Genetic sprint – all the way down through the Americas

It was more of a race than a migration when people began to spread across the Americas – from Alaska in the North to the southernmost tip of South America – around 15,000 years ago.

The continent is approximately 15,000 km long and, to date, science has assumed that this process took a very long time – millennia, in fact. But an international research project, recently published in the prestigious scientific journal Science, proves this not to be the case.

“On the contrary, it went extremely fast. It took only a few centuries to populate the entire Americas,” says Professor and DNA detective Eske Willerslev from the Centre for GeoGenetics at the University of Copenhagen. He headed the scientific study, which involved 54 of the world’s best researchers in analyses of archaeological DNA. This huge research team represents universities from ten countries, including Brazil, Argentina, the USA, the UK and Germany. The Danish contingent was made up of experts from the Technical University of Denmark.

The study, which was funded by, among others, the Lundbeck Foundation, the Danish National Research Foundation, the European Research Council and National Institutes of Health (USA), is based on DNA analyses of human bone material from 15 ancient skeletons found at various sites across the Americas. Six of the skeletons are more than 10,000 years old.

The scientists were able to isolate so much DNA from this material that they could reconstruct the entire genome of these people. And they could date the ancient bones using carbon-14 dating.

Eske Willerslev explains that this information – added to numerous more recent DNA profiles which were included as comparative data – enables us to piece together the history of the genetic sprint across the Americas, which took place around 15,000 years ago:

“We don’t yet know why the spread was so rapid, but it’s remarkable because the American continent is so huge! And the spread brings another fascinating aspect to light: mankind’s immense ability to adapt. Within the space of a few centuries, people settled in all of the continent’s climate zones: on both sides of mountain ranges and other geographical barriers. And each location posed its own special challenges, which needed to be resolved before man could settle and live there.”

Iconic bones spoke
Eske Willerslev explains that several of the over 10,000-year-old skeletons included in the study have an almost iconic history:

“An example is one of the 20 or so human skulls excavated by the Danish natural scientist P W (Peter Wilhelm) Lund – one of the world’s first palaeontologists – from a stalactite cave near the town of Lagoa Santa, Brazil, during the first half of the 19th century.”

P W Lund (1801-1880), who also discovered bones from many extinct animals, including a giant sloth, in the cave, sent the human skulls and a few bones from human limbs home to Copenhagen. And this evidence of early settlements in South America has lain in storage here ever since:

“Many scientists have tried, without success, to extract analysable DNA from this material over the past 15 to 20 years. And we’ve also tried at the Centre for GeoGenetics. When we were about to begin the study for Science, one of my colleagues, Peter de Barros Damgaard, suggested seeing whether the Lagoa Santa bones could be used – if we worked with part of the inner ear, the so-called petrous bone. This is a very compact bone and when we removed it from one of the skulls P W Lund had sent home to Copenhagen from Lagoa Santa, there was sufficient DNA to create the entire genome of an individual who lived around 10,400 years ago.”

The unique thing about this DNA image is that it carries traces of an Australasian genetic signal, most closely related to people currently living on the Andaman Islands, in the Bay of Bengal. And this is highly weird,” Eske Willerslev points out. “There’s no doubt that the signal is Australasian. But it’s not been found anywhere else on the American continent in archaeological DNA material. Neither in our study, nor previously.”

According to Eske Willerslev, the findings raise several questions – among others, whether the Americas were populated even earlier than 15,000 years ago:

“This has been suggested by the Brazilian anthropologist Walter Neves, although he’s not been able to support this convincingly. If there’s some truth in it, these people – in some way or other – most likely came to the American continent by sea. If such an ancient settlement took place, for example 30-50,000 years ago, this would be huge, because we’d have to rewrite the entire history of human settlement of the continent. But before we can study this more closely – and we’d very much like to do so – we need to find more prehistoric DNA with Australasian components in other locations on the American continent. That’s the challenge.”

And a mummy also gave a hand
The scientists were given a massive helping hand by an 11,000-year-old mummy – Spirit Cave Man – discovered in Nevada, USA.

This mummy is the world’s oldest mummified human being and was found in the 1950s in an area native to the Indian Fallon Paiute-Shoshones tribe. The mummy was sent to the Nevada State Museum and the Indian tribe was unable to get it back. Many scientists over the years have wanted to subject it to DNA analyses.

The chiefs of the tribe have consistently refused to allow DNA analysis, but three years ago they gave Eske Willerslev permission to try – on one condition: “The agreement was that if the mummy were genetically proven to represent an early Indian – and many scientists had their doubts – it should be reburied.”

When the analyses were finished, the answer came from the Centre for GeoGenetics in Copenhagen: genetically, the mummy was an Indian. The museum therefore had to hand over the mummy and, in the summer, it was buried at a grand ceremony in which Eske Willerslev also participated. As the DNA professor says, it was both formal and extremely moving: “The tribe put their ancestors to rest, nothing less. And it was a good feeling that we, at the Centre for GeoGenetics, helped it happen.”

Genetically, Spirit Cave Man is very closely related to the individual who lived in Lagoa Santa at approximately the same time – although without the Australasian signal.