There are many examples around the world of two distinct ethnic groups living side by side.

Sometimes these groups co-exist peacefully. Other times they do not.

Often two groups’ differences – along with circumstantial factors – lead to tension between them and sometimes violence. The Hutus and Tutsis of Rwanda, the Sunnis and Shiites of Iraq, and the Croats and Serbs of former Yugoslavia all illustrate how cultural distinctions – like language and religion – can contribute to tensions and conflict around the globe.

But do these cultural and ethnic distinctions translate to biological distinctions as well? Exactly how biologically distinct are two ethnic groups living side by side? Anthropologist Evelyn Heyer and an international team of researchers set out to answer these and many other questions by studying the adjacent – and culturally very different – Tajik and Turkic speakers along the Silk Road of Central Asia. Their results are published in this week’s BMC Genetics.

The authors focused on the Tajik and Turkic speakers because those groups offered a unique perspective on how two groups living in such close proximity can be so different from each other.

The Turks are largely nomadic herders. They speak Indo-Iranian languages like Azerbaijani, Turkish, and Altay. Their society is organized into clans, or “descent groups,” whose membership is passed down from father to children.

The Tajiks are, conversely, agriculturalists. They speak various dialects of the the Tajik, or Tajik Persian, language that may have arrived with Muslim invaders 1,000 years ago. Their society is largely patrilocal – meaning that when couples marry they put up residence near the husband’s family; and first cousin marriages are encouraged.

The two societies are supposedly closed, and members of both groups are said to rarely leave their clan or village. This cultural isolation made them perfect candidates for Heyer and her team to study.

So the researchers collected both maternally inherited mitochondrial DNA and paternally inherited Y chromosome DNA from more than 1,000 individuals spanning 24 Turkic and Tajik populations.

What they found was that these two ethnic groups weren’t so different after all.

Genetically, the Tajiks and the Turks were virtually indistinguishable. The authors found the overall level of genetic diversity between the two groups to be less than 1% overall — so small that there was a greater amount of diversity within each group than between the two.

Their analysis also shed some light on the origins of these these two ethnic groups. The modern-day people of Central Asia maintain their own origin stories that are unique to their particular group. In part, it is these unique origin stories that distinguish them from one another. But Heyer’s analysis proves that these groups actually share the same roots; they are simply a hodgepodge of the clans, tribes, and villages that have called Central Asia their home for thousands of years. Over many generations they banded together to form larger groups until they consolidated into just two major divisions: the Tajiks and the Turks.

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Just over 20 years ago, the first study using mitochondrial DNA (mtDNA) to trace prehistoric human migrations was published. In this seminal study, scientists managed to determine that all humans alive today can trace their maternal ancestry back to one woman who lived about 200,000 years ago in Africa. The findings were revolutionary, and the idea that we could use genetics as a new tool to examine human prehistory was staggering.

One of the main reasons mtDNA was originally used was that it is passed down relatively intact, from mother to children, without recombining with any other bit of the human genome. And even more importantly, there was a small section of the mtDNA that did not code for any genes and was thus unaffected by natural selection. Any changes in this bit of mtDNA over time had to be caused by random mutations, which were believed to occur at a regular, clock-like rate. By counting and analyzing the mutations distinguishing any two individuals – or any two groups of people – scientists reasoned they could discover when their most recent maternal ancestor lived.

But there were a few problems with this idea. First, the small section of mtDNA scientists were analyzing, known as the Hyper-Variable Region (HVR), represents only about 2.5% of the entire mitochondrial genome. Some scientists argued that analyzing the genome in full would yield more accurate information about our past. Second, the idea that the mutations in our mtDNA accumulate like clockwork has been questioned continually. Just last year, 23andMe scientists reported that the mtDNA mutation rate may have been accelerating since the end of the Last Ice Age about 15,000 years ago, when human populations around the world began to grow and diversify at much faster rates. It soon seemed that many of the time estimates scientists had been calculating for years had been less accurate than previously thought.

