For as long as humans have lived in complex communities, cities and civilizations, they have divided and classified their societies. Those divisions have been based on age, gender, appearance or – in many cases – occupation. In many traditional societies artisans would share the same social status; as would soldiers, priests and workers in any number of other occupations.

In antiquity, the status of a family rarely changed. If you were a farmer, your sons would be farmers, and so on. While today social status barriers are crumbling in many societies, in others they remain largely unchanged.

India’s complex social stratification, known as the caste system, has been one of the traditional cornerstones of society. Though urban Indians are shedding the caste labels of their parents and grandparents, many rural Indians – who make up 72% of the entire population – hold steadfast to the system. In small villages and towns, the Brahmin caste – consisting of scholars and priests – is still revered as one of the highest social strata. And members of the Dalit caste – formerly known as “Untouchables” – are still viewed as unclean and remain separated from others.

The rigidity of the system still present in rural India has made many wonder exactly how long castes have existed. Historical records are unclear, as early Hindu scriptures like the Bhagavad Gita are somewhat ambiguous when it comes to the topic. Some historians even propose that the caste system as we know it today is largely a construct of the English Colonial Era, arguing that the development of such a system could have been deemed necessary to instill order.

Genetic analysis has also proven inconclusive, as analysis of small segments of the human genome has yielded different results. But a new study by geneticist David Reich and colleagues, published in the September 24 issue of Nature, takes a new approach to understanding the genetic history of India.

The core difference between Reich’s genetic analysis and previous studies is in the sheer amount of genetic material analyzed. Reich’s team examined more than 550,000 points across all segments of the human genome. In doing so, they hoped to obtain a more complete picture of Indian genetic history.

The research team analyzed the DNA of 132 individuals from India and neighboring regions, dividing them into 25 distinct groups based on geography, caste and language. They calculated how genetically ‘closed’ each of these groups were. In the caste system it is rare to marry someone from another class, making caste societies very closed, or ‘endogamous.’ If this endogamy continues over many generations, it will leave a behind a genetic signature for scientists to discover.

Reich and his team found such a signature, indicating a long history of endogamy in several of the groups. In fact, the research team calculated that the DNA of six of the groups can be traced back to just a few individuals who lived anywhere from 30 to more than 100 generations ago. Assuming a generation time of 25 years, that establishes the existence of the caste system in the range of 750 to more than 2,500 years ago — long before the British colonial era.

In a second analysis, Reich and his team examined how ancient migrations could have influenced the formation of castes. First the researchers divided the Indian groups into language families: Indo-European and Dravidian. Dravidian tongues, like Tamil and Malayalam, are mainly spoken in southern India and are believed to be a remnant of languages spoken by some of the earliest inhabitants of the region. Indo-European languages, like Punjabi and Urdu, are more common in the north. They are believed to have arrived with a migration of farmers from southwestern Asia or the Near East about 9,000 years ago.

Reich and his colleagues then compared the genetics of each of the Dravidian and Indo-European groups to a sample of European DNA. The team reasoned that, if Indo-European groups were really descended from the farmers, they would show more genetic similarity to the Europeans than the Dravidians.

Not surprisingly, the authors’ hypothesis held true. The Indo-European speakers, like the Kashmiri Pandit and Vaish, were more genetically similar to Europeans. And because the majority of the upper castes speak Indo-European languages, while the lower ones tend to be Dravidian speakers, there could be a relationship between the arrival of Indo-European people and the formation of caste structure. Further evidence that an ancient caste system has permeated through India for thousands of years.

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In 1974, scientists digging in the dry lake bed of Lake Mungo in southeastern Australia uncovered the skeleton of a man preserved in the deep layers of sand and clay. Dating techniques eventually revealed that this individual died about 40,000 years ago.

Scientists and the popular press dubbed the individual “Mungo Man.” Why did he make such a splash?  Not only because he was – and remains – one of the oldest and most complete skeletons of the earliest Australians, but because his appearance shattered the previously held notion that humans had first set foot in Australia less than 10,000 years ago. It was so far from where humans arose in Africa, and so remote.  So of course humans arrived there so much later than everywhere else, many experts reasoned. With the discovery of Mungo Man, this idea lost support, and scientists now concede that Australia was settled much earlier than many other parts of the world, including the Americas and parts of Europe.

While this discovery initially answered many questions regarding the peopling of Australia, it left many more unanswered — especially how people could have reached an island continent so soon after humans first expanded beyond Africa about 60,000 years ago.

