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Doctors and forensic scientists have long known that certain medical procedures turn people into chimeras, but where exactly a donor’s DNA shows up — beyond blood — has rarely been studied with criminal applications in mind.

The average doctor does not need to know where a donor’s DNA will present itself within a patient. That’s because this type of chimerism is not likely to be harmful. Nor should it change a person. “Their brain and their personality should remain the same,”

for a forensic scientist, it’s a different story. The assumption among criminal investigators as they gather DNA evidence from a crime scene is that each victim and each perpetrator leaves behind a single identifying code — not two

In 2004, investigators in Alaska uploaded a DNA profile extracted from semen to a criminal DNA database. It matched a potential suspect. But there was a problem: The man had been in prison at the time of the assault. It turned out that he had received a bone marrow transplant. The donor, his brother, was eventually convicted.

she heard about another disconcerting scenario since then. It involved police investigators who were skeptical of a sexual assault victim’s account because she said there had been one attacker, though DNA analysis showed two. Eventually the police determined that the second profile had come from her bone marrow donor.

In 2008, he was trying to identify the victim of a traffic accident for the National Forensic Service in Seoul, South Korea. Blood showed that the individual was female. But the body appeared to be male, which was confirmed by DNA in a kidney, but not in the spleen or the lung, which contained male and female DNA. Eventually, he figured out that the victim had received a bone marrow transplant from his daughter.

Fraternal twins sometimes acquire each other’s DNA in the womb; in at least one case that led to unfounded fears of infidelity when a man’s child did not seem to be his. In another case, a mother nearly lost custody of her children after a DNA test.

instead of allowing it to cement racialized ways of thinking, we can use these tests to highlight how meaningless genetic ancestry is compared with the many other factors that shape our experience of ourselves and our communities.

For most of human history, the concept of peoplehood — of belonging to a group larger than one’s extended family — has been largely determined by shared cultural practices (such as religion, customs and language) or political institutions.

This more capacious notion of belonging is how heritage is lived day to day for most people. I didn’t need a DNA test to identify as a Jew of European ancestry.

race science got a new lease on life when, in the 1950s, scientists discovered the molecular structure of DNA. By the 1980s DNA testing could reliably prove paternity

More than 26 million people are estimated to have taken genealogical DNA tests.

The answer goes back to the 18th and 19th centuries, when European colonialism and the slave trade birthed the modern concept of race.

As societies were built and genocides committed on the basis of racial hierarchy, it became imperative for racists to prove the biological existence of race. And so race “science” emerged, seeking to dislodge cultural heritage as the prime difference between groups of people.

“Aryan” became synonymous with “German,” excluding the many Jewish and Slavic speakers of the language.

et, no matter how strong it was, this sense of cultural heritage didn’t feel like enough for me. In a society that determines so much based on blood — money, connections, assumptions about character — culture by itself felt like an unreliable narrator of my identity.

Ironically, as academics were reaching the consensus that race is a social construct with no basis in biology — about 94 percent of human genetic variation occurs within so-called racial groups, with racial difference accounting for only 6 percent — the popularity of DNA testing was helping undermine that very idea.

almost 53 percent of Americans think biology at least somewhat determines their racial identity

Two-thirds of white Americans believe that their racial identity is determined by their DNA, compared with about half of black, Latino and Asian Americans

Only 35 percent of those surveyed believe that shared history or culture determines their racial identity.

DNA tests merely compare patterns in your genome with those of groups of people who have been identified as belonging to different ethnicities based on traditional genealogical research (vital records, family trees, etc.)

“If your 23andMe test says you’re 29 percent British, it’s because 29 percent of the pieces of your DNA were most likely to have come from a group that 23andMe’s reference library has labeled ‘British.’ ”

This means ethnicity estimates from companies like 23andMe and AncestryDNA tell a much narrower story than consumers — led along by marketing campaigns — read into them

such estimates are only as good as the companies’ pattern-matching algorithms and DNA reference libraries, which can be incomplete and haphazardly assembled

In fact, these companies’ ethnicity estimates for the same person can vary substantially.

In a society that continues to structure lives along the hierarchy of race, it is difficult to leave behind the reassuring neatness of blood ancestry — of, in an age of turbulence and uncertainty, knowing exactly and objectively who we are — for the reality of mixing, moving and contradiction that makes up our history on this planet.

As Chuck Hoskin Jr., then the Cherokee Nation’s secretary of state, wrote in response to Warren’s DNA testing controversy, “We are [tribal] citizens through historical documentation, adopted laws and a shared language and culture that make us unique.” DNA tests, he said, are “useless to determine tribal citizenship.”

Their study, published on Thursday in the journal Cell, concludes that a wave of modern humans departed Africa far earlier than had been known: some 200,000 years ago.

The ancestors of humans and Neanderthals lived about 600,000 years ago in Africa

The scientists also found evidence that people living somewhere in western Eurasia moved back to Africa and interbred with people whose ancestors never left. The new study suggests that all Africans have a substantially greater amount of Neanderthal DNA than earlier estimates.

The research offers a view of human history “almost as a spider web of interactions, rather than a tree with distinct branches.”

These people interbred with Neanderthals, the new study suggests. As a result, Neanderthals were already carrying genes from modern humans when the next big migration from Africa occurred, about 140,000 years later.

