Many thousands of years ago, not a single human being lived in the Americas.
This only changed during the last Ice Age. It was a time when most of North America was covered with a thick sheet of ice, which made the Americas difficult to inhabit.
But at some point during this time, adventurous humans started their journey into a new world.
They probably came on foot from Siberia across the Bering Land Bridge, which existed between Alaska and Eurasia from the end of the last Ice Age until about 10,000 years ago. The area is now submerged by water.
There is still debate about when these first Americans actually arrived and where they came from. But we are now getting closer to uncovering the original narrative, and finding out who these first Americans really were.
During the peak of the last Ice Age about 20,000 years ago, a journey from Asia into the Americas would not have been particularly desirable. North America was covered in icy permafrost and tall glaciers. But, paradoxically, the presence of so much ice meant that the journey was, in a way, easier than it would be today.
The abundance of ice meant that sea levels were much lower than they are now, and a stretch of land emerged between Siberia and Alaska. Humans and animals could simply walk from Asia to North America. The land bridge was called Beringia.
People were using the woody shrubs from the land bridge to ignite bones on the landscape
At some point around this time – known as the Last Glacial Maximum – groups of hunter-gatherers moved east from what is now Siberia to set up camp there.
"The first people who arrived in Beringia were probably small, highly mobile groups evolving in a large landscape, probably depending on the availability of seasonal resources," says Lauriane Bourgeon of the University of Montreal, Canada.
These people did well to seek refuge there. Central Beringia was a much more desirable environment than the icy lands they had left behind. The climate was a bit damper. Vegetation, in the form of woody shrubs, would have given them access to wood that they could burn to keep warm.
Beringia was also an ideal environment for large grazing mammals, giving early hunter-gathers something to hunt, says Scott Elias of Royal Holloway, University London in the UK, who reconstructs past climates.
"Our hypothesis is that people were using the woody shrubs from the land bridge to ignite bones on the landscape. The bones of big animals contain lots of fatty deposits of marrow, and they will burn."
When humans got to Beringia, they would have had little choice but to set up camp there. The vast Laurentide and Cordilleran ice sheets further east cut them off from North America.
This standstill helped these isolated groups of people to become genetically distinct from those they had left behind
It is now becoming clear that they made Beringia their home, staying put for several thousand years. This idea is called the Beringian Standstill Hypothesis. This standstill helped these isolated groups of people to become genetically distinct from those they had left behind, according to a 2007 study.
This long standstill therefore meant that the people who arrived in the Americas – when the ice finally retreated and allowed entry – were genetically different to the individuals who had left Siberia thousands of years earlier. "Arguably one of the most important parts of the process is what happened in Beringia. That's when they differentiated from Asians and started becoming Native Americans," says Connie Mulligan of the University of Florida in Gainesville, US, who took part in this early analysis.
Since then, other genetic insights have further supported the standstill hypothesis. Elias and colleagues even propose that people stayed in Beringia for as long as 10,000 years.
When the ice finally started to retreat, groups of people then travelled to different pockets of the Americas.
There has long been debate over whether these early settlers arrived from several migrations from different areas, or just one.
There's been no turnover or change in the population group as some people had previously hypothesised
Over 20 years ago, Mulligan proposed that there was just one migration from Beringia into the "New World". She came to this conclusion by analysing the genetic variation in the DNA of modern-day Native Americans and comparing it with the variation in Asia. The same rare pattern appeared in all the Native Americans she studied, but very rarely appeared in modern-day Asians. This meant Native Americans likely arose from a single population of people who had lived in Beringia, isolated for many years.
In 2015, a study using more advanced genetic techniques came to a similar conclusion. Rasmus Nielsen of the University of California, Berkeley, US, and colleagues found that the "vast majority" of Native Americans must have originated from just one colonisation event.
"There's been no turnover or change in the population group as some people had previously hypothesised," says Nielsen. In fact, about 80% of Native Americans today are direct descendants of the Clovis people, who lived across North America about 13,000 years ago. This discovery came from a 2014 genetic study of a one-year-old Clovis boy who died about 12,700 years ago.
But we now know there must have been staggered migrations from Beringia.
That is because there are small groups of people in the Amazonian region of South America – such as the Suruí and Karitiana – with additional mysterious "arctic gene flow", unrelated to the Clovis boy. Another 2015 study therefore proposed there was more than one "founding population of the Americas".
The indigenous populations of the Americas, the team found, have distant genetic links in common with people of Australia, Papua New Guinea and the Andaman Islands.
People came into Beringia over different times during the standstill
This means, says Pontus Skoglund of Harvard University in Boston, Massachusetts, that people came into Beringia over different times during the "standstill" and went on to populate different parts of the Americas. Those early dispersals are still reflected by differences in the genomes of people living today.