The thinking is that after leaving Africa, one or more groups of humans journeyed from their homeland in East Africa into Arabia via the Red Sea. Over the next several thousand years, their descendants continued along the coasts of Arabia and India, eventually heading south into present-day Indonesia and finally to Australia, which was joined with the island of New Guinea at the time.

There has been some archaeological and genetic evidence of such a migration, but most of it has been indirect or circumstantial. Some scientists remain unconvinced because researchers have not been able to show a direct link between modern Australian Aborigines and modern people living along the coastal route from Africa. But now, in a new study led by the Anthropological Survey of India, geneticists believe they’ve found the first concrete evidence of such a link. Their results are reported in the July 21 issue of BMC Evolutionary Biology.

The team, led by Satish Kumar, reasoned that if the hypothesis of an ancient migration along the Indian Ocean coast toward Australia was accurate, there would be evidence in the DNA of modern people living along that path. So they compared the DNA of modern Australian Aborigines to that of tribes from India, such as the Baiga of central India and the Birhor of eastern India. These groups are often called “relic populations” because they are believed to share many cultural, linguistic, physical and genetic features with the region’s ancient inhabitants.

Experts have long noticed that the Baiga, Birhor and other relic populations share physical similarities with native Australians. Kumar and his team reasoned that there could be DNA similarities too.

Kumar led the extraction and analysis of mitochondrial DNA (mtDNA) from nearly 1,000 individuals from Indian relic populations. For comparison, they used Australian Aboriginal DNA data that had already been analyzed and published by colleagues. After comparing the two groups, they came to a startling conclusion: two specific genetic mutations on the mtDNA of the Indian and aboriginal samples matched perfectly. Not only that, but these particular mutations do not exist elsewhere in the world; they are shared exclusively between a few isolated tribes in India and native Australians.

Kumar and his colleagues concluded the two groups must share a common ancestry. To lend further credence to their theory, they calculated the date when the ancestors of the Indian tribes and Aborigines must have split.

Their calculations produced a date of 55,000 years ago, a time when early humans in India were probably hunting wildlife and gathering plant foods. Some of their descendants eventually formed tribes like the Baiga and Birhor; others moved eastward, traversing southeastern Asian and then using maritime technology to cross nearly 60 miles of open ocean between Indonesia and New Guinea.

After arriving in Australia, they moved into the heart of the Australian Outback. A few thousand years later, a direct descendant of these ancient explorers was laid to rest along the shores of Lake Mungo.

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A special supplement to Nature Reviews Genetics, published online today, details how four countries with emerging economies – Mexico, India, Thailand and South Africa – are taking steps to build capacity for studies of human genomic variation and its applications to healthcare.

Following completion of the HapMap Project, several consortiums in the developed world have undertaken large-scale genotyping projects. The hope is that these efforts will lead to advances in personalized diagnostics and therapeutics that could one day improve health outcomes.

The authors of the six-part series, all from the McLaughlin-Rotman Centre for Global Health (MRC) in Toronto, conclude that similar initiatives in genomic medicine have great potential to help developing countries by strengthening local research infrastructure and local intellectual property regimes, addressing local health needs and reducing health-care costs.
(The reports are available free of charge from Nature Genetics with registration)

The highlighted initiatives:

• Mexico
  Mexico has formed the National Institute of Genomic Medicine (INMEGEN), the first Mexican institute to systematically describe the genomic diversity of the Mexican population. INMEGEN will allow Mexico to focus on the link between genomic variation in the Mexican population, disease susceptibility and drug-response variability.
• India
  The Indian Genome Variation (IGV) database is a collaborative network that will capture essential data about disease predisposition, adverse drug reactions (ADRs) and population migration within India.
• Thailand
  The Thai SNP Discovery Project and the Thai Centre for Excellence in Life Sciences Pharmacogenomics Project will help Thailand understand the genomic diversity of its population and explore the role that this diversity has in drug response and disease susceptibility.
• South Africa
  The South African government is in the early stages of planning a national genomic medicine research program. Public engagement in genomic issues is spear-headed by The Africa Genome Education Institute.

“The world has reached an historic moment on the path to genomic medicine – the point where theory is about to be translated into practice,” said the project’s principal investigator, Professor Abdallah Daar, in a statement.

“Benefits of this emerging science cannot be an exclusive luxury reserved for wealthier industrialized countries. Instead it must be universally advanced by developed and developing countries alike to prevent an increased widening of already huge difference in global health care quality,” said project leader Dr. Béatrice Séguin.

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