The Neanderthal lineage left the continent; the fossils of what we describe as Neanderthals range from 200,000 years to 40,000 years in age, and are found in Europe, the Near East and Siberia.

The human genome is detailed in units called base pairs, about 3 billion such pairs in total. The scientis

ts found that Europeans on average had 51 million base pairs that matched Neanderthal DNA, and East Asians had 55 million.Dr. Akey’s previous research had indicated that East Asians carried far more Neanderthal ancestry than did Europeans.Africans on average had 17 million base pairs that matched Neanderthal DNA — far higher than predicted by the original models describing how humans and Neanderthals interbred.

They concluded that a group of modern humans expanded out of Africa perhaps 200,000 years ago and interbred with Neanderthals. Those modern humans then disappeared. But Neanderthals who lived after that disappearance inherited some modern human DNA.

sitation over the analysis of African DNA, Dr. Reich said the new findings do make a strong case that modern humans departed Africa much earlier than thought

It’s possible that humans and Neanderthals interbred at other times, and not just 200,000 years ago and again 60,000 years ago. But Dr. Akey said that these two migrations accounted for the vast majority of mixed DNA in the genomes of living humans and Neanderthal fossils.

In recent years, Dr. Reich and other researchers have found evidence that ancient people from the Near East moved back into Africa in the past few thousand years and spread their DNA to many African populations.

The Swahili Coast is a narrow strip of land that stretches some 2,000 miles along the Eastern African seaboard — from modern-day Mozambique, Comoros and Madagascar in the south, to Somalia in the north

In its medieval heyday, the region was home to hundreds of port towns, each ruled independently, but with a common religion (Islam), language (Kiswahili) and culture.

Many towns grew immensely wealthy thanks to a vibrant trading network with merchants who sailed across the Indian Ocean on the monsoon winds. Middle Eastern pottery, Asian cloths 0c 0c and other luxury goods came in. African gold, ivory and timber 0c 0c went out — along with a steady flow of enslaved people, who were shipped off and sold across the Arabian Peninsula and Persian Gulf. (Slave trading later took place between the Swahili coast and Europe as well.)

A unique cosmopolitan society emerged that blended African customs and beliefs with those of the foreign traders, some of whom stuck around and assimilated.

Islam, for example, arrived from the Middle East and became an integral part of the Swahili social fabric, but with coral-stone mosques built and decorated in a local, East African style

Or consider the Kiswahili language, which is Bantu in origin but borrows heavily from Indian and Middle Eastern tongues

The arrival of Europeans, beginning around 1500, followed by Omani sailors some 200 years later, changed the character of the region

over the past 40 years, archaeologists, linguists and historians have come to see Swahili society as predominantly homegrown — with outside elements adopted over time that had only a marginal impact.

That African-centric version of Swahili roots never sat well with the Swahili people themselves, though

They generally preferred their own origin story, one in which princes from present-day Iran (then known as Persia) sailed across the Indian Ocean, married local women and enmeshed themselves into East African society. Depending on the narrative source, that story dates to around 850 or 1000 — the same period during which genetic mixing was underway, according to the DNA analysis.

“It’s remarkably spot on,” said Mark Horton, an archaeologist at the Royal Agricultural University of England

“This oral tradition was always maligned,”

“Now, with this DNA study, we see there was some truth to it.”

The ancient DNA study is the largest of its kind from Africa, involving 135 skeletons dating to late-medieval and early-modern times, 80 of which have yielded analyzable DNA.

To figure out where these people came from, the researchers compared genetic signatures from the dug-up bones with cheek swabs or saliva samples taken from modern-day individuals living in Africa, the Middle East and around the world.

The burial-site DNA traced back to two primary sources: Africans and present-day Iranians. Smaller contributions came from South Asians and Arabs as well, with foreign DNA representing about half of the skeletons’ genealogy

“It’s surprising that the genetic signature is so strong

Gene sequences from tiny power factories inside the cell, known as mitochondria, were overwhelmingly African in origin. Since children inherit these bits of DNA only from their mothers, the researchers inferred that the maternal forbearers of the Swahili people were mostly of African descent.

By comparison, the Y chromosome, passed from father to son, was chock-full of Asian DNA that the researchers found was common in modern-day Iran. So, a large fraction of Swahili ancestry presumably came from Persian men

Dr. Reich initially assumed that conquering men settled the region by force, displacing the local males in the process. “My hypothesis was that this was a genetic signature of inequality and exploitation,”

hat turned out to be a “naïve expectation,” Dr. Reich said, because “it didn’t take into account the cultural context in this particular case.”

In East Africa, Persian customs never came to dominate. Instead, most foreign influences — language, architecture, fashion, arts — were incorporated into a way of life that remained predominantly African in character, with social strictures, kinship systems and agricultural practices that reflected Indigenous traditions.

“Swahili was an absorbing society,” said Adria LaViolette, an archaeologist at the University of Virginia who has worked on the East African coast for over 35 years. Even as the Persians who arrived influenced the culture, “they became Swahili,”

One major caveat to the study: Nearly all the bones and teeth came from ornamental tombs that were located near grand mosques, sites where only the upper class would have been laid to rest.

the results might not be representative of the general populace.