So geneticists at the University of Leeds took decided to create a new method of calculation based on the best possible evidence. In the June 12 issue of the American Journal of Human Genetics, the team led by Pablo Soares and Martin Richards describes how they developed a way to estimate the maternal ancestry of the world’s people that is more accurate and precise.

The revised method used by these geneticists centered around the idea of natural selection. Up to now natural selection had not played a major role in calculating time estimates, mainly because scientists only used the HVR to make these calculations, which was thought to be immune from natural selection. Natural selection, an evolutionary force first described by Charles Darwin, gradually removes harmful genetic mutations and does appear to act on much of the mitochondrial genome. Failing to account for this evolutuionary force, according to Soares, has led to inaccurate and imprecise time estimates. “What we’ve done is work out a formula that corrects this effect,” says Soares. Using this newly developed mathematical formula, “we can [now] date any migration for which we have the available data.”

After developing this formula, Soares and his colleagues set about to test current theories on prehistoric human migrations patterns by comparing previous time estimates to those calculated using their new and improved formula. The results have already cleared up some problems with the previous estimates. Using their formula to calculate the time when humans first made their away across the Bering Strait to the Americas, they came up with a date of about 15,000 years, a full 2,000 years later than previous estimates. This new estimate can put to rest a longstanding discrepancy between genetic data and the archaeological record.

According to Richards, “We can settle the debate regarding mankind’s expansion through the Americas. Researchers have been estimating dates from mtDNA that are too old for the archaeological evidence, but our calculations confirm the date to be some 15,000 years ago, around the time of the first unequivocal archaeological remains.”

The 23andMe Maternal Line feature already puts that event at 15,000 years ago, because our scientists rely on both archaeological and genetic evidence when estimating the dates of past events.

Soares and colleagues found similar differences when recalculating the time estimates of other important prehistoric migration events, like the peopling of Polynesia just a few thousand years ago and the earliest migration of modern humans out of Africa nearly 70,000 years ago. But if anything, these corrections bolster the use of mtDNA in dating prehistoric events. Contrary to what some scientists have long feared, this study reveals that the principle underlying the technique was always sound; it was the specific method scientists used to analyze mtDNA that required a bit of fine-tuning.

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It seems like new discoveries about the peopling of the Americas are a dime a dozen these days.  Without a doubt, the journey those first Americans took from the frozen wastelands of Asia down the Pacific coast into the Americas has been an active research subject for many decades.  Archaeologists, linguists, and now geneticists have all analyzed the data in their respective fields, and while we have seen progress in figuring out the overall timing and migration routes across the Bering Strait during the height of the Ice Age, many questions remain unanswered.  For example, there is still disagreement over whether there was a single wave of migrations into the New World around 18,000 years ago (a scenario generally favored by geneticists), or whether there were several separate migrations, each bringing across the Bering Strait its own distinct culture and languages (more popular among linguists).

Now, an international team of geneticists has added to the debate by trying things a bit differently.  While the majority of genetic studies have focused on the four most common mitochondrial DNA (mtDNA) types, or ‘haplogroups,’ among Native Americans, these authors switched things up a bit. In study published online Thursday by Current Biology, they focused instead on two of the most rare and localized mtDNA haplogroups in the New World: D4h3 and X2a.

Mitochondrial DNA is passed down from mother to child exclusively. So by comparing the mtDNA of different populations, geneticists can estimate where and when their female lines diverged from one another.

Haplogroup D4h3 is usually found along the Pacific coast of South America, while X2a has been found only in north-central North America.  The authors sampled a total of 55 individuals who fell into either one of these two groups.  They sequenced the entire mitochondrial genome for each, thereby adding to the considerable lack of knowledge on these haplogroups; this was the first time that anyone completely sequenced representatives of either D4h3 or X2a.

The analyses of haplogroups D4h3 and X2a revealed two distinct genetic histories. That difference suggests they may have come from separate regions of Asia and expanded in the New World in very different directions, even though they both may have arrived around the same time.

