannia during the ascendance of the Anglo-Saxons. Using computer analysis, the researchers explored how such a pattern could have arisen and concluded that a massive replacement of the native fourth-century male Britons had taken place. Between 50 percent and 100 percent of indigenous English men today, the researchers estimate, are descended from Anglo-Saxons who arrived on England’s eastern coast 16 centuries ago.

So what happened? Mass killing, or “population replacement,” is one possible explanation. Mass migration of Anglo-Saxons, so that they swamped the native gene pool, is another.

Yet no archaeological or historical evidence from the fifth and sixth centuries hints at the immense scale of violence or migration that would be necessary to explain this genetic legacy. The science hinted at an untold story.

across entire fields of inquiry, the traditional boundaries between history and prehistory have been melting away as the study of the human past based on the written record increasingly incorporates the material record of the natural and physical sciences.

The study of the human past, in other words, has entered a new phase in which science has begun to tell stories that were once the sole domain of humanists.

Thomas had found that genetically, not one of the English towns he sampled was significantly different from the others. Welsh towns, on the other hand, were significantly different from each other and from the English towns.

Most importantly, he found that inhabitants of  the Dutch province of Friesland were indistinguishable genetically from the English town-dwellers. Friesland is one of the known embarkation points of the Angl0-Saxons--and the language spoken there is the closest living relative to English.

The implications are profound: “Suddenly, we have all these genuine historical observations that need to be taken on board by historians and archaeologists and they raise a whole series of new questions, focusing particularly on…what is going on at the intimate level of this new civilization that is being born in the ruins of the Roman empire. The history of Europe will never be the same.”

But most archaeologists and historians who understand the economic capacity of the era, he noted, “find such massive contributions to the English gene pool to be completely unacceptable.

“But still, the genetic data are quite robust,” Thomas pointed out. “This is where the idea of an apartheid-like social structure comes in.” He has advanced a theory that a sexually biased, ethnically driven reproductive pattern, in which Anglo-Saxon males fathered children with Anglo-Saxon females and possibly Celtic females, while the reproductive activities of Romano-Celtic males were more restricted, is the most plausible explanation for the demographic, archaeological, and genetic patterns seen today. 

In an attempt to explain the remarkable similarity between Frisian and English towns, Thomas and colleagues constructed a population simulation model on a computer. He tested many theories: common ancestry dating back to the Neolithic age; background migration over centuries and even millennia; and a mass-migration event that, he calculated, would have had to involve at least 50 percent replacement--the movement, in other words, of a million people.

Simulating such an advantage, and choosing an arbitrary figure of 10 percent migration, Thomas found that the Y chromosomes of native Britons could have been replaced in the general population in as few as five generations. 

by the 1970s, he continues, scholars began to realize there never was a homogenous “nation” of Germans in northern Europe, just small tribes that coalesced along the Roman frontier in what were political and cultural, rather than biological, federations, as their very names suggest: Alemanni, meaning “all men”; Goths, meaning “good guys.”

The Romans, scholars believed, provided a common enemy, and that unified the disparate Germanic tribes. This line of reasoning led historians to a further thought: maybe the Anglo-Saxon identity was similarly socially constructed, and not biological after all

More recent historical scholarship, therefore, has increasingly emphasized discovering the extent to which the barbarian migrations were really a process of ethnogenesis--the creation of new ethnic identities, as the merchant’s story illustrates.

“There is lots of evidence for it,” McCormick says. “But now you have Mark Thomas telling us that you could actually study mating patterns. That is utterly unanticipated.” The work raises a host of new questions: What was women’s role in the barbarian settlements? Were Anglo-Saxon men mating with Celtic women? Or were there women in those invading boats, and if so, how many? What happened to the Romano-Celtic men? Were they killed? 