Protocols for disinterring, sampling and reburying human remains were established in consultation with local religious leaders and community stakeholders. Under Islamic law, exhumations are permitted if they serve a public interest, including that of determining ancestry,

Comparing genomes, scientists concluded that today’s humans outside Africa carry an average of 2.5 percent Neanderthal DNA, and that people from parts of Oceania also carry about 5 percent Denisovan DNA. A study published in November found that Southeast Asians carry about 1 percent Denisovan DNA in addition to their Neanderthal genes.

as few as six couplings all those tens of thousands of years ago might have led to the current level of ancient immune alleles.

This means that our modern era, since H. floresiensis died out, is the only time in the four-million-year human history that just one type of human has been alive,

Were they romantic couplings? More likely they were aggressive acts between competing human groups, Dr. Stringer said. For a model, he pointed to modern hunter-gatherer groups that display aggressive behavior among tribes.

The value of the interbreeding shows up in the immune system, Dr. Parham’s analysis suggests. The Neanderthals and Denisovans had lived in Europe and Asia for many thousands of years before modern humans showed up and had developed ways to fight the diseases there

When modern humans mated with them, they got an injection of helpful genetic immune material, so useful that it remains in the genome today. This suggests that modern humans needed the archaic DNA to survive. The downside of archaic immune material is that it may be responsible for autoimmune diseases like diabetes, arthritis and multiple sclerosis, Dr. Parham said, stressing that these are preliminary results.

little is known about the Denisovans — the only remains so far are the pinky bone and the tooth, and there are no artifacts like tools. Dr. Reich and others suggest that they were once scattered widely across Asia, from the cold northern cave to the tropical south. The evidence is that modern populations in Oceania, including aboriginal Australians, carry Denisovan genes.

the new studies were “a major game-changer. To me, it marks a new phase in ancient DNA research.”

Until about 9,000 years ago, Europe was home to a genetically distinct population of hunter-gatherers, the researchers found. Then, between 9,000 and 7,000 years ago, the genetic profiles of the inhabitants in some parts of Europe abruptly changed, acquiring DNA from Near Eastern populations.

Archaeologists have long known that farming practices spread into Europe at the time from Turkey. But the new evidence shows that it wasn’t just the ideas that spread — the farmers did, too.

the Yamnaya, who left behind artifacts on the steppes of western Russia and Ukraine dating from 5,300 to 4,600 years ago. The Yamnaya used horses to manage huge herds of sheep, and followed their livestock across the steppes with wagons full of food and water.

“You have groups which are as genetically distinct as Europeans and East Asians. And they’re living side by side for thousands of years.”

Between 7,000 and 5,000 years ago, however, hunter-gatherer DNA began turning up in the genes of European farmers. “There’s a breakdown of these cultural barriers, and they mix,”

About 4,500 years ago, the final piece of Europe’s genetic puzzle fell into place. A new infusion of DNA arrived — one that is still very common in living Europeans, especially in central and northern Europe.

The hunter-gatherers didn’t disappear, however. They managed to survive in pockets across Europe between the farming communities.

The closest match to this new DNA, both teams of scientists found, comes from skeletons found in Yamnaya graves in western Russia and Ukraine.

it was likely that the expansion of Yamnaya into Europe was relatively peaceful. “It wasn’t Attila the Hun coming in and killing everybody,”

the most likely scenario was that the Yamnaya “entered into some kind of stable opposition” with the resident Europeans that lasted for a few centuries. But then gradually the barriers between the cultures eroded.

the Yamnaya didn’t just expand west into Europe, however. The scientists examined DNA from 4,700-year-old skeletons from a Siberian culture called the Afanasievo. It turns out that they inherited Yamnaya DNA, too.

was surprised by the possibility that Yamnaya pushed out over a range of about 4,000 miles. “

For decades, linguists have debated how Indo-European got to Europe. Some favor the idea that the original farmers brought Indo-European into Europe from Turkey. Others think the language came from the Russian steppes thousands of years later.

he did think the results were consistent with the idea that the Yamnaya brought Indo-European from the steppes to Europe.

The eastward expansion of Yamnaya, evident in the genetic findings, also supports the theory, Dr. Willerslev said. Linguists have long puzzled over an Indo-European language once spoken in western China called Tocharian. It is only known from 1,200-year-old manuscripts discovered in ancient desert towns. It is possible that Tocharian was a vestige of the eastern spread of the Yamnaya.

the new studies were important, but were still too limited to settle the debate over the origins of Indo-European. “I don’t think we’re there yet,” he said.

Dr. Heggarty speculated instead that early European farmers, the second wave of immigrants, may have brought Indo-European to Europe from the Near East. Then, thousands of years later, the Yamnaya brought the language again to Central Europe.

They coexisted in Europe for thousands of years, apparently trading tools and sharing cultures. Some groups survived the Ice Age, while others vanished,

when farmers arrived in Europe about 8,000 years ago, they encountered the descendants of this long history, with light-skinned, dark-eyed people to the east, and possibly dark-skinned and blue-eyed people to the west.

“We lack still an understanding of why these movements were triggered. What happened here, why it happened — it’s strange.”