Specifically, the authors argue that that, even though it appears that both D4h3 and X2a individuals arrived in the New World at about the same time period – between 14,000 and 17,000 years ago – they took very different routes to get there. The authors argue that D4h3 individuals crossed from Asia to the Americas via a coastal route; the same path the ancestors of most people bearing the major haplogroups are believed to have taken. Then, they continued down the Pacific coast, settling in various places along the way.

However, X2a individuals seemed to have embarked on a different journey.  Avoiding the coast entirely, this group of people traveled through a small inland corridor between two major North American ice sheets, about 15,000 years ago.  Then, they continued into the heart of North America, settling in what is now central Canada, where their descendants still reside today.

The results presented by the authors provide – perhaps for the first time – clear evidence that at least two separate routes were used by the earliest Paleo-Indians as they left East Asia and entered the Americas.  But, and maybe even more importantly, it shows that the migrants who took these routes may have come from two different source populations.  We can only hope that future research on these two haplogroups can reveal where in East Asia they originated, and maybe even what made them cross the frozen landscape in the first place – by land and by sea.

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One of the reasons genetics is such a powerful tool for telling us about the past is that mutations that accumulate over the generations can be used as a clock, allowing scientists to calculate when different events occurred in the prehistoric past.

By counting the number of mutations differentiating one person from another, we can determine when in the past they shared a common ancestor. Each mutation is like a genetic tick of the clock, equivalent to a certain period of time. But figuring out how frequently mutations occur, and thus how long each tick of the genetic clock takes, is a major enterprise.

In a new paper appearing this month in Molecular Biology and Evolution, scientists from 23andMe propose that the genetic clock may have started running faster after the Ice Age ended about 15,000 years ago. If so, they may have resolved a discrepancy that has perplexed researchers for a number of years.

DNA mutation rates are estimated in two different ways. Geneticists can either count the number of mutations that occur in one generation (the changes between parents and their children), or they can count the number of differences between human and chimpanzee DNA and calculate a mutation rate based on the number of years since the two species diverged — about 6 million years.

It turns out that mitochondrial DNA mutation rates measured by these two methods described differ almost 10-fold. This difference is seen not only in humans, but also in species such as birds and fish. The discrepancy hints at the possibility that what we are measuring might be more complicated than we previously thought.

Working with Marcus Feldman of Stanford University, 23andMe scientists set out to find a new way of estimating the human DNA mutation rate by comparing the genetic diversity of lineages derived from the first human inhabitants of various continents and islands with archaeological finds that give precise information about when those colonists arrived (such as Polynesia).

We focused our studies on mitochondrial DNA data because there are a large number of mitochondrial sequences available in databases and the mutation rate of this DNA is known to be higher than in the autosomal DNA found in the 23 pairs of chromosomes,

What we found was surprising. Younger lineages (i.e. groups of people that have more recent common ancestors) had higher rates of mutation than older lineages, meaning that the molecular clock is not constant for human mtDNA: it slows down as lineages get older.

But why?

In our study the estimated mutation rates dropped off quickly around the end of the Ice Age — 15,000-20,000 years ago — suggesting that population history may play an important role in explaining the reduction in genetic diversity, and thus the changing pace of the DNA clock.

Prior to 20,000 years ago, humans lived in small hunter-gatherer groups that likely experienced boom and bust cycles: periodic climatic shifts or food shortages probably caused frequent and sudden population collapses. When populations decrease in size, or “bottleneck,” some genetic lineages are lost, which decreases the genetic diversity of the population. Since mutation rates are calculated using genetic diversity, the mutation rate estimates from these older lineages are slower.

After the climate warmed about 15,000 years ago and humans subsequently invented agriculture, populations grew dramatically. This resulted in more stable genetic diversity and faster mutation rate estimates.

More research needs to be done before we can determine whether population history or other factors, such as natural selection changing mutation rates, are primarily responsible for the human molecular clock slowdown. For example, we could simulate genetic diversity changes when a population size decreases and see if this matches the empirical mutation rate estimates.

Using our new data, which trace the slowdown in mutation rate back in time, scientists can identify the number that is most appropriate to use for studies of particular population events.