There is some support for this in ancient English laws, which indicate that Britons and Anglo-Saxons were legally and economically different even in the seventh century, long after the initial migration. Thomas cited wergild (blood money) payments as one example: “Killing an Anglo-Saxon was a costly business, but killing a native Briton was quite cheap.” This points to differences in economic status. And differences in wealth “almost always result in differences in reproductive output,” he said. “Sometimes two- and three-fold differences.” To the extent Anglo-Saxons were able to have and support more children, this could lead to a gradual replacement of the indigenous Y-chromosome over many generations

The Y-chromosome can be a particularly revealing signature of the past when compared to other kinds of genetic data. Among African Americans in the United States, for example, Y-chromosomes are about 33 percent European, he says, though the proportion varies from city to city. But those same African Americans’ mitochondrial DNA, which comes from the female line, is only about 6 percent European. And that, says Reich, “tells you about the history of this country, in which men contributed about three-fourths of the European ancestry that is present in the African-American population data. The data speak to a history in which white male slaveowners exploited women of African descent--a fact that is well documented in the historical record. That there is evidence of this in genetic data should be no surprise.”

Most Americans associate Medellín with the drug cartels of that isolated region. But the remoteness has also preserved a genetic legacy that can be traced to the conquistadores. As described in a paper by Andrés Ruiz-Linares of University College London, the Y-chromosomes of men in Medellín are 95 percent European, while the mitochondrial DNA of the women is 95 percent Native American. Spanish men and Native American women created a new population--confirming the recorded history of the region.

The pattern of sexual exploitation by a dominant group seen in the preceding examples is not at all unusual in the human genetic record, says Reich’s frequent collaborator, Nick Patterson, a senior research scientist at the Broad Institute of MIT and Harvard. The Icelandic sagas record that the exiles who settled that island raided Scotland and Ireland, kidnapping Celtic women. And the genes corroborate this account. The mitochondrial DNA of the women is Celtic, the Y-chromosomes are Nordic

Fortunately, the science of the human past has progressed in these other areas no less than in the field of genetics. Innovations in archaeological analysis have had a profound impact

After the fall of the Roman empire, “you get this layer called ‘dark earth’” in the archaeological stratigraphy, he says. “People thought the empire fell and the cities turned into garden [plots]. That is how dark earth was understood up until about five years ago,

“In the Roman excavations,” says McCormick, “there were pots and stone buildings and columns.” But then suddenly you get a layer of nothing but dark, humus-looking soil. What actually happened, Galinié and others have found, is that people shifted to organic building materials. “They had thatched roofs and wooden houses, they didn’t have Roman garbage removal, and they just dumped the ashes and charcoal from their hearths out in the road and all of that compacted. It is extremely rich, extremely dense,

Scientists say that a national surveillance program would be able to determine just how widespread the new variant is and help contain emerging hot spots, extending the crucial window of time in which vulnerable people across the country could get vaccinated.

That would cost several hundred million dollars or more. While that may seem like a steep price tag, it’s a tiny fraction of the $16 trillion in economic losses that the United States is estimated to have sustained because of Covid-19.

With such a system in place, health officials could warn the public in affected areas and institute new measures to contend with the variant — such as using better masks, contact tracing, closing schools or temporary lockdowns — and do so early, rather than waiting until a new surge flooded hospitals with the sick.

Experts point to Britain as a model for what the U.S. could do. British researchers sequence the genome — that is, the complete genetic material in a coronavirus — from up to 10 percent of new positive samples

But the U.S. falls far short of that goal now. Over the past month, American researchers have only sequenced a few hundred genomes a day,

In March, Britain started what many American experts yearn for: a well-run national program to track mutations of the new coronavirus. The country invested 20 million pounds — roughly $27 million — to create a scientific consortium that enlisted hospitals across the country, giving them standard procedures for sending samples to dedicated labs that would sequence their viruses.

Using cloud computing, experts analyzed the mutations and figured out where each lineage of the virus fit on an evolutionary tree.