Modern humans arose in Africa and expanded to other continents about 60,000 years ago

These early Europeans have almost no genetic link to younger remains of hunter-gatherers. It appears that the first modern humans in Europe may have disappeared along with the Neanderthals

he oldest DNA of modern humans in Europe, dating back 45,000 years, undermines such a simple story. It comes from people who belonged to a lost branch of the human family tree. Their ancestors were part of the expansion out of Africa, but they split off on their own before the ancestors of living Europeans and Asians split apart.

About 33,000 years ago, as the climate turned cold, a new culture called the Gravettian arose across Europe. Gravettian hunters made spears to kill woolly mammoths and other big game. They also made so-called Venus figurines that might have represented fertility.

When the glaciers retreated, some descendants of the Fournol continued living in Iberia. But others expanded north as a new population, which Dr. Posth and his colleagues called GoyetQ2. “It really seems like a peopling of Europe after the last glacial maximum,

Dr. Posth and his colleagues found DNA in Gravettian remains scattered across Europe. The scientists had expected all of the individuals to have come from the same genetic population, but instead found two distinct groups: one in France and Spain, and another in Italy, the Czech Republic and Germany.

“They were very distinct, and this was a very big surprise to us because they practiced the same archaeological culture,”

Dr. Posth and his colleagues named the western population the Fournol people, and found a genetic link between this group and 35,000-year-old Aurignacian remains in Belgium.

They called the eastern group Vestonice, and discovered that they share an ancestry with 34,000-year-old hunter-gatherers who lived in Russia.

That genetic gulf led Dr. Posth and his colleagues to argue that the Fournol and Vestonice belonged to two waves that migrated into Europe separately. After they arrived, they lived for several thousand years sharing the Gravettian culture but remaining genetically distinct.

It’s clear from the new study that they were not isolated entirely from each other. In Belgium, the scientists found 30,000-year-old remains with a mix of Fournol and Vestonice ancestry.

About 26,000 years ago, the two groups faced a new threat to their survival: an advancing wall of glaciers. During the Ice Age, from 26,000 to 19,000 years ago, European hunter-gatherers were shut out of much of the continent, surviving only in southern refuges.

the refuge of the Iberian Peninsula, the region now occupied by Spain and Portugal, by studying DNA in the teeth of a 23,000-year-old man found in a cave in southern Spain. His DNA revealed that he belonged to the Fournol people who lived in Iberia before the Ice Age. The researchers also found genetic markers linking him to a 45,000-year-old skeleton discovered in Bulgaria.

When these groups arrived in Europe, Neanderthals had already been living across the continent for more than 100,000 years. The Neanderthals disappeared about 40,000 years ago, perhaps because modern humans outcompeted them with superior tools.

The Vestonice, by contrast, did not survive the Ice Age. When the glaciers were at their most expansive, the Vestonice may have endured for a time in Italy. But Dr. Posth and his colleagues found no Vestonice ancestry in Europeans after the Ice Age. Instead, they discovered a population of hunter-gatherers that appeared to have expanded from the Balkans, known as the Villabruna. They moved into Italy and replaced the Vestonice.

For several thousand years, the Villabruna were limited to southern Europe. Then, 14,000 years ago, they crossed the Alps and encountered the GoyetQ2 people to the north. A new population emerged, its ancestry three parts Villabruna to one part GoyetQ2.

This new people, which Dr. Posth and his colleagues called Oberkassel, expanded across much of Europe, replacing the old GoyetQ2 population.

another climate shift could explain this new wave. About 14,000 years ago, a pulse of strong warming produced forests across much of Europe. The Oberkassel people may have been better at hunting in forests, whereas the GoyetQ2 retreated with the shrinking steppes.

To the east, the Oberkassel ran into a new group of hunter-gatherers, who probably arrived from Russia. The scientists named this group’s descendants, who lived in Ukraine and surrounding regions, the Sidelkino.

in Iberia, there were no great sweeps of newcomers replacing older peoples. The Iberians after the Ice Age still carried a great deal of ancestry from the Fournol people who had arrived there thousands of years before the glaciers advanced. The Villabruna people moved into northern Spain, but added their DNA to the mix rather than replacing those who were there before.

When the first farmers arrived in Europe from Turkey about 8,000 years ago, three large groups of hunter-gatherers thrived across Europe: the Iberians, the Oberkassel and the Sidelkino. Living Europeans carry some of their gene

The Sidelkino people in the east had genes associated with dark eyes and light skin. The Oberkassel in the west, in contrast, probably had blue eyes and may have had dark skin

These three groups of hunter-gatherers remained isolated from each other for about 6,000 years, until the farmers from Turkey arrived. After this advent of agriculture, the three groups began mixing, the scientists found. It’s possible that the spread of farmland forced them to move to the margins of Europe to survive. But over time, they were absorbed into the agricultural communities that surrounded them.

every continent will likely have its own history of hunter-gatherer migrations.

it is now possible to extract human DNA from cave sediments rather than searching for bones and teeth.

One of these groups can be traced as far back as 35,000 years, thanks to a skeleton discovered at a site in Belgium called Goyet. The Goyet-related people spread across Europe, only to be replaced on much of the continent near the end of the Ice Age by a genetically distinct population. The earliest sign of the second group appears 14,000 years ago, known to researchers by DNA in a skeleton at an Italian site called Villabruna.

the Goyet and Villabruna people coexisted. Hunter-gatherers across the peninsula had a mixture of ancestry from the two peoples.