To date, the timing of most population events in human evolutionary genetics was estimated has used a rate close to the slower one we see for older lineages, before the end of the Ice Age. So our understanding of the genetic history of early human evolution shouldn’t change very much. But the timing of the splits between mitochondrial lineages associated with relatively recent events, such as agricultural expansions, may need revision. Using our newly calibrated mitochondrial mutation rates, researchers will be better able to correlate genetic, archaeological and linguistic data, leading to a more accurate understanding of human prehistory.

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One of the most infamous emperors of Chinese antiquity was the very first:  Qin Shi Huang.  Also known as Ying Zheng, he ruled the Chinese state of Qin from 247-210 BC.  When he came to power, various Chinese kingdoms were engaged in a struggle against each other for superiority; by the time he died in 210 BC he had managed to unify these warring states, becoming the First Emperor of China and ushering in the Qin Dynasty.

Under the rule of Qin Shi Huang, massive construction projects were undertaken to help build the infrastructure needed in an imperial China, often at the expense of many lives.  The first version of the Great Wall of China was built during this period, as was the first national road system.

But the most famous remnant of Qin Shi Huang’s rule was his mausoleum in the Shaanxi province of central China.  Construction began in 246 BC — in addition to the tomb that would house the body of the emperor, there were large palaces, valuables, rivers flowing with mercury, and scenes of the earth and sky painted on the ceiling.  But the most spectacular feature of the mausoleum was the emperor’s guards:  the Terracotta Army.  These 8,000 soldiers, crafted using molds and clay, stand guard over the Emperor Qin’s tomb so they may aid him in the afterlife.

The construction of the mausoleum, like other construction projects during the emperor’s rule, involved a huge work force.  An estimated 700,000 workers were involved in the mausoleum construction alone. Until now the identity of these workers has remained a mystery, but a study published last week in PLoS ONE has attempted to discover who these workers were by examining the DNA preserved in their bones.

In 2003, hundreds of skeletal remains were unearthed near the mausoleum.  They were believed to be the remains of the workers who built the monument, and a preliminary examination of their bones revealed that these men were engaged in heavy manual labor up to their death.  But due to poor preservation of the remains, scientists could not determine the ethnic origins of these workers.  Since the Qin Dynasty controlled a vast territory and encompassed 22 million people, these workers could have come from anywhere.  So Chinese scientists decided to examine the mitochondrial DNA of these workers directly to discover their ethnic origins.

These scientists started by collecting DNA samples from 50 thigh bones for analysis.  Because ancient DNA analysis is prone to contamination, the researchers took extreme precautions in performing their analysis.  In the end, the state of decay of the samples meant the DNA of only 19 individuals could be analyzed.

What the researchers found was that these workers had come from a variety of places across East Asia.  In fact, the 19 individuals fell into 16 unique maternal haplogroups.  The most common haplogroup was N9a, which is thinly spread throughout central and eastern Asia today.  Other maternal haplogroups represented included M8a, A, and D5, which are all present among East Asian populations today.

Only four of the 19 specimens could be considered ethnically Han, which is the most common ethnic group in China today.  Interestingly, seven individuals came from southern China, and a smaller number came from northern China.  One individual even carried the maternal haplogroup usually seen only among certain Japanese populations, M7a.

The authors conclude that the overall diversity of mitochondrial genetic types seen in these 19 individuals indicates that workers were taken from across China, something that is also asserted in historical records from the period.  While further analysis on a greater number of remains is necessary, this initial study has shown that workers from all parts of the burgeoning Qin Empire may have taken part in building some of the first structures of Imperial China.

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Despite mounting genetic evidence that modern humans are not descended from Neanderthals, there are still some who argue that our two species interbred when both roamed Europe about 35,000 years ago.

A report appearing tomorrow in the journal Cell puts another nail in that theory’s coffin. Svante Paabo’s group at the Max Planck Institute for Anthropology in Germany has produced the first-ever complete sequence of a Neanderthal’s mitochondrial genome. Their analysis shows that the last common ancestor of humans and Neanderthals walked the Earth on the order of 660,000 years ago – hundreds of millennia earlier than the most recent common ancestor of all humans living today.

