The Guardian, Monday 25 April 2011:
The earliest humans are silent witnesses: they testify only through their bones and tools.
But modern humans carry within their tissues a different kind of evidence. DNA serves as a lineal history, a family album, a passport that bears the marks of both origin and journey. Mitochondrial DNA (mtDNA) is inherited only from the mother.
Every few generations, a random mutation creeps into this familial signature. So comparison of two samples of mtDNA will show degrees of kinship and ancestral origin. Conversely, the Y chromosome – a twisted rope composed entirely of DNA – is inherited by males from the father. Random infrequent changes once again provide a way of estimating the number of generations back to a shared ancestor. The evidence of DNA reveals that all humans are very closely related. A Scot, a Japanese and an Australian Aborigine are far more closely linked by family inheritance than any three chimpanzees from different African groups. DNA research suggests that all surviving humans are descended from one woman who lived perhaps 200,000 years ago. Research also shows that the story begins in Africa, home to the greatest variation in human DNA, and therefore the oldest location. Accordingly the woman was promptly dubbed "the African Eve".
Not surprisingly, people of the same ethnic and linguistic group turn out to be genetically more closely related to each other than to the rest of the planet, but the same research shows a great deal of mixing of populations as well. Studies of telltale markers in the DNA sequence have been used to reconstruct the journeys of ancient human groups around the globe, and not just ancient humans. Along the way from East Africa to Easter Island, early human voyagers picked up fellow travellers such as the stomach ulcer bug Helicobacter pylori. This bacterium also carries a DNA signature of its origins.
In 2009, it delivered an answer to one of the great mysteries of the human migration: all the settlers on the islands of Polynesia and Melanesia carried stomach bugs that their ancestors could only have picked up in Taiwan. So that became the jumping-off point for the colonisation of the Pacific. Long before agriculture, metalwork, settlement, writing, nationality and the idea of history, long before formal territorial identity or ethnic tradition, all humans had a scrapbook, a set of passport stamps that are now beginning to reveal some twists in the great human journey. The beginning remains a mystery. But the blood and flesh of humans and chimpanzees holds a molecular story-so-far, a cryptic chemical summary of the 6m-year human thriller.
DNA is a new way of telling: the secrets of its decryption exposed in less than one human lifetime. As they read it, the characters on the latest page are beginning to see what must have happened in the earlier chapters. There is more to come.