But whatever solitude Iberia might have offered came to an end about 7,500 years ago, when new people arrived with crops and livestock. These first farmers, originally from Anatolia, brought with them a distinctive genetic signature.

Ninety percent of the DNA from the later skeletons derives from the Anatolian farmers; 10 percent comes from the hunter-gatherers.

Starting about 6,000 years ago, Dr. Olalde and his colleagues found, hunter-gatherer ancestry in Iberian farmers actually increased to 20 percent.

A skeleton from an elaborate grave in central Spain about 4,400 years old belonged to a man whose ancestry was 100 percent North African

findings suggest that people were moving into Iberia from Africa more than 3,000 years before the rise of the Roman Empire. “These are cosmopolitan places,”

About 4,500 years ago, still another wave of people arrived, profoundly altering the makeup of Iberia.A few centuries earlier, nomads from the steppes of what is now Russia turned up in Eastern Europe with horses and wagons. They spread across the continent, giving up nomadic life and intermarrying with European farmers.

DNA from the men, however, all traced back to the steppes. The Y chromosomes from the male farmers disappeared from the gene pool.

When they finally reached Iberia, these people spread out far and wide. “They really have an impact on the whole peninsula,”

To archaeologists, the shift is a puzzle.“I cannot say what it is,

he ruled out wars or massacres as the cause. “It’s not a particularly violent time,”

Iberian farmers originally lived in egalitarian societies, storing their wealth together and burying their dead in group graves.

over several centuries, palaces and fortresses began to rise, and power became concentrated in the hands of a few. Dr. Risch speculated that the cultural shift had something to do with the genetic shift

The Bronze Age in Iberia was followed by the Iron Age about 2,800

Iron Age Iberians could trace some of their ancestry to new waves of people arriving from northern and Central Europe, possibly marking the rise of so-called Celtiberian culture on the peninsula.

scientists found a growing amount of North African ancestry in skeletons from the Iron Age. That may reflect trade around the Mediterranean, which brought North Africans to Iberian towns, where they settled down.

North African ancestry increased in Iberia even more after Romans took control. Now the peninsula was part of an empire that thrived on widespread trade. At the same time, people from southern Europe and the Near East also began leaving an imprint

The Basque speak a language that is unrelated to other European tongues. Some researchers have speculated that they descended from a population that had been distinct since the Bronze Age or earlier.Genetically, at least, that doesn’t seem to be the case. Before the Roman era, the Basque had DNA that was indistinguishable from that of other Iron Age Iberians

After the fall of Rome, ancient DNA in Iberia reflects its medieval history. Skeletons from the Muslim era show growing ancestry from both North Africa and sub-Saharan Africa.

The researchers were also able to group Spaniards into five genetic clusters. On a map, these groups form five strips running north to south. Those strips line up neatly with history.

In 2006, the man’s remains were exhumed, but DNA extracted from his teeth yielded no match with a list of people Navy officials thought might be his descendants. With few leads, the scientist who conducted the DNA test, Jeremy Austin, told the Navy about an emerging technique that could predict a person’s ancestry and physical traits from genetic material.

In Australia, forensic scientists are repurposing the technique to help link missing persons with unidentified remains in the hope of resolving long-running mysteries. In the case of the sailor, Dr. Austin sent the sample to researchers in Europe, who reported back that the man was of European ancestry and most likely had red hair and blue eyes.

That alone wasn’t enough to identify the sailor, but it narrowed the search. “In a ship full of 645 white guys, you wouldn’t expect to see more than two or three with this pigmentation,”

This forensic tool, which has been slowly advancing since the mid-2000s, is similar to genetic tests that estimate risks for certain diseases. About five years ago, scientists with the Australian Federal Police began developing their own version of the technology, which combines genomics, big data and machine learning. It became available for use last year.

The predictions from DNA phenotyping — whether a person had, say, brown hair and blue eyes — will be brought to life by a forensic artist, combining the phenotype information with renderings of bone structure to generate a three-dimensional digital facial reconstruction.

“It’s an investigative lead we’ve never had before,”

In the United States, police departments have for years been using private DNA phenotyping services, like one from the Virginia-based Parabon NanoLabs, to try to generate facial images of suspects. The images are sometimes distributed to the public to assist in investigations.

Many scientists, however, are skeptical of this application of the technology. “You cannot do a full facial prediction right now,” said Susan Walsh, a professor of biology at Indiana University-Purdue University Indianapolis who developed some of the earliest phenotyping methods for eye and hair color. “The foundation of the genetics is absolutely not there.”

Facial image prediction has been condemned by human rights organizations, including the A.C.L.U., which suggest that it risks being skewed by existing social prejudices.

The same DNA was then linked to dozens of serious crimes across Western Europe, prompting a theory that the perpetrator was a serial offender from a traveling Roma community.It turned out that the recurring genetic material belonged to a female Polish factory worker who had accidentally contaminated the cotton swabs used to collect the samples.

“The families want any and all techniques applied to these cases if it’s going to help answer the question of what happened,” she said.

