杏吧原创

It鈥檚 not exactly bringing back the dead but it鈥檚 close. A person鈥檚 genome has been partially pieced together from fragments of his DNA found in hundreds of his modern-day descendants. It is the first time a dead person鈥檚 genome has been reconstructed without DNA extracted from their remains.

The person in question is , who is something of an icon in Iceland. His fame was bolstered by a biography, .

Jonatan was born in 1784 on the island of St Croix, then part of the Danish West Indies and now one of the US Virgin Islands in the Caribbean. His mother, Emilia Regina, was a black woman who was a house slave. His father was a Danish man called Hans Gram, a secretary on a plantation. Jonatan became a slave on the plantation.

Following a downturn in business, the plantation鈥檚 Danish owner returned to Denmark in 1789, taking Regina and later Jonatan.

After fighting for the Danish navy in the Napoleonic wars, Jonatan declared himself a free man, as slavery was illegal in Denmark. In a key court case in 1801, Jonatan鈥檚 lawyer argued he could no longer be kept as a slave. But in 1802 the judge ruled that he should be sent back to the Danish West Indies, where slavery was still legal.

However, Jonatan escaped.

In search of freedom

That same year he arrived at Dj煤pivogur, a small fishing town in east Iceland. There he found immediate sanctuary and acceptance. 鈥淗e was the first black man to set foot on Icelandic soil and was received with open arms, showing that racism was not innate in this insular 19th-century community of Icelanders,鈥� says K谩ri Stef谩nsson of in Reykjavik, Iceland.

Jonatan became a merchant and peasant farmer, and ran the trading post in the village. He had two surviving children with his Icelandic wife, Katr铆n Anton铆usd贸ttir.

Jonatan died in 1827. 鈥淗e was buried in an unmarked grave, so his remains may exist, but we don鈥檛 know where they are,鈥� says Agnar Helgason of deCODE.

Now, almost two centuries later, deCODE has reconstructed a big chunk of the DNA Jonatan inherited from his mother. To do so, the team took DNA samples from 182 of his Icelandic descendants going back four or five generations, starting with some of his great-great-grandchildren.

The analysis reveals that Jonatan鈥檚 African family hailed from a region of West Africa encompassing modern Nigeria, Cameroon and 鈥� most likely 鈥� Benin.

Back from the grave

deCODE was able to rebuild parts of Jonatan鈥檚 genome due to two special circumstances.

First, Iceland has kept meticulous records of family trees since it was first colonised more than 1000 years ago. More recently, deCODE has built a database of DNA from over 150,000 Icelanders, around half the current population. This rich information means it is easy to work back through family trees.

And secondly, Hans Jonatan was the first inhabitant of Iceland with African heritage and the only one for more than 100 years. 鈥淭here was no African ancestry in Iceland, apart from Hans Jonatan, prior to around 1920,鈥� says Stef谩nsson.

The DNA Jonatan inherited from his mother was distinct from that of his fellow Icelanders, which was almost entirely of European origin. Its distinctness allowed deCODE to track fragments of African DNA from Jonatan鈥檚 genome through his descendants and reassemble as many as possible.

deCODE identified 182 of Jonatan鈥檚 788 known descendants by screening their DNA against known markers of African DNA. They identified 593 fragments that could be traced back to Jonatan and Regina. In all, they managed to reconstruct 38 per cent of the material in Regina鈥檚 chromosomes, equivalent to 19 per cent of Jonatan鈥檚 genome.

deCODE confirmed that Regina was of African origin by screening her reconstructed DNA against reference DNA from Europe, Africa and Asia. By comparing it with samples from specific geographic regions, they pinned her origins to Benin, Cameroon or Nigeria, with Benin the best fit. Either she, or her ancestors, was probably abducted from there.

Reconstructing lost people

鈥淭his is an amazing piece of work,鈥� says at the University of Warwick, UK. 鈥淏ut this seems to be the sort of analysis you could only do under particular circumstances, when an immigrant genome is of a very rare type.鈥�

Still, there are possible uses. 鈥淚t鈥檚 the sort of study that could, for instance, be used to recover genomes of explorers who had interbred with isolated native communities,鈥� says Allaby.

Stef谩nsson is more optimistic. 鈥淚t鈥檚 all a question of the amount of data you have,鈥� he says. 鈥淚n principle, it could be done anywhere with any ancestors, but what made it easy in Iceland was that there were no other Africans.鈥�

deCODE鈥檚 methods could, in theory, be used to build 鈥渧irtual ancient DNA鈥� of long-dead historical figures, or family members. 鈥淎ny historic figure born after 1500 who has known descendants could be reconstructed,鈥� says Helgason. But the quantity, scale and detail of DNA required may mean this is generally impractical. Identifiable fragments of a person鈥檚 DNA get smaller and more diffuse with every generation of descendants.

And while the study may conjure up scary prospects of reviving dead dictators by reassembling their genomes, this is pure fantasy. Apart from the monumental challenges involved, people aren鈥檛 wholly shaped by their genes.