The history of all humankind begins approximately 2,00,000 years ago when the first anatomically-modern humans are thought to have appeared in Africa. Then, around 60,000 years ago, a band of people ventured out of Africa, into the Middle East, branched out into India and Europe, and ultimately settled all over the planet, replacing other early human populations. These new settlers changed as they adapted to different conditions, as they migrated and interbred in complex ways. Eventually, they gave rise to the wide variety of humans found today across the earth: dark-skinned, light-skinned, red-haired, blue-eyed, able to digest milk as adults, able to resist some diseases but prone to others, and so on.
In 2005, K. Thangaraj and his colleagues at CCMB published their findings about the origin of Andaman islanders in the journal Science. The Onge turned out to have surprisingly unmixed origins. They had likely lived isolated in the islands since the arrival here of the first group of humans out of Africa. There were mutations in their mtDNA that were found nowhere else in the world. These mutations must have originated here and not spread. The Onge were an untouched link to the earliest humans who settled the planet.
Among those who noticed CCMB’s work was David Reich, a geneticist at Harvard Medical School with an interest in studying how human populations had mixed in the past. He approached CCMB with a view to working together on the genetic history of indigenous Andamanese.
“This has been a wonderful collaboration,” Reich writes by email when asked about working with CCMB. Their comparison of people such as the Onge with diverse ethnic groups on the Indian mainland has now led to important insights into the ancient history of Indians.
All humans carry DNA from both parents in their body cells. The nucleus in a cell contains two sets of 23 chromosomes, one from our mother and one from our father. When a man’s body produces sperm, or when a woman’s produces eggs, bits of DNA from that person’s parents are spliced together to create a single recombined version. Their child will then have DNA assembled from the DNA of his four grandparents, and thus, generation to generation, we carry our entire family tree in our genes.
It is relatively straightforward to draw conclusions about our origins by looking at mtDNA because it passes unchanged from mother to daughter. Every change observed is a significant marker. This is also the case with another part of DNA called Y-DNA, that passes unchanged from father to son. But the rest of DNA recombines every generation, and it is far trickier to make sense of ancestry from the wealth of jumbled information that is the entire human genome.
Two people from different places or ethnic groups will possess characteristically different markers in their DNA. “You can think of their DNAs as being two long strips of paper, one red and one green,” K. Thangaraj tells me on the phone from Hyderabad. If those two people have a child together, the recombined DNA can be thought of as a single strip with alternating stretches of red and green, a sort of bar code of history and ancestry.
By looking at the lengths of stretches of the two colours and how often they occur, it can be estimated how much ancestry each colour contributed, and how many generations ago those colours interbred. “Of course in reality it’s a little more complicated than that,” says the CCMB researcher. “And that’s where David Reich comes in.”
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