Such was the case with the mystery sailor. After his genotype was sequenced and his phenotype predicted, a team of scientists across several Australian institutions, including Dr. Ward’s program, used this information to track down a woman they believed to be a living relative of the soldier. They checked her DNA and had a match.

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.

After that, you see for the first time people related to later European hunter-gatherers who have contributed a little bit to present-day Europeans. That happens beginning 35,000 to 37,000 years ago. Then the ice sheets descend across northern Europe and a lot of these populations are chased into these refuges in the southern peninsulas of Europe. After the Ice Age, there’s a repeopling of northern Europe from the southwest, probably from Spain, and then also from the southeast, probably from Greece and maybe even from Anatolia, Turkey

Again, after 9,000 years ago, there’s a mass movement of farmers into the region which almost completely replaces the hunter-gatherers with a small amount of mixture

And then again, after 5,000 years ago, there’s this mass movement at the beginning of the Bronze Age of people from the steppe, who also probably bring these languages that are spoken by the great majority of Europeans today.

Reich: Archaeology has always been political, especially in Europe. Archaeologists are very aware of the misuse of archaeology in the past, in the 20th century. There’s a very famous German archaeologist named Gustaf Kossinna, who was the first or one of the first to come up with the idea of “material culture.” Say, you see similar pots, and therefore you’re in a region where there was shared community and aspects of culture.

He went so far as to argue that when you see the spread of these pots, you’re actually seeing a spread of people and there’s a one-to-one mapping for those things. His ideas were used by the Nazis later, in propaganda, to argue that a particular group in Europe, the Aryans, expanded in all directions across Europe.

This was used to justify their expansionism in the propaganda that the Germans used in the run-up to the Second World War.

So after the Second World War, there was a very strong reaction in the European archaeological community—not just the Germans, but the broad continental European archaeological community—to the fact that their discipline had been used for these terrible political ends. And there was a retreat from the ideas of Kossinna.

one of the things the ancient DNA is showing is actually the Corded Ware culture does correspond coherently to a group of people. [Editor’s note: The Corded Ware made pottery with cord-like ornamentation and according to ancient DNA studies, they descended from steppe ancestry.] I think that was a very sensitive issue to some of our coauthors

Our results are actually almost diametrically opposite from what Kossina thought because these Corded Ware people come from the East, a place that Kossina would have despised as a source for them.

it is true that there’s big population movements, and so I think what the DNA is doing is it’s forcing the hand of this discussion in archaeology, showing that in fact, major movements of people do occur. They are sometimes sharp and dramatic, and they involve large-scale population replacements over a relatively short period of time. We now can see that for the first time.

Zhang: As you say, the genetics data is now often ahead of the archaeology, and you keep finding these big, dramatic population replacements throughout human history that can’t yet be fully explained. How should we be thinking about these population replacements? Is there a danger in people interpreting or misinterpreting them as the result of one group’s superiority over another?

When you see these replacements of Neanderthals by modern humans or Europeans and Africans substantially replacing Native Americans in the last 500 years or the people who built Stonehenge, who were obviously extraordinarily sophisticated, being replaced from these people from the continent, it doesn’t say something about the innate potential of these people. But it rather says something about the different immune systems or cultural mismatch.

Zhang: On the point of immune systems, one of the hypotheses for why people from the steppe were so successful in spreading through Europe is that they brought the bubonic plague with them. Since the plague is endemic to Central Asia, they may have built up immunity but the European farmers they encountered had not.

ut there have been profound and Earth-shattering events, again and again, every few thousand years in our history and that’s what ancient DNA is telling us.

if you actually take any serious look at this data, it just confounds every stereotype. It’s revealing that the differences among populations we see today are actually only a few thousand years old at most and that everybody is mixed.

I think that if you pay any attention to this world, and have any degree of seriousness, then you can’t come out feeling affirmed in the racist view of the world. You have to be more open to immigration. You have to be more open to the mixing of different peoples. That’s your own history.

Modern Egyptians, by comparison, share much more DNA with sub-Saharan populations.

The findings have turned years of theory on its head

Heat and high humidity in tombs, paired with some of the chemicals involved in mummification, all contribute to DNA degradation

They found that the sample set showed a strong connection with a cluster of ancient non-African populations based east of the Mediterranean Sea

This is not just the DNA of one person. It's the DNA of the parents, grandparents, grandparents' parents, grand-grand-grandparents' parents and so forth

Strict social structures and legal incentives to marry along ethnic lines within these communities may have played a part in the Egyptians' genetic stasis

A lot of people has assumed foreign invaders ... brought a lot of genetic ancestry into the region," Krause said. "People expected that through time, Egypt would become more European, but we see the exact opposite

Modern Egyptians were found to "inherit 8% more ancestry from African ancestors" than the mummies studied. The paper cites increased mobility along the Nile, increased long-distance commerce and the era of the trans-Saharan slave trade as potential reasons why

The team's findings do come with one obvious caveat: "All our genetic data (was) obtained from a single site in Middle Egypt and may not be representative for all of ancient Egypt

It yielded a full DNA profile, and it’s not Cooper’s or any of the victims’ — but it hasn’t been matched to anyone else. Match that DNA, and we may quickly solve these murders.