“What the U.K. has done with sequencing is, to me, the moonshot of the pandemic,”

“They decided they were going to do sequencing and they just stood up an absolutely incredible program from scratch.”

In the U.S., a constellation of labs, mostly at universities, have been analyzing coronavirus genomes since the spring. Many of them spend their own modest funds to do the work

Previous research had found that modern humans and Neanderthals shared a common ancestor that lived 600,000 years ago. Neanderthals expanded across Europe and Asia, interbred with modern humans coming out of Africa, and then became extinct about 40,000 years ago.

Human DNA also points to Africa. Living Africans have a vast amount of genetic diversity compared with other people. That’s because humans lived and evolved in Africa for thousands of generations before small groups — with comparatively small gene pools — began expanding to other continents.

Brenna Henn, a geneticist at the University of California, Davis, and her colleagues developed software to run large-scale simulations of human history. The researchers created many scenarios of different populations existing in Africa over different periods of time and then observed which ones could produce the diversity of DNA found in people alive today.

The researchers analyzed DNA from a range of African groups, including the Mende, farmers who live in Sierra Leone in West Africa; the Gumuz, a group descended from hunter-gatherers in Ethiopia; the Amhara, a group of Ethiopian farmers; and the Nama, a group of hunter-gatherers in South Africa.

The researchers compared these Africans’ DNA with the genome of a person from Britain. They also looked at the genome of a 50,000-year-old Neanderthal found in Croatia

The researchers concluded that as far back as a million years ago, the ancestors of our species existed in two distinct populations. Dr. Henn and her colleagues call them Stem1 and Stem2.

Even after these mergers 120,000 years ago, people with solely Stem1 or solely Stem2 ancestry appear to have survived

About 600,000 years ago, a small group of humans budded off from Stem1 and went on to become the Neanderthals. But Stem1 endured in Africa for hundreds of thousands of years after that, as did Stem2.

If Stem1 and Stem2 had been entirely separate from each other, they would have accumulated a large number of distinct mutations in their DNA. Instead, Dr. Henn and her colleagues found that they had remained only moderately different — about as distinct as living Europeans and West Africans are today. The scientists concluded that people had moved between Stem1 and Stem2, pairing off to have children and mixing their DNA.

it’s possible that bands of these two groups moved around a lot over the vast stretches of time during which they existed on the continent.

About 120,000 years ago, the model indicates, African history changed dramatically.

In southern Africa, people from Stem1 and Stem2 merged, giving rise to a new lineage that would lead to the Nama and other living humans in that region

Elsewhere in Africa, a separate fusion of Stem1 and Stem2 groups took place. That merger produced a lineage that would give rise to living people in West Africa and East Africa, as well as the people who expanded out of Africa.

It’s possible that climate upheavals forced Stem1 and Stem2 people into the same regions, leading them to merge into single groups.

Paleoanthropologists and geneticists have found evidence pointing to Africa as the origin of our species. The oldest fossils that may belong to modern humans, dating back as far as 300,000 years, have been unearthed there. So were the oldest stone tools used by our ancestors.

The DNA of the Mende people showed that their ancestors had interbred with Stem2 people just 25,000 years ago. “It does suggest to me that Stem2 was somewhere around West Africa,”

She and her colleagues are now adding more genomes from people in other parts of Africa to see if they affect the models.

It’s possible they will discover other populations that endured in Africa for hundreds of thousands of years, ultimately helping produce our species as we know it today.

Dr. Scerri speculated that living in a network of mingling populations across Africa might have allowed modern humans to survive while Neanderthals became extinct. In that arrangement, our ancestors could hold onto more genetic diversity, which in turn might have helped them endure shifts in the climate, or even evolve new adaptations.

