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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When scientific research is published, the authors often confess that they wish they’d collected more data. Critical reviews of research studies often say the same thing. Indeed, if there’s anything scientists love, it’s more data.

Which is why the members of an international team of genetic anthropologists led by Sarah Tishkoff of the University of Pennsylvania are probably quite pleased with themselves. In a new study published this week in Science, the team took the concept of “more is more” to heart by collecting and analyzing the DNA of thousands of people, mostly from Africa, so that they might uncover more clues to not only the genetic make-up of modern Africans, but also the genetic history of Africans and non-Africans alike.

The scientists’ first step was to collect DNA from a diverse set of Africans. Africa is the most culturally and linguistically diverse place on Earth, so it was important to take a wide sample of individuals from all corners of the continent. In total, they collected 2,432 DNA samples from 113 diverse and distinct groups of people from across the African continent as well as 60 non-African groups. They sampled everyone from the Mozabite Berbers of Morocco to the hunter-gatherer San of the Kalahari Desert, and many in between.

But the hard work didn’t stop there. The scientists then examined 1,327 genetic markers across the human genome for each individual studied. While many studies focus on a particular part of the genome such the mitochondrial DNA or the Y chromosome, this study took a comprehensive approach. Finally, the researchers used sophisticated statistical techniques, piecing together how these populations from Africa and around the world were the same, and how they were different.

The results confirmed that Africa has the highest genetic diversity of any continent, as many scientists have proposed. In fact, the authors found genetic diversity to decrease the further one traveled away from Africa. Genetic diversity is often used as a measure of how long ago humans inhabited a region — conventional wisdom places the earliest humans in East Africa, which had exceptionally high genetic diversity according to this study, though an analysis by the researchers put the origin of the human expansion farther south near the border of Namibia and Angola.

The study also shed light on the incredible genetic diversity among African populations, said Roy King, a professor of psychiatry and anthropological geneticist from Stanford University:

Not only did farming and pastoral communities differ from hunter-gatherers, but within the broad range of agricultural populations of West and West-Central Africa — from which many African Americans derive their ancestry — the authors also found some genetic diversity. For example, the Dogon of Mali, although geographically near the Mandinka of Senegal, cluster with North African Berber populations. Thus, this study supports the notion that not only is Africa varied in culture — art, music, religion and language — but also harbors a rich genetic diversity across its multitude of ethnic groups.

The authors also found a loose connection between the genetics of a population and its language. However, there were a few exceptions, most often the result of a population adopting a new language within the last few thousand years.

The sheer size and diversity of the DNA samples collected allowed the researchers to construct a human family tree based on their analyses. Not unexpectedly, the tree they constructed fits well with current theories on the genetic relationship between Africans and non-Africans; namely that all non-Africans are descended from a particular group or groups of people who were the first humans to migrate out of Africa tens of thousands of years ago.

This study is important for a multitude of reasons. It has been able to confirm theories from the archaeological, cultural, and linguistic records on the origins and movements of Africans and non-Africans.

“It fits nicely with earlier genetic studies, while subverting the early 20th century colonialist idea of sub-Saharan Africa as constituting a homogeneous genetic an cultural unit,” King said.

It also creates a new resource that historians, linguists, archaeologists and scientists from a range of other disciplines can use in their own work. If we are lucky, this study will bring forth a flurry of activity surrounding the origins and history of the African continent, and the people who live there.

Credit: istockphoto/Erie

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If you take two members of the human race at random and ask how much their genomes differ, you’ll get a surprising answer: they’re almost identical.

On average, for every 1,000 DNA bases you have, 999 or so of them are exactly the same between you and your neighbor – and for that matter, between you and your neighbors on the other side of the planet. Over your entire 6 billion DNA base pair genome, however, that one difference in a thousand adds up to several millions of differences.

In studies published by Science and Nature this week, scientists have taken their most detailed look yet at these genetic differences. In this blog post, we’ll take a brisk stroll through their findings.

Both studies are based on around 600,000 SNPs genotyped in individuals (i.e., people) from the Human Genome Diversity Panel(HGDP-CEPH). HGDP-CEPH is a remarkable scientific resource. It consists of immortalized cell lines from 1064 individuals in 51 populations scattered around the globe. The idea guiding the creation of the panel was to take a wide-angle snapshot of human genetic diversity. This explains the presence in the panel of little-known populations like the Uygur, the Surui, and the Xibo, alongside more familiar populations like the Japanese, Palestinians, and French. This polyglot collection reposes at the Fondation Jean Dausset in Paris, as it has now for nearly a decade.

The Science study peers closely into those one-per-thousand differences between people, asking: Of all the genetic diversity seen in the panel, how much is found between people from the same population, how much between people from different populations in the same geographic region, and how much between people from different geographic regions?

For example, if there were no within-population diversity that would mean that all Russians are genetically identical, all Surui are identical, and so on, and therefore that all human genetic diversity must exist at the population and regional levels.

The paper finds nearly the opposite. About 90% of human diversity exists within populations, with most of the remaining 10% existing between geographic regions. This strongly confirms a decades-old result in human genetics: of those very few DNA bases which differ between people, a small minority of these differ between peoples.

Even so, 10% of several million differences is still a lot of differences between populations. Both studies zoom in on these differences, mustering some mathematical machinery called Bayesian cluster analysis, and ask: how easy is it to guess someone’s ethnicity based on their genotype? The answer the two papers find is that it’s pretty easy, at least on the regional scale. The following figure, drawn from earlier work done by many of the same researchers that was published in the open-access journal PLoS Genetics in 2005, illustrates the results of the analysis; these are qualitatively the same as the results shown in the fancifully-priced Science and Nature figures.

Each one of the (very) thin vertical lines in the figure represents a person, and the colors comprising each line correspond to the inferred proportion of ancestry from each of seven world regions. The key here is that the cluster analysis has no notion of a region or a population. It is simply told to divide up the genetic diversity into seven clusters (or six or eight – the results don’t change much), and then to guess which cluster or clusters each individual belongs to. The ethnic and regional labels are only applied once the analysis is through and, as is plain, the agreement between the donor-supplied ethnic label and the assignment is quite strong.

The papers go much further than we’ve seen here, looking into the history of human migration and exploring what happens when you use DNA insertions and deletions instead of SNPs to ask the same questions.

It’s worth noting that 23andMe is proud to have cosponsored the genotyping of the HGDP-CEPH that was conducted by the authors of the Science paper. The genotypes are available, gratis, at the CEPH website. We downloaded them ourselves, and now our customers can compare themselves to these very same populations using our Global Similarity feature.

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