Lee came to the attention of the authorities during the investigation after his girlfriend, Diana Roper, fingered him as the killer: She reported that he had returned home late on the night of the killings wearing bloody coveralls, in a car that resembled the Ryens’ station wagon

The DNA on the orange towel is not Lee’s, and he has denied to me that he was involved in the Ryen murders. It’s important not to try to free one innocent man by rushing to judgment about another. Still, the DNA testing and these witnesses add to the evidence that Cooper is an innocent man on death row.

The deans of four law schools and a former president of the American Bar Association have expressed concerns about the case. An excellent book, “Scapegoat,” has been written about it, and a documentary is in the works. Kim Kardashian visited Cooper in 2019 and has embraced his cause.

The D.A.’s office notes, for example, that Cooper’s blood was found on that tan T-shirt apparently worn by the murderer. That’s true: Testing years ago confirmed that it is Cooper’s blood — but also suggests that sheriff’s deputies spilled the blood on the shirt to frame him.

Cooper’s blood on that shirt had elevated EDTA in it; in other words, it seemed to have come not from his body directly, but from a test tube.

Relatives of the victims are convinced that Cooper is guilty, and San Bernardino authorities argue that there is plenty of forensic evidence against him — a bloodstain, shoe prints, cigarette butts, and so on. If you trust the sheriff’s office, it’s compelling.

So as he mounted this attack, Cooper was juggling three or four weapons? Pausing in midassault to change shirts? And how could a 155-pound man like Cooper enter the house, with the Ryens’ dogs presumably sounding an alarm, and then overpower both Doug Ryen, a former military policeman, and Peggy Ryen, strong and athletic — each of whom had a loaded gun by the side of the bed?

Yet California for almost 38 years has prepared to ignore all this and execute a Black man who is probably innocent. Democratic and Republican politicians alike have mostly averted their eyes, presumably in part for fear of offending voters who might be upset at the exoneration of a Black man convicted of a brutal crime against a white family.

Conservative law enforcement officials in San Bernardino County blocked comprehensive DNA testing for years, but so did Democratic politicians like Jerry Brown and Kamala Harris when they were attorneys general of California.

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,”

But only a handful of cancers—of the breast, lung, colon and cervix—have screening tests recommended by the U.S. Preventive Services Task Force

Many companies are developing blood tests that can detect cancer signals before symptoms occur, and Grail’s is the most advanced. A study found it can identify more than 50 types of cancer 52% of the time and the 12 deadliest cancers in Stages I through III 68% of the time.

There’s a hitch. The test costs $949 and isn’t covered by Medicare or most private insurance.

The trouble is that this cancer is almost never caught early. There’s no routine screening for it, and symptoms don’t develop until it is advanced. Mr. Royse, 64, had no idea he was sick until he took a blood test called Galleri, produced by the Menlo Park, Calif., startup Grail. He had surgery and chemotherapy and is now cancer-free.

Mr. Royse visited Grail’s website, which referred him to a telemedicine provider who ordered a test. Another telemedicine doctor walked him through his results, which showed a cancer signal likely emanating from the pancreas, gallbladder, stomach or esophagus.

An MRI revealed a suspicious mass on his pancreas, which a biopsy confirmed was cancerous. Mr. Royse had three months of chemotherapy, surgery and another three months of chemotherapy, which ended last February. Because pancreatic cancer often recurs, he gets CT and MRI scans every three months. In addition, he has signed up for startup Natera’s Signatera customized blood test, which checks DNA specific to the patient’s cancer and can signal its return before signs are visible on the scans

Grail’s test likewise looks for DNA shed by cancer cells, which is tagged by molecules called methyl groups that are specific to a cancer’s origin. Grail uses genetic sequencing and machine learning to recognize links between DNA methyl groups and particular cancers

The test “is based on how much DNA is being shed by tumor,” Grail’s president, Josh Ofman, says. “Some tumors shed a lot of DNA. Some shed almost none.

ut slow-growing tumors typically aren’t shedding a lot of DNA.” That reduces the probability that Grail’s test will identify indolent cancers that pose no immediate danger.

Grail’s test has a roughly 0.5% false-positive rate, meaning 1 in 200 patients who don’t have cancer will get a positive signal

Its positive predictive value is 43%, so that of every 100 patients with a positive signal, 43 actually have cancer

the legislation’s price tag could reduce political support. According to one private company’s estimate, the test could cost the government $39 billion to $145 billion over a decade. Mr. Goldman counters that analysts usually overestimate the costs and underestimate the benefits of medical interventions.

Because Grail uses machine learning to detect DNA-methylation cancer linkages, the Grail test’s accuracy should improve as more tests and patient data are collected

regulators may balk at approving the test, and insurers at covering it, until it becomes cheaper and more reliable.

How would the FDA weigh the risk that a false positive on a test like Grail’s could require invasive follow-up testing against the dire but hard-to-quantify risk that a deadly cancer wouldn’t be caught until it’s much harder to treat? It’s unclear.

some experts urge the FDA to require large randomized controlled trials before approving blood cancer tests. “Multicancer screening would entail tremendous costs and potentially substantial harms,” H. Gilbert Welch and Tanujit Dey of Brigham and Women’s Hospital wrote

Dr. Welch and Mr. Dey also suggested that companies should be required to prove their tests reduce overall mortality, even though the FDA doesn’t require drugmakers to prove their products reduce deaths or extend life. Clinical trials for the mRNA Covid vaccines didn’t show they reduced deaths.