“This diversity at the root of our species may have been ultimately the key to our success,”

The study’s conclusions need to be tempered by the idea that these 300 ancient genomes may not be wholly representative of the region’s overall population

Many of the genomes used in the new analysis were collected from individuals uncovered in burial grounds, grave fields and churchyards. But some samples came from people who died in unusual circumstances—including sailors from a Swedish warship that sank off the country’s southeastern coast in 1676, and inhabitants of a settlement known as Sandby borg who were likely massacred during an organized attack in the fifth century.

“There is a question of how much you can call it population genomics as opposed to kind of lots of little vignettes,

The Mal’ta people are not related to the Asians who live in the region today. But before they disappeared, they also passed down their DNA to Europeans. Later research revealed the route those genes took from Asia to Europe

Dr. Willerslev and his colleagues discovered Mal’ta-like DNA in Bronze Age nomads called the Yamnaya, who lived 4,300 to 5,500 years ago in what is now southwestern Russia. About 5,000 years ago, the Yamnaya expanded into Europe, where they added their DNA to the gene pool.

In 2010, he found a piece of hair collected in Australia in the 1920s at the University of Cambridge. He and his colleagues retrieved DNA from the hair and reconstructed the owner’s genome.Their analysis revealed that the ancestors of aboriginal Australians split off from other non-Africans about 70,000 years ago. That finding supports the idea that the first settlers in Australia were the ancestors of today’s aboriginals

“Aboriginal people feel exonerated in showing the broader community that they are by far the oldest continuous civilization in the world,” the council said in a statement.

The Kennewick Man genome, like the Anzick child’s, revealed an ancient continuity between living Native Americans and the earliest people in the New World. A

Since the Kennewick Man project, Dr. Willerslev has hosted visits from a number of Native American tribes to his laboratory in Copenhagen. His guests have helped him see how differently he, as a European, treats history than they do.

Dr. Willerslev once proudly showed off a collection of ancient Danish skulls to Native American visitors, only to find them upset by the sight.“‘How can you treat your ancestors like that, so disrespectfully?” he recalls them asking.

Such information would allow detection of so-called Mendelian disorders, like cystic fibrosis, Tay-Sachs disease and Marfan syndrome, which are caused by mutations in a single gene.

But the geneticists see no trace of the Danelaw, the Danish rule over northern England from the ninth to the 11th century, nor of the Norman Conquest of England in 1066. The numbers of invaders may have been too small to leave a demographic imprint

in the case of the Normans, who had previously emigrated from southern Denmark to Normandy, it is hard to distinguish their genes from those of the earlier Danish invaders.

The researchers found that the modern British population falls into 17 clusters altogether, based on genetic relatedness. Though very similar, the groups are genetically distinguishable, and even the main population cluster, that of southern and central England, is distinguishable from the populations of France, Germany and other European countries.

The people of the southern and central parts of England form a homogeneous population, but all around the Celtic periphery, in Cornwall, Wales and Scotland, lie small clusters of genetically different populations that have maintained their identity over the generations. This is a surprise, given that the Celtic peoples who ruled most of England until Caesar’s invasion in B.C. 55 were assumed to be fairly homogeneous.

Dr. Donnelly and his colleagues managed to sidestep this recent churning of the population history by seeking out elderly people who lived in rural areas and whose grandparents had been born locally. Because individual genomes are composed of random samples of the four grandparents’ DNA, the researchers were in effect looking two generations into the past and testing the population of the late 19th century.

They analyzed the DNA of their 2,000 subjects at 500,000 sites along the genome, and then organized them into the 17 genetic clusters. They also analyzed the genomes of 6,000 Europeans in the same way, and could thus identify the source populations in Europe from which each of the 17 British clusters was derived.

The migrations revealed in that way match the known historical record but also point to events that have not been recorded, such as a major migration from northern France that accounts for about one-third of the ancestry of the average person in Britain.

“History is written by the winners, and archaeology studies the burials of wealthy people,” Dr. Donnelly said. “But genetic evidence is interesting because it complements that by showing what is happening to the masses rather than the elite.”