"It wasn't simply a single homogenous founding population. There must have been some type of patchwork of people, and maybe there were multiple pulses," says Skoglund.
In other words, the Beringian inhabitants did not all arrive or leave at the same time.
This makes sense when you consider that Beringia was not a narrow land bridge with ocean on either side. "It was a huge region about twice the size of Texas," says Elias. The people living there would have had no idea that it was a land bridge at all. "There were no sign posts saying they were leaving Siberia."
This makes it highly likely that there were different groups of Beringian inhabitants that never met.
A study published in February 2017 strengthens this idea further. After examining the shapes of 800- to 500-year-old skulls from Mexico, researchers found they were so distinct, the people the skulls belonged to must have remained genetically isolated for at least 20,000 years.
To understand who the first Americans really were, we have to consider when they arrived. While the exact timing is hard to pin down. Nielsen's work gives some insight. By sequencing the genomes of people from the Americas, Siberia, and Oceania, he and colleagues could understand when these populations diverged. The team concludes that the ancestors of the first Americans came to Beringia at some point between 23,000 years and 13,000 years ago.
We found cut marks on bones from horse, caribou and wapiti so we know that humans were relying on those species
We now have archaeological evidence to suggest that the people who left Siberia – and then Beringia – did so even earlier than the 23,000-year-limit proposed by Nielsen and colleagues. In January 2017, Lauriane Bourgeon and her team found evidence of people living in a cave system in the northern Yukon Territory of western Canada, called the Bluefish Caves, that dates to as early as 24,000 years ago. It was previously believed that people had only arrived in this area 10,000 years later.
"They reached Beringia as early as 24,000 years ago, and they remained genetically and geographically isolated until about 16-15,000 years ago, before dispersing south of the ice sheets that covered most of North America during this period," says Bourgeon.
The caves "were only used on brief occasions for hunting activities", she says. "We found cut marks on bones from horse, caribou and wapiti, so we know that humans were relying on those species."
This work provides further evidence that people were in the Beringia area at this early date. But it does not reveal the exact dates these people first ventured further south.
For that, we can turn to archaeological evidence. For decades, stone tools left by the Clovis people have been found throughout North America. Some date to as early as 13,000 years ago. This might suggest that humans moved south very late. But in recent years evidence has begun to emerge that questions this idea.
Most preserved remains are stone tools and sometimes bones of animals
For instance, at a site called Monte Verde in southern Chile, there is evidence of human occupation that dates between 14,500 and 18,500 years ago. We know these people built fires, ate seafood and used stone tools – but because they did not leave any human remains behind, much about this early group remains mysterious.
"We really know little about them, because most preserved remains are stone tools and sometimes bones of animals, thus technology and diet," explains Tom Dillehay of Vanderbilt University in Tennessee, US, who is studying these people. "Monte Verde in south-central Chile, where I am at present, has several organic remains – animal hide, meat, plant remains that reveal a wider diet, wood technology – but these types of sites are rare to find."
Another conundrum remains. Ice sheets still covered North America 18,500 years ago, making journeying south difficult. How did people arrive in southern Chile so early?
A leading idea had been that an ice-free corridor opened up, which allowed humans to travel south. However, the latest evidence suggests this corridor only opened about 12,600 years ago, long after these early Chileans arrived.
Elias also points out how difficult this journey would have been. "Even if there was a small gap in between these enormous ice sheets, the environment left in that gap would have been so horrible, with mud, ice, meltwater and slush. It would not have been a habitable place for people or the animals they would have wanted to follow," he says.
These early people could have travelled by boat
There is an alternative. These early people could have travelled by boat, taking a route along the Pacific coast. There is no archaeological evidence to support this idea, but that is not entirely unexpected: wooden boats are rarely preserved in the archaeological record.
There are still many unanswered questions, but Mulligan says that studying how and when early hunter-gatherers spread across the Americas can help us to understand the process of migration itself. That is, how population sizes change and which genetic traits persist.
In many ways, the peopling of America presents scientists with a golden opportunity to study these processes. There have been multiple migrations both into and out of other regions of the world – Africa, Europe and Asia, for instance. But the people who moved into the Americas were on a one-way journey. "We know the original inhabitants came from Asia into the New World with no other people there, and no major back migrations, so it's the simplest model you can conceive of."
That it was a one-way journey, coupled with the increased interest in studying the genetics of these ancient people, means we should soon understand even more about who these first Americans really were, and exactly when they arrived.
Melissa Hogenboom is BBC Earth's associate editor. She is @melissasuzanneh on Twitter.
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