One alternative is to rely on real-world studies, which Grail is already doing. One study of patients 50 and older without signs of cancer showed that the test doubled the number of cancers detected.

One recurring problem he has seen: “Epidemiologists are always getting cancer wrong,” he says. “Epidemiologists a decade ago said U.S. overtreats cancers. Well, no, the EU undertreats cancer.”

A 2012 study that he co-authored found that the higher U.S. spending on cancer care relative to Europe between 1983 and 1999 resulted in significantly higher survival rates for American patients than for those in Europe

By his study’s calculation, U.S. spending on cancer treatments during that period resulted in $556 billion in net benefits owing to reduced mortality.

He expects Galleri and other multicancer early-detection tests to reduce deaths and produce public-health and economic benefits that exceed their monetary costs

Expanding access to multicancer early-detection tests could also help solve the chicken-and-egg problem of drug development. Because few patients are diagnosed at early stages of some cancers, it’s hard to develop treatments for them

the positive predictive value for some recommended cancer screenings is far lower. Fewer than 1 in 10 women with an abnormal finding on a mammogram are diagnosed with breast cancer.

Mr. Royse makes the same point with personal force. “I would be dead right now if not for multicancer early-detection testing,” Mr. Royse told an FDA advisory committee last fall. “The longer the FDA waits, the more people are going to die. It’s that simple.”

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 new law, sponsored by Democratic lawmakers, also dictates that the technique be used only for serious crimes, such as murder and sexual assault. And it states that investigators may only use websites with strict policies around user consent.

The laws “demonstrate that people across the political spectrum find law enforcement use of consumer genetic data chilling, concerning and privacy-invasive,” said Natalie Ram, a law professor at the University of Maryland who championed the Maryland law

“This bill strikes a balance between this very important technology to identify people that do the very worst things to our Marylanders, yet it balances that against the privacy concerns and the trust that we need from the public,” John Fitzgerald, the chief of the Chevy Chase Village Police Department

For one thing, the law states that by 2024, genealogists working on such cases must be professionally certified — a credential that does not yet exist.

Investigators may use only genealogy companies that have explicitly informed the public and their customers that law enforcement uses their databases, and that have asked for their customers’ consent to participate.

“We know well that most people do not read these kinds of forms closely,” Ms. Ram said. “This is likely to generate unwitting inclusion rather than actual consent.”

Unlike 23andMe and Ancestry, which have kept their immense genetic databases unavailable to law enforcement without a court order, GEDmatch and FamilyTreeDNA are eager to cooperate

Investigators cannot use any of the genetic information collected, whether from the suspect or third parties, to learn about a person’s psychological traits or disease predispositions

These searches are “the equivalent of the government going through all of your medical records and all of your family records just to identify you,” said Leah Larkin

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.)

After acquiring the ability to infect fleas, Yersinia pestis may have begun to spread more readily from one rodent to another, eventually causing widespread epidemics.

Christopher Columbus traveled more after death than many people do in life! In 1537, his bones and those of his son Diego were sent from Spain to Santo Domingo to lie in the cathedral there. As time went on, Santo Domingo became less important to the Spanish Empire and in 1795 Spain ceded all of Hispaniola, including Santo Domingo, to France as part of a peace treaty.

Columbus' remains were judged too important to fall into French hands, so they were sent to Havana. But in 1898, Spain went to war with the United States, and the remains were sent back to Spain lest they fall to the Americans.

In 1877, workers in the Santo Domingo cathedral found a heavy leaden box inscribed with the words “Illustrious and distinguished male, don Cristobal Colon.” Inside was a set of human remains and everyone assumed they belonged to the legendary explorer. Columbus was returned to his resting place and the Dominicans have claimed ever since that the Spanish hauled the wrong set of bones out of the cathedral in 1795. Meanwhile, the remains sent back to Spain via Cuba were interred in an imposing tomb in the Cathedral in Seville. But which city had the real Columbus?

The man whose remains are in the box in the Dominican Republic shows signs of advanced arthritis, an ailment from which the elderly Columbus was known to have suffered. There is, of course, the inscription on the box, which no one suspects is false. It was Columbus’ wish to be buried in the New World and he founded Santo Domingo: it’s not unreasonable to think that some Dominican passed off some other bones as those of Columbus in 1795.

The Spanish have two solid arguments. First of all, the DNA contained in the bones in Seville is an extremely close match to that of Columbus’ brother Diego, who is also buried there. The experts who did the DNA testing believe the remains are those of Christopher Columbus. The Dominican Republic has refused to authorize a DNA test of their remains. The other strong Spanish argument is the well-documented travels of the remains in question: had the lead box not been discovered in 1877, there would be no controversy.

The tourism factor alone is huge: many tourists would like to take their picture in front of Christopher Columbus’ tomb. This is probably why the Dominican Republic has refused all DNA tests: there is too much to lose and nothing to gain for a small nation that depends heavily on tourism.

The Dominicans refuse to acknowledge the DNA test done on the Spanish bones and refuse to allow one to be done on theirs: until they do, it will be impossible to know for sure.