Mitochondrial DNA is passed down intact from mother to child. All people living today can use mitochondrial DNA to trace their maternal line back to the Mother of all Mothers (MoM), who probably lived about 175,000 years ago in eastern Africa.

When working with ancient DNA, researchers have to contend with several technical problems. Contamination by DNA from laboratory workers must be carefully avoided. And even if contamination is controlled, the inevitable chemical breakdown of DNA that has been buried for thousands of years can skew results. Lead author Richard Green and his colleagues avoided both of these pitfalls by sequencing the mitochondrial DNA nearly 35 times over.

“For the first time, we’ve built a sequence from ancient DNA that is essentially without error,” Green said in a statement.

The authors say that their success at sequencing the mitochondrial genome of a Neanderthal will help in their ultimate goal of sequencing the species’ much larger and more complicated nuclear genome, which could then be compared with a modern human genome to identify genes that were important in the emergence of Homo sapiens.

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Just about everything else is subject to debate: who the people were, where they originated, when they migrated, how numerous they were and what they did once they crossed into the Americas.

Finally, a solution to the decades-old dispute may be at hand. That’s because genetic analyses are beginning to converge on a scenario that fits in with all the archaeological, linguistic, climatological and ecological data relevant to the issue – and also seems to make sense.

In this month’s issue of the American Journal of Human Genetics, researchers from Brazil and California argue that the present-day pattern of genetic diversity among Native Americans suggests a rapid southward migration along the Pacific coast that began about 18,000 years ago and took only a few thousand years to reach the tip of South America.

It was only later, the researchers claim, that people made their way inland in pursuit of large game animals such as woolly mammoth. The traditional view is that these mammoth hunters, who appear in the archaeological record about 13,000 years ago, were the first Americans.

This is not the first time a migration down the Pacific coast has been proposed. But the researchers were able to show by comparing the mitochondrial DNA of 86 Native Americans that all five of the major genetic branches in the New World trace back to an ancestral population of a few thousand people that began expanding rapidly around 18,000 years ago.

At that time – not long after the peak of the Ice Age – glaciers thousands of feet thick blocked the land route from Alaska into the heart of North America. But the coast was clear – recent research indicates the Pacific rim of North America was relatively ice-free by about 19,000 years ago.

The DNA evidence also offers information about the ancestral Asian populations that gave rise to the first Americans. The researchers compared the Native American DNA samples to their closest known relatives in Asia, and found that the Asian and American lines appear to have split about 5,000 years before the rapid population expansion on the American side.

That suggests a long period of isolation, probably in the region that is now around (and underneath) the Bering Strait, before the first people flooded into the New World.

Two recent papers in the open-access journal Public Library of Science One suggest a similar scenario, albeit with slightly different dates. One study, published last month by University of Florida researchers, analyzed mitochondrial DNA donated by 77 Native Americans. That study found that the overall pattern of diversity was most consistent with a population that has experienced two episodes of rapid growth – one about 40,000 years ago and another about 15,000 years ago.

A second study, published last year by a global team of researchers, found that all of the major genetic branches of Native Americans appear to have started diversifying about 14,000 years ago, after being isolated for a long period of time.

The beauty of these genetic studies is their ability to explain the vexing archaeological evidence for the migration of people into the Americas. There are signs of humans living in Siberia, far from the major population centers of the time, as early as 30,000 years ago. And there is evidence of people at the Monte Verde archaeological site in southern Chile as early as 14,500 years ago.

Yet there are no archaeological sites intermediate between those two very distant locations in time and space. The genetic evidence suggest two reasons why: First, a small number of people migrating very quickly down the Pacific coast wouldn’t have left many artifacts behind. And second, if the migration happened shortly after the peak of the Ice Age, as the genetic evidence suggests, then the coastline it followed is now well offshore – sea level has risen 120 